Crystals were obtained by different methods under conditions where nucleation and growth occur fr... more Crystals were obtained by different methods under conditions where nucleation and growth occur from precipitated macromolecular material. The phenomenon was observed with compounds of different size and nature, such as thaumatin, concanavalin A, an α-amylase, a thermostable aspartyl-tRNA synthetase, the nucleo-protein complex between a tRNAAsp transcript and its cognate yeast aspartyl-tRNA synthetase, and tomato bushy stunt virus. In each system,
Poster Sessions interactions. NMR shows that there is a reduction in the backbone mobility on the... more Poster Sessions interactions. NMR shows that there is a reduction in the backbone mobility on the regions of the PHD that participate in the peptide binding, and binding affinities differ depending on histone tail lengths. Thermodynamic analysis reveals that the discrimination in favor of methylated lysine is entropy driven, contrary to what has been described for chromodomains. The molecular basis of H3K4me3 recognition by ING4 differs from that of ING2, which is consistent with their different affinities for methylated histone tails. These differences suggest a distinct role in transcriptional regulation for these two ING family members due to the antagonistic effect of the complexes that they recruit onto chromatin. Our results illustrate the versatility of PHD fingers as readers of the histone code.
We show here that small RNA helices which recapitulate part or all of the acceptor stem of yeast ... more We show here that small RNA helices which recapitulate part or all of the acceptor stem of yeast aspartate tRNA are efficiently aminoacylated by cognate class II aspartyl-tRNA synthetase. Aminoacylation is strongly dependent on the presence of the single-stranded G73 'discriminator' identity nucleotide and is essentially insensitive to the sequence of the helical region. Substrates which contain as few as 3 bp fused to G73CCAoH are aspartylated. Their charging is insensitive to the sequence of the loop closing the short helical domains. Aminoacylation of the aspartate mini-helix is not stimulated by a hairpin helix mimicking the anticodon domain and containing the three major anticodon identity nucleotides. A thermodynamic analysis demonstrates that enzyme interactions with G73 in the resected RNA substrates and in the whole tRNA are the same. Thus, if the resected RNA molecules resemble in some way the earliest substrates for aminoacylation with aspartate, then the contemporary tRNAAsP has quantitatively retained the influence of the major signal for aminoacylation in these substrates.
Transfer RNA (tRNA) is a small nucleic acid (typically 76 nucleotides) that forms binary complexe... more Transfer RNA (tRNA) is a small nucleic acid (typically 76 nucleotides) that forms binary complexes with proteins, such as aminoacyl tRNA synthetases (RS) and Trbp111. The latter is a widely distributed structure-specific tRNA-binding protein that is incorporated into cell signaling molecules. The structure of Trbp111 was modeled onto to the outer, convex side of the L-shaped tRNA. Here we present RNA footprints that are consistent with this model. This binding mode is in contrast to that of tRNA synthetases, which bind to the inside, or concave side, of tRNA. These opposite locations of binding for these two proteins suggest the possibility of a ternary complex. The formation of a tRNA synthetase--tRNA--Trbp111 ternary complex was detected by two independent methods. The results indicate that the tRNA is sandwiched between the two protein molecules. A thermodynamic and functional analysis is consistent with the tRNA retaining its native structure in the ternary complex. These results may have implications for how the translation apparatus is linked to other cellular machinery.
Proceedings of the National Academy of Sciences of the United States of America, Jan 24, 2003
The genetic code was established through aminoacylations of RNA substrates that emerged as tRNAs.... more The genetic code was established through aminoacylations of RNA substrates that emerged as tRNAs. The 20 aminoacyl-tRNA synthetases (one for each amino acid) are ancient proteins, the active-site domain of which catalyzes formation of an aminoacyl adenylate that subsequently reacts with the 3' end of bound tRNA. Binding of tRNA depends on idiosyncratic (to the particular synthetase) domains and motifs that are fused to or inserted into the conserved active-site domain. Here we take the domain for synthesis of alanyl adenylate and fuse it to "artificial" peptide sequences (28 aa) that were shown previously to bind to the acceptor arm of tRNAAla. Certain fusions confer aminoacylation activity on tRNAAla and on hairpin microhelices modeled after its acceptor stem. Aminoacylation was sensitive to the presence of a specific G:U base pair known to be a major determinant of tRNAAla identity. Aminoacylation efficiency and specificity also depended on the specific peptide seque...
