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 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 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 P_RM. 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 P_RM, facilitating the interaction of region 4 with the promoter -35 element and participating directly in the protein-protein interaction with λcI.