Extended —10 Promoters

E. coli RNA polymerase (RNAP) is a multisubunit enzyme with a molecular mass of nearly half a million. The major form of RNAP in cells consists of core enzyme (subunit composition ββ′α2) in complex with the σ70 factor. σ70 is 613 amino acids in length: sequence comparisons show that it shares four regions of amino acid sequence similarity with other σ factors (regions 1, 2, 3 and 4: Fig. 1; Gross et al. 1992). It has long been known that RNAP containing σ70, is competent to initiate transcription at many promoters in the absence of any activator protein and that the σ70 subunit is essential for recognition of these promoters.