The σ⁷⁰ subunit of RNA polymerase is contacted by the λQ antiterminator during early elongation

The Q protein of bacteriophage λ is a transcription antiterminator that modifies the elongation properties of E. coli RNA polymerase (RNAP). To do this, DNA-bound λQ must first engage a paused elongation complex. Here we show that this engagement of λQ with RNAP involves an interaction between λQ and σ⁷⁰, demonstrating that σ⁷⁰ can be a target of regulation during elongation. Furthermore, we provide evidence that this interaction between λQ and σ⁷⁰ stabilizes a conformation of RNAP that requires the disengagement of a segment of σ⁷⁰ from the core enzyme. Recent structure-based models posit that the transition from the initiation to the elongation phase of transcription involves the staged displacement of σ⁷⁰ from the RNAP core. Our findings provide support for this proposal.