Structural Basis for the Action of an All-Purpose Transcription Anti-termination Factor

Bacteriophage lambda N protein, a model anti-termination factor, binds nascent RNA and host Nus factors, rendering RNA polymerase resistant to all pause and termination signals. A 3.7-angstrom-resolution cryo-electron microscopy structure and structure-informed functional analyses reveal a multi-pronged strategy by which the intrinsically unstructured lambda N directly modifies RNA polymerase interactions with the nucleic acids and subverts essential functions of NusA, NusE, and NusG to reprogram the transcriptional apparatus. lambda N repositions NusA and remodels the beta subunit flap tip, which likely precludes folding of pause or termination RNA hairpins in the exit tunnel and disrupts termination-supporting interactions of the alpha subunit C-terminal domains. lambda N invades and traverses the RNA polymerase hybrid cavity, likely stabilizing the hybrid and impeding pause- or termination-related conformational changes of polymerase. lambda N also lines upstream DNA, seemingly reinforcing anti-backtracking and anti-swiveling by NusG. Moreover, lambda N-repositioned NusA and NusE sequester the NusG C-terminal domain, counteracting rho-dependent termination. Other anti-terminators likely utilize similar mechanisms to enable processive transcription.