Protein–Protein and Protein–DNA Interactions of σ70 Region 4 Involved in Transcription Activation by λcI

The cI protein of bacteriophage λ (λcI) activates transcription from promoter PRM through an acidic patch on the surface of its DNA-binding domain. Genetic evidence suggests that this acidic patch stimulates transcription from PRM through contact with the C-terminal domain (region 4) of the σ70 subunit of Escherichia coli RNA polymerase. Here, we identify two basic residues in region 4 of σ70 that are critical for λcI-mediated activation of transcription from PRM. On the basis of structural modeling, we propose that one of these σ70 residues, K593, facilitates the interaction between λcI and region 4 of σ70 by inducing a bend in the DNA upstream of the −35 element, whereas the other, R588, interacts directly with a critical acidic residue within the activating patch of λcI. Residue R588 of σ70 has been shown to play an important role in promoter recognition; our findings suggest that the R588 side-chain has a dual function at PRM, facilitating the interaction of region 4 with the promoter −35 element and participating directly in the protein–protein interaction with λcI.