In vitro transcription has proven to be a successful tool for preparation of functional RNAs, esp... more In vitro transcription has proven to be a successful tool for preparation of functional RNAs, especially in the tRNA field, in which, despite the absence of post-transcriptional modifications, transcripts are correctly folded and functionally active. Human mitochondrial (mt) tRNALys deviates from this principle and folds into various inactive conformations, due to the absence of the post-transcriptional modification m1A9 which hinders base-pairing with U64 in the native tRNA. Unavailability of a functional transcript is a serious drawback for structure/function investigations as well as in deciphering the molecular mechanisms by which point mutations in the mt tRNALys gene cause severe human disorders. Here, we show that an engineered in vitro transcribed “pseudo-WT“ tRNALys variant is efficiently recognized by lysyl-tRNA synthetase and can substitute for the WT tRNA as a valuable reference molecule. This has been exploited in a systematic analysis of the effects on aminoacylation o...
Human tyrosyl-tRNA synthetase from mitochondria (mt-TyrRS) presents dual sequence features charac... more Human tyrosyl-tRNA synthetase from mitochondria (mt-TyrRS) presents dual sequence features characteristic of eubacterial and archaeal TyrRSs, especially in the region containing amino acids recognizing the N1-N72 tyrosine identity pair. This would imply that human mt-TyrRS has lost the capacity to discriminate between the G1-C72 pair typical of eubacterial and mitochondrial tRNATyr and the reverse pair C1-G72 present in archaeal and eukaryal tRNATyr. This expectation was verified by a functional analysis of wild-type or mutated tRNATyr molecules, showing that mt-TyrRS aminoacylates with similar catalytic efficiency its cognate tRNATyr with G1-C72 and its mutated version with C1-G72. This provides the first example of a TyrRS lacking specificity toward N1-N72 and thus of a TyrRS disobeying the identity rules. Sequence comparisons of mt-TyrRSs across phylogeny suggest that the functional behavior of the human mt-TyrRS is conserved among all vertebrate mt-TyrRSs.
Progress in Biophysics and Molecular Biology, 2009
Medium-sized single crystals with perfect habits and no defect producing intense and well-resolve... more Medium-sized single crystals with perfect habits and no defect producing intense and well-resolved diffraction patterns are the dream of every protein crystallographer. Crystals of biological macromolecules possessing these characteristics can be prepared within a medium in which mass transport is restricted to diffusion. Chemical gels (like polysiloxane) and physical gels (such as agarose) provide such an environment and are therefore suitable for the crystallisation of biological macromolecules. Instructions for the preparation of each type of gel are given to urge crystal growers to apply diffusive media for enhancing crystallographic quality of their crystals. Examples of quality enhancement achieved with silica and agarose gels are given. Results obtained with other substances forming gel-like media (such as lipidic phases and cellulose derivatives) are presented. Finally, the use of gels in combination with capillary tubes for counter-diffusion experiments is discussed. Methods and techniques implemented with proteins can also be applied to nucleic acids and nucleoprotein assemblies such as viruses.
The crystallization of transfer RNA (tRNA) was investigated using atomic force microscopy (AFM) o... more The crystallization of transfer RNA (tRNA) was investigated using atomic force microscopy (AFM) over the temperature range from 4 to 16_C, and this produced the first in situ AFM images of developing nucleic acid crystals. The growth of the (110) face of hexagonal yeast tRNA Phe crystals was observed to occur at steps on vicinal hillocks generated by multiple screw dislocation sources in the temperature range of 13.5-16_C. Two-dimensional nucleation begins to dominate at 13.5_C, with the appearance of threedimensional nuclei at 12_C. The changes in growth mechanisms are correlated with variations in supersaturation which is higher in the low temperature range. Growth of tRNA crystals was characterized by a strong anisotropy in the tangential step movement and transformation of growth modes on single crystals were directly observed by AFM over the narrow temperature range utilized. Finally, lattice resolution images of the molecular structure of surface layers were recorded. The implications of the strong temperature dependence of tRNA Phe crystal growth are discussed in view of improving and better controlling crystallization of nucleic acids.
