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

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