The minus 35-recognition region of Escherichia coli sigma 70 is inessential for initiation of transcription at an "extended minus 10" promoter.

It is known that Escherichia coli promoters having the major minus 10 consensus sequence TATAAT, but lacking significant resemblance to consensus in the minus 35 region, allow transcriptional initiation in vivo and in vitro if they have an additional TGn motif immediately upstream of minus 10. To determine whether region 4.2 of sigma 70, whose normal role is sequence recognition at minus 35, is unnecessary for initiation of transcription at such "extended minus 10" promoters, we modified the sigma 70 gene so as to generate a carboxy-truncated polypeptide lacking the last 84 amino acids and therefore missing region 4.2. Our results show that both the intact and truncated sigma 70 allow purified RNA polymerase to initiate efficiently and specifically at an extended minus 10 promoter, whereas only the intact sigma 70 permits efficient initiation at normal promoters (defined by minus 35 and minus 10 hexamer sequences).