Direct sequencing of human mitochondrial tRNA Lys shows the absence of editing and the occurrence... more Direct sequencing of human mitochondrial tRNA Lys shows the absence of editing and the occurrence of six modified nucleotides (m 1 A9, m 2 G10, Ψ27, Ψ28 and hypermodified nucleotides at positions U34 and A37). This tRNA folds into the expected cloverleaf, as confirmed by structural probing with nucleases. The solution structure of the corresponding in vitro transcript unexpectedly does not fold into a cloverleaf but into an extended bulged hairpin. This non-canonical fold, established according to the reactivity to a large set of chemical and enzymatic probes, includes a 10 bp aminoacyl acceptor stem (the canonical 7 bp and 3 new pairs between residues 8-10 and 65-63), a 13 nt large loop and an anticodon-like domain. It is concluded that modified nucleotides have a predominant role in canonical folding of human mitochondrial tRNA Lys. Phylogenetic comparisons as well as structural probing of selected in vitro transcribed variants argue in favor of a major contribution of m 1 A9 in this process.
The solution conformation of eight leucine tRNAs from Phaseolus vulgars, baker's yeast and Escher... more The solution conformation of eight leucine tRNAs from Phaseolus vulgars, baker's yeast and Escherichia coil, characterized by long variable regions, and the interaction of four of them with bean cytoplasmic leucyl-tRNA synthetase were studied by phosphate mapping with ethyinitrosourea. Phosphate reactivities in the variable regions agree with the existence of RNA helices closed by miniloops. At the junction of these regions with the T-stem, phosphate 48 is strongly protected, in contrast to small variable region tRNAs where P49 is protected. The constant protection of P22 is another characteristics of leucine tRNAs. Conformational differences between leucine isoacceptors concern the anticodon region, the D-arm and the variable region. In several parts of free tRNALOU species, e.g. in the T-loop, phosphate reactivities are similar to those found in tRNAs of other specificities, indicating conformational similarities among tRNAs. Phosphate alkylation of four leucine tRNAs complexed to leucyl-tRNA synthetase indicates that the 3'-side of the anticodon stem, the D-stem and the hinge region between the anticodon and D-stems are in contact with the plant enzyme.
The influence of nine synthetic polyamines on in vitro transcription with T7 RNA polymerase has b... more The influence of nine synthetic polyamines on in vitro transcription with T7 RNA polymerase has been studied. The compounds used were linear or macrocyclic tetra-and hexaamine, varying in their size, shape and number of protonated groups. Their effect was tested on different types of templates, all presenting the T7 RNA promoter in a double-stranded form followed by sequences encoding short transcripts (25 to 35-mers) either on single-or double-stranded synthetic oligodeoxyribonucleotides. All polyamines used stimulate transcription of both types of templates at levels dependent on their size, shape, protonation degree, and concentration. For each compound, an optimal concentration could be defined; above this concentration, transcription inhibition occurred. Highest stimulation (up to 12-fold) was obtained by the largest cyclic compound called [38JN6C10.
Systematic crystallization studies on baker's yeast aspartyl-tRl~A synthetase (Mr 114,000, dimeri... more Systematic crystallization studies on baker's yeast aspartyl-tRl~A synthetase (Mr 114,000, dimeric structure} have yielded two tetragonal crystal forms (space group P41212) differing in their unit cell dimensions. The first form (a = b = 92 ~, c ~ 185 A) contains one subunit per asymmetric unit, whereas the second one crystallized in presence of the cognate tRNA (a = b ~-89.1 A, c ~ 480 A) contains the whole enzyme molecule in the asymmetric unit. The diffraction pattern extends to 3 A of resolution with synchrotron radiation. From the crystallographic data it is shown that aspartyl-tRNA synthetase has an elongated shape.
