Abstract
The σ initiation factor σ
70
of
Escherichia coli
acts not only in promoter recognition and DNA strand opening, but also to mediate the transformation of RNA polymerase (RNAP) to an antiterminating form by the phage λ gene Q protein. Q is able to bind and modify RNAP when α
70
, still present in the initially elongating enzyme, recognizes a repeat of the −10 promoter element and induces a transcription pause. We have isolated mutations in the
rpoD
gene for σ
70
that impair Q function because they reduce the ability of σ
70
to recognize the downstream pause site. These mutations identify a locus of σ
70
that is important for the formation and stability of open promoter complex, likely because it mediates protein interactions with RNAP core.