Molecular Mechanisms of an All-Purpose Transcription Anti-Termination Factor

Abstract

Bacteriophage λN protein, a paradigmatic anti-termination factor, binds nascent RNA and host Nus factors, rendering RNA polymerase resistant to all pause and termination signals. A 3.7 A-resolution cryo-electron microscopy structure and structure-informed functional analyses reveal an all-it-takes strategy, in which the intrinsically unstructured λN directly modifies RNA polymerase interactions with the nucleic acids and subverts essential functions of NusA, NusE and NusG to reprogram the transcriptional apparatus. λN repositions NusA and remodels the β-subunit flap tip to preclude folding of pause/termination RNA hairpins in the exit tunnel and disrupt termination-supporting interactions of the α-subunit C-terminal domains. λN invades and traverses the RNA polymerase catalytic cavity, stabilizing the hybrid and impeding pause/termination-related conformational changes. λN also binds upstream DNA, enhancing the weak anti-backtracking and anti-swiveling activities of NusG. Moreover, λN-repositioned NusA and NusE sequester the NusG C-terminal domain, counteracting ρ-dependent termination. Other anti-terminators likely utilize similar mechanisms to enable processive transcription.