Dimeric class II aspartyl-tRNA synthetase (AspRS) from yeast has a modular architecture and inclu... more Dimeric class II aspartyl-tRNA synthetase (AspRS) from yeast has a modular architecture and includes an N-terminal appendix of 70 amino acid residues that protrudes from the anticodon-binding module. This extension, of predicted helical structure, is not essential for aminoacylation but contains an RNA-binding motif that promotes non-specific interactions with tRNAs. As shown here, this protein extension can also interact with the 5 0 end of the AspRS mRNA. In vitro, optimal binding occurs on an mRNA domain comprising part of the 87 nucleotide long 5 0 UTR and the sequence encoding the N-terminal appendix. At the protein side, only the appendix and the anticodon-binding module participate in the interaction between AspRS and the mRNA domain. Binding is specific, since only tRNA Asp can dissociate the complex. In vivo, AspRS also binds specifically this mRNA domain and in doing so triggers a reduced translation of a fused GFP mRNA. From that, a mechanism for the regulation of this eukaryotic aminoacyl-tRNA synthetase is proposed. Implications for aspartylation accuracy in yeast are given.
This study evaluates the role of the N-terminal extension from yeast aspartyl-tRNA synthetase in ... more This study evaluates the role of the N-terminal extension from yeast aspartyl-tRNA synthetase in tRNA aspartylation. The presence of an RNA-binding motif in this extension, conserved in eukaryotic class IIb aminoacyl-tRNA synthetases, provides nonspecific tRNA binding properties to this enzyme. Here, it is assumed that the additional contacts the 70 amino acid-long appendix of aspartyl-tRNA synthetase makes with tRNA could be important in expression of aspartate identity in yeast. Using in vitro transcripts mutated at identity positions, it is demonstrated that the extension grants better aminoacylation efficiency but reduced specificity to the synthetase, increasing considerably the risk of noncognate tRNA mischarging. Yeast tRNA Glu(UUC) and tRNA Asn(GUU) were identified as the most easily mischarged tRNA species. Both have a G at the discriminator position, and their anticodon differs only by one change from the GUC aspartate anticodon.
Phenylalanine identity of yeast tRNA Phe is governed by five nucleotides including residues A73, ... more Phenylalanine identity of yeast tRNA Phe is governed by five nucleotides including residues A73, G20, and the three anticodon nucleotides (Sampson et al., 1989, Science 243, 1363-1366). Analysis of in vitro transcripts derived from yeast tRNA Phe and Escherichia coli tRNA Ala bearing these recognition elements shows that phenylalanyl-tRNA synthetase is sensitive to additional nucleotides within the acceptor stem. Insertion of G2-C71 has dramatic negative effects in both tRNA frameworks. These effects become compensated by a second-site mutation, the insertion of the wobble G3-U70 pair, which by itself has no effect on phenylalanylation. From a mechanistic point of view, the G2-C71/G3-U70 combination is not a "classical" recognition element since its antideterminant effect is compensated for by a second-site mutation. This enlarges our understanding of tRNA identity that appears not only to be the outcome of a combination of positive and negative signals forming the so-called recognition/identity set but that is also based on the presence of nonrandom combinations of sequences elsewhere in tRNA. These sequences, we name "permissive elements," are retained by evolution so that they do not hinder aminoacylation. Likely, no nucleotide within a tRNA is of random nature but has been selected so that a tRNA can fulfill all its functions efficiently.
Orthorhombic crystals of the enzyme aspartyl-tRNA synthetase (AspRS) were prepared in agarose gel... more Orthorhombic crystals of the enzyme aspartyl-tRNA synthetase (AspRS) were prepared in agarose gel, a chemical alternative to microgravity or nano-volume drops. Besides providing a convection-free medium, the network of the polysaccharide improved the stability of the crystalline lattice during soaking with L-aspartol adenylate, a synthetic and non-hydrolysable analog of the catalytic intermediate aspartyl adenylate. When crystals were embedded in the polysaccharide matrix the ligand reached their surfaces more uniformly. Gel-grown crystals exhibited well defined reflections even at high resolution and low mosaicity values, despite their fairly high solvent content and the relatively harsh flash cooling procedure. By contrast, soaked AspRS crystals prepared in solution broke apart within 10–30 s after the ligand was introduced into the mother liquor, and subsequently these fragments became an amorphous precipitate. A general objection to the use of gels in protein crystallization is ...
