Escherichia coli NusA is required for efficient RNA binding by phage HK022 Nun protein

The Nun protein of phage HK022 is an RNA binding protein of the arginine-rich motif family. Nun binds the phage λ boxB RNA sequence (BOXB) on nascent λ transcripts and arrests transcription elongation. Binding to BOXB is inhibited by Zn(2+) and stimulated by the Escherichia coli NusA protein. Deletion of the Nun C-terminal region enhances BOXB binding and makes it independent of Zn(2+) and NusA. The C terminus of Nun thus appears to interfere with the N-terminal RNA binding motif. NusA relieves this interference by binding to the Nun C terminus and forming a complex with Nun and BOXB. However, NusA also inhibits transcription arrest in vitro, in the absence