Region 2.5 of the Escherichia coli RNA polymerase sigma(70) subunit is responsible for the recognition of the 'extended -10' motif at promoters

At some bacterial promoters, a 5'-TG-3' sequence element, located one base upstream of the -10 hexamer element, provides an essential motif necessary for transcription initiation, We have identified a mutant of the Escherichia coli RNA polymerase sigma(70) subunit that has an altered preference for base sequences in this 'extended -10' region. We show that this mutant sigma(70) subunit substantially increases transcription from promoters bearing 5'-TC-3' or 5'-TT-3' instead of a 5'-TG-3' moth, located one base upstream of the -10 hexamer, The mutant results from a single base pair substitution in the rpoD gene that causes a Glu to Gly change at position 458 of sigma(70). This substitution identifies a functional region in sigma(70) that is immediately adjacent to the well-characterized region 2.4 (positions 434-453, previously shown to contact the -10 hexamer). From these results, we conclude that this region (which we name region 2.5) is involved in contacting the 5'-TG-3' motif found at some bacterial promoters: thus, extended -10 regions are recognized by an extended region 2 of the RNA polymerase sigma(70) subunit.