Crystals were obtained by different methods under conditions where nucleation and growth occur fr... more Crystals were obtained by different methods under conditions where nucleation and growth occur from precipitated macromolecular material. The phenomenon was observed with compounds of different size and nature, such as thaumatin, concanavalin A, an α-amylase, a thermostable aspartyl-tRNA synthetase, the nucleo-protein complex between a tRNAAsp transcript and its cognate yeast aspartyl-tRNA synthetase, and tomato bushy stunt virus. In each system,
Poster Sessions interactions. NMR shows that there is a reduction in the backbone mobility on the... more Poster Sessions interactions. NMR shows that there is a reduction in the backbone mobility on the regions of the PHD that participate in the peptide binding, and binding affinities differ depending on histone tail lengths. Thermodynamic analysis reveals that the discrimination in favor of methylated lysine is entropy driven, contrary to what has been described for chromodomains. The molecular basis of H3K4me3 recognition by ING4 differs from that of ING2, which is consistent with their different affinities for methylated histone tails. These differences suggest a distinct role in transcriptional regulation for these two ING family members due to the antagonistic effect of the complexes that they recruit onto chromatin. Our results illustrate the versatility of PHD fingers as readers of the histone code.
We show here that small RNA helices which recapitulate part or all of the acceptor stem of yeast ... more We show here that small RNA helices which recapitulate part or all of the acceptor stem of yeast aspartate tRNA are efficiently aminoacylated by cognate class II aspartyl-tRNA synthetase. Aminoacylation is strongly dependent on the presence of the single-stranded G73 'discriminator' identity nucleotide and is essentially insensitive to the sequence of the helical region. Substrates which contain as few as 3 bp fused to G73CCAoH are aspartylated. Their charging is insensitive to the sequence of the loop closing the short helical domains. Aminoacylation of the aspartate mini-helix is not stimulated by a hairpin helix mimicking the anticodon domain and containing the three major anticodon identity nucleotides. A thermodynamic analysis demonstrates that enzyme interactions with G73 in the resected RNA substrates and in the whole tRNA are the same. Thus, if the resected RNA molecules resemble in some way the earliest substrates for aminoacylation with aspartate, then the contemporary tRNAAsP has quantitatively retained the influence of the major signal for aminoacylation in these substrates.
Transfer RNA (tRNA) is a small nucleic acid (typically 76 nucleotides) that forms binary complexe... more Transfer RNA (tRNA) is a small nucleic acid (typically 76 nucleotides) that forms binary complexes with proteins, such as aminoacyl tRNA synthetases (RS) and Trbp111. The latter is a widely distributed structure-specific tRNA-binding protein that is incorporated into cell signaling molecules. The structure of Trbp111 was modeled onto to the outer, convex side of the L-shaped tRNA. Here we present RNA footprints that are consistent with this model. This binding mode is in contrast to that of tRNA synthetases, which bind to the inside, or concave side, of tRNA. These opposite locations of binding for these two proteins suggest the possibility of a ternary complex. The formation of a tRNA synthetase--tRNA--Trbp111 ternary complex was detected by two independent methods. The results indicate that the tRNA is sandwiched between the two protein molecules. A thermodynamic and functional analysis is consistent with the tRNA retaining its native structure in the ternary complex. These results may have implications for how the translation apparatus is linked to other cellular machinery.
Proceedings of the National Academy of Sciences of the United States of America, Jan 24, 2003
The genetic code was established through aminoacylations of RNA substrates that emerged as tRNAs.... more The genetic code was established through aminoacylations of RNA substrates that emerged as tRNAs. The 20 aminoacyl-tRNA synthetases (one for each amino acid) are ancient proteins, the active-site domain of which catalyzes formation of an aminoacyl adenylate that subsequently reacts with the 3' end of bound tRNA. Binding of tRNA depends on idiosyncratic (to the particular synthetase) domains and motifs that are fused to or inserted into the conserved active-site domain. Here we take the domain for synthesis of alanyl adenylate and fuse it to "artificial" peptide sequences (28 aa) that were shown previously to bind to the acceptor arm of tRNAAla. Certain fusions confer aminoacylation activity on tRNAAla and on hairpin microhelices modeled after its acceptor stem. Aminoacylation was sensitive to the presence of a specific G:U base pair known to be a major determinant of tRNAAla identity. Aminoacylation efficiency and specificity also depended on the specific peptide seque...
