Structural basis of Q-dependent antitermination

Lambdoid bacteriophage Q protein mediates the switch from middle to late bacteriophage gene expression by enabling RNA polymerase (RNAP) to read through transcription terminators preceding bacteriophage late genes. Q loads onto RNAP engaged in promoter-proximal pausing at a Q binding element (QBE) and an adjacent sigma-dependent pause element (SDPE) to yield a \"Q-loading complex,\" and Q subsequently translocates with RNAP as a pausing-deficient, termination-deficient \"Q-loaded complex.\" Here, we report high-resolution structures of four states on the pathway of antitermination by Q from bacteriophage 21 (Q21): Q21, the Q21-QBE complex, the Q21-loading complex, and the Q21-loaded complex. The results show that Q21 forms a torus--a \"nozzle\"--that narrows and extends the RNAP RNA-exit channel, extruding single-stranded RNA and preventing formation of pause and terminator hairpins.\n\nOne Sentence SummaryQ forms a \"nozzle\" that narrows the RNA polymerase RNA-exit channel and extrudes ssRNA, preventing formation of RNA hairpins.