Role of E. coli NusA in Phage HK022 Nun-mediated Transcription Termination

The 109 amino acid residue Nun protein expressed from prophage HK022 excludes superinfecting phage λ by arresting transcription on the λ chromosome near the λ nut sites. In vitro, the Nun N terminus binds to nascent λ nutRNA, whereas the C terminus interacts with RNA polymerase and DNA template. Escherichia coli host factors, NusA, NusB, NusE (S10), and NusG, stimulate Nun-arrest. NusA binds the Nun C terminus and enhances formation of the Nun–nutRNA complex. Because of these in vitro activities of NusA, and since a nusA mutation (nusAE136K) blocked Nun in vivo, we assumed that NusA was required for Nun activity. However, using a nusAts strain, we find that NusA is required for termination at nutR but not at nutL. Furthermore, nusAE136K is dominant to nusA+ for Nun-arrest, both in vitro and in vivo. NusAE136K shows increased affinity for Nun and, unlike NusA+, can readily be recovered in a ternary complex with Nun and nutRNA. We propose NusAE136K suppresses Nun-arrest when it is a component of the transcription elongation complex, perhaps, in part, by blocking interactions between the Nun C terminus and RNA polymerase and DNA. We also find that in contrast to Nun-arrest, antitermination by λ N requires NusA.