In vitro transcription has proven to be a successful tool for preparation of functional RNAs, esp... more In vitro transcription has proven to be a successful tool for preparation of functional RNAs, especially in the tRNA field, in which, despite the absence of post-transcriptional modifications, transcripts are correctly folded and functionally active. Human mitochondrial (mt) tRNALys deviates from this principle and folds into various inactive conformations, due to the absence of the post-transcriptional modification m1A9 which hinders base-pairing with U64 in the native tRNA. Unavailability of a functional transcript is a serious drawback for structure/function investigations as well as in deciphering the molecular mechanisms by which point mutations in the mt tRNALys gene cause severe human disorders. Here, we show that an engineered in vitro transcribed “pseudo-WT“ tRNALys variant is efficiently recognized by lysyl-tRNA synthetase and can substitute for the WT tRNA as a valuable reference molecule. This has been exploited in a systematic analysis of the effects on aminoacylation o...
Human tyrosyl-tRNA synthetase from mitochondria (mt-TyrRS) presents dual sequence features charac... more Human tyrosyl-tRNA synthetase from mitochondria (mt-TyrRS) presents dual sequence features characteristic of eubacterial and archaeal TyrRSs, especially in the region containing amino acids recognizing the N1-N72 tyrosine identity pair. This would imply that human mt-TyrRS has lost the capacity to discriminate between the G1-C72 pair typical of eubacterial and mitochondrial tRNATyr and the reverse pair C1-G72 present in archaeal and eukaryal tRNATyr. This expectation was verified by a functional analysis of wild-type or mutated tRNATyr molecules, showing that mt-TyrRS aminoacylates with similar catalytic efficiency its cognate tRNATyr with G1-C72 and its mutated version with C1-G72. This provides the first example of a TyrRS lacking specificity toward N1-N72 and thus of a TyrRS disobeying the identity rules. Sequence comparisons of mt-TyrRSs across phylogeny suggest that the functional behavior of the human mt-TyrRS is conserved among all vertebrate mt-TyrRSs.
Progress in Biophysics and Molecular Biology, 2009
Medium-sized single crystals with perfect habits and no defect producing intense and well-resolve... more Medium-sized single crystals with perfect habits and no defect producing intense and well-resolved diffraction patterns are the dream of every protein crystallographer. Crystals of biological macromolecules possessing these characteristics can be prepared within a medium in which mass transport is restricted to diffusion. Chemical gels (like polysiloxane) and physical gels (such as agarose) provide such an environment and are therefore suitable for the crystallisation of biological macromolecules. Instructions for the preparation of each type of gel are given to urge crystal growers to apply diffusive media for enhancing crystallographic quality of their crystals. Examples of quality enhancement achieved with silica and agarose gels are given. Results obtained with other substances forming gel-like media (such as lipidic phases and cellulose derivatives) are presented. Finally, the use of gels in combination with capillary tubes for counter-diffusion experiments is discussed. Methods and techniques implemented with proteins can also be applied to nucleic acids and nucleoprotein assemblies such as viruses.
The crystallization of transfer RNA (tRNA) was investigated using atomic force microscopy (AFM) o... more The crystallization of transfer RNA (tRNA) was investigated using atomic force microscopy (AFM) over the temperature range from 4 to 16_C, and this produced the first in situ AFM images of developing nucleic acid crystals. The growth of the (110) face of hexagonal yeast tRNA Phe crystals was observed to occur at steps on vicinal hillocks generated by multiple screw dislocation sources in the temperature range of 13.5-16_C. Two-dimensional nucleation begins to dominate at 13.5_C, with the appearance of threedimensional nuclei at 12_C. The changes in growth mechanisms are correlated with variations in supersaturation which is higher in the low temperature range. Growth of tRNA crystals was characterized by a strong anisotropy in the tangential step movement and transformation of growth modes on single crystals were directly observed by AFM over the narrow temperature range utilized. Finally, lattice resolution images of the molecular structure of surface layers were recorded. The implications of the strong temperature dependence of tRNA Phe crystal growth are discussed in view of improving and better controlling crystallization of nucleic acids.
