Escherichia coli σ(70) senses sequence and conformation of the promoter spacer region

In bacteria, promoter identification by RNA polymerase is mediated by a dissociable σ factor. The housekeeping σ(70) factor of Escherichia coli recognizes two well characterized DNA sequence elements, known as the ‘−10’ and ‘−35’ hexamers. These elements are separated by ‘spacer’ DNA, the sequence of which is generally considered unimportant. Here, we use a combination of bioinformatics, genetics and biochemistry to show that σ(70) can sense the sequence and conformation of the promoter spacer region. Our data illustrate how alterations in spacer region sequence can increase promoter activity. This stimulatory effect requires σ(70) side chain R451, which is located in close proximity to the non-template strand at promoter position −18. Conversely, R451 is not required to mediate transcriptional stimulation by improvement of the −10 element. Mutation of σ(70) residue R451, which is highly conserved, results in reduced growth rate, consistent with a central role in promoter recognition.