Protein-protein and Protein-DNA Interactions of Sigma70 Region 4 Involved in Transcription Activation by LambdacI

The cI protein of bacteriophage lambda (lambdacI) activates transcription from promoter P(RM) through an acidic patch on the surface of its DNA-binding domain. Genetic evidence suggests that this acidic patch stimulates transcription from P(RM) through contact with the C-terminal domain (region 4) of the sigma(70) subunit of Escherichia coli RNA polymerase. Here, we identify two basic residues in region 4 of sigma(70) that are critical for lambdacI-mediated activation of transcription from P(RM). On the basis of structural modeling, we propose that one of these sigma(70) residues, K593, facilitates the interaction between lambdacI and region 4 of sigma(70) 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 lambdacI. Residue R588 of sigma(70) has been shown to play an important role in promoter recognition; our findings suggest that the R588 side-chain has a dual function at P(RM), facilitating the interaction of region 4 with the promoter -35 element and participating directly in the protein-protein interaction with lambdacI.