Direct sequencing of human mitochondrial tRNA Lys shows the absence of editing and the occurrence... more Direct sequencing of human mitochondrial tRNA Lys shows the absence of editing and the occurrence of six modified nucleotides (m 1 A9, m 2 G10, Ψ27, Ψ28 and hypermodified nucleotides at positions U34 and A37). This tRNA folds into the expected cloverleaf, as confirmed by structural probing with nucleases. The solution structure of the corresponding in vitro transcript unexpectedly does not fold into a cloverleaf but into an extended bulged hairpin. This non-canonical fold, established according to the reactivity to a large set of chemical and enzymatic probes, includes a 10 bp aminoacyl acceptor stem (the canonical 7 bp and 3 new pairs between residues 8-10 and 65-63), a 13 nt large loop and an anticodon-like domain. It is concluded that modified nucleotides have a predominant role in canonical folding of human mitochondrial tRNA Lys. Phylogenetic comparisons as well as structural probing of selected in vitro transcribed variants argue in favor of a major contribution of m 1 A9 in this process.
The solution conformation of eight leucine tRNAs from Phaseolus vulgars, baker's yeast and Escher... more The solution conformation of eight leucine tRNAs from Phaseolus vulgars, baker's yeast and Escherichia coil, characterized by long variable regions, and the interaction of four of them with bean cytoplasmic leucyl-tRNA synthetase were studied by phosphate mapping with ethyinitrosourea. Phosphate reactivities in the variable regions agree with the existence of RNA helices closed by miniloops. At the junction of these regions with the T-stem, phosphate 48 is strongly protected, in contrast to small variable region tRNAs where P49 is protected. The constant protection of P22 is another characteristics of leucine tRNAs. Conformational differences between leucine isoacceptors concern the anticodon region, the D-arm and the variable region. In several parts of free tRNALOU species, e.g. in the T-loop, phosphate reactivities are similar to those found in tRNAs of other specificities, indicating conformational similarities among tRNAs. Phosphate alkylation of four leucine tRNAs complexed to leucyl-tRNA synthetase indicates that the 3'-side of the anticodon stem, the D-stem and the hinge region between the anticodon and D-stems are in contact with the plant enzyme.
The influence of nine synthetic polyamines on in vitro transcription with T7 RNA polymerase has b... more The influence of nine synthetic polyamines on in vitro transcription with T7 RNA polymerase has been studied. The compounds used were linear or macrocyclic tetra-and hexaamine, varying in their size, shape and number of protonated groups. Their effect was tested on different types of templates, all presenting the T7 RNA promoter in a double-stranded form followed by sequences encoding short transcripts (25 to 35-mers) either on single-or double-stranded synthetic oligodeoxyribonucleotides. All polyamines used stimulate transcription of both types of templates at levels dependent on their size, shape, protonation degree, and concentration. For each compound, an optimal concentration could be defined; above this concentration, transcription inhibition occurred. Highest stimulation (up to 12-fold) was obtained by the largest cyclic compound called [38JN6C10.
Systematic crystallization studies on baker's yeast aspartyl-tRl~A synthetase (Mr 114,000, dimeri... more Systematic crystallization studies on baker's yeast aspartyl-tRl~A synthetase (Mr 114,000, dimeric structure} have yielded two tetragonal crystal forms (space group P41212) differing in their unit cell dimensions. The first form (a = b = 92 ~, c ~ 185 A) contains one subunit per asymmetric unit, whereas the second one crystallized in presence of the cognate tRNA (a = b ~-89.1 A, c ~ 480 A) contains the whole enzyme molecule in the asymmetric unit. The diffraction pattern extends to 3 A of resolution with synchrotron radiation. From the crystallographic data it is shown that aspartyl-tRNA synthetase has an elongated shape.
Dimeric class II aspartyl-tRNA synthetase (AspRS) from yeast has a modular architecture and inclu... more Dimeric class II aspartyl-tRNA synthetase (AspRS) from yeast has a modular architecture and includes an N-terminal appendix of 70 amino acid residues that protrudes from the anticodon-binding module. This extension, of predicted helical structure, is not essential for aminoacylation but contains an RNA-binding motif that promotes non-specific interactions with tRNAs. As shown here, this protein extension can also interact with the 5 0 end of the AspRS mRNA. In vitro, optimal binding occurs on an mRNA domain comprising part of the 87 nucleotide long 5 0 UTR and the sequence encoding the N-terminal appendix. At the protein side, only the appendix and the anticodon-binding module participate in the interaction between AspRS and the mRNA domain. Binding is specific, since only tRNA Asp can dissociate the complex. In vivo, AspRS also binds specifically this mRNA domain and in doing so triggers a reduced translation of a fused GFP mRNA. From that, a mechanism for the regulation of this eukaryotic aminoacyl-tRNA synthetase is proposed. Implications for aspartylation accuracy in yeast are given.
This study evaluates the role of the N-terminal extension from yeast aspartyl-tRNA synthetase in ... more This study evaluates the role of the N-terminal extension from yeast aspartyl-tRNA synthetase in tRNA aspartylation. The presence of an RNA-binding motif in this extension, conserved in eukaryotic class IIb aminoacyl-tRNA synthetases, provides nonspecific tRNA binding properties to this enzyme. Here, it is assumed that the additional contacts the 70 amino acid-long appendix of aspartyl-tRNA synthetase makes with tRNA could be important in expression of aspartate identity in yeast. Using in vitro transcripts mutated at identity positions, it is demonstrated that the extension grants better aminoacylation efficiency but reduced specificity to the synthetase, increasing considerably the risk of noncognate tRNA mischarging. Yeast tRNA Glu(UUC) and tRNA Asn(GUU) were identified as the most easily mischarged tRNA species. Both have a G at the discriminator position, and their anticodon differs only by one change from the GUC aspartate anticodon.
Phenylalanine identity of yeast tRNA Phe is governed by five nucleotides including residues A73, ... more Phenylalanine identity of yeast tRNA Phe is governed by five nucleotides including residues A73, G20, and the three anticodon nucleotides (Sampson et al., 1989, Science 243, 1363-1366). Analysis of in vitro transcripts derived from yeast tRNA Phe and Escherichia coli tRNA Ala bearing these recognition elements shows that phenylalanyl-tRNA synthetase is sensitive to additional nucleotides within the acceptor stem. Insertion of G2-C71 has dramatic negative effects in both tRNA frameworks. These effects become compensated by a second-site mutation, the insertion of the wobble G3-U70 pair, which by itself has no effect on phenylalanylation. From a mechanistic point of view, the G2-C71/G3-U70 combination is not a "classical" recognition element since its antideterminant effect is compensated for by a second-site mutation. This enlarges our understanding of tRNA identity that appears not only to be the outcome of a combination of positive and negative signals forming the so-called recognition/identity set but that is also based on the presence of nonrandom combinations of sequences elsewhere in tRNA. These sequences, we name "permissive elements," are retained by evolution so that they do not hinder aminoacylation. Likely, no nucleotide within a tRNA is of random nature but has been selected so that a tRNA can fulfill all its functions efficiently.
Orthorhombic crystals of the enzyme aspartyl-tRNA synthetase (AspRS) were prepared in agarose gel... more Orthorhombic crystals of the enzyme aspartyl-tRNA synthetase (AspRS) were prepared in agarose gel, a chemical alternative to microgravity or nano-volume drops. Besides providing a convection-free medium, the network of the polysaccharide improved the stability of the crystalline lattice during soaking with L-aspartol adenylate, a synthetic and non-hydrolysable analog of the catalytic intermediate aspartyl adenylate. When crystals were embedded in the polysaccharide matrix the ligand reached their surfaces more uniformly. Gel-grown crystals exhibited well defined reflections even at high resolution and low mosaicity values, despite their fairly high solvent content and the relatively harsh flash cooling procedure. By contrast, soaked AspRS crystals prepared in solution broke apart within 10–30 s after the ligand was introduced into the mother liquor, and subsequently these fragments became an amorphous precipitate. A general objection to the use of gels in protein crystallization is ...
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Papers by Richard Giegé