The bacteriophage λQ anti-terminator protein regulates late gene expression as a stable component of the transcription elongation complex

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The Q protein of bacteriophage λ (λQ) is a transcription anti-terminator required for the expression of the phage's late genes under the control of promoter PR′. To effect terminator read-through, λQ must gain access to RNA polymerase (RNAP) via a promoter-restricted pathway. In particular, λQ modifies RNAP by binding a specific DNA site embedded in PR′ and interacting with RNAP in the context of a specific paused early elongation complex. The resultant λQ-modified transcription elongation complex is competent to read through downstream termination signals. Here we use a chromatin-immunoprecipitation assay to test the hypothesis that λQ functions as a stable component of the transcription elongation complex. Our results indicate that, in vivo, the λQ-modified transcription elongation complex contains Q as a stably associated subunit. Furthermore, we find that in the physiologically relevant context of an induced λ lysogen, Q remains stably associated with RNAP as it transcribes at least 22 kb of the phage late operon. Thus, our findings suggest that the promoter-specific pathway leading to λQ-mediated terminator read-through results in the formation of a highly stable λQ-containing transcription elongation complex capable of traversing the entire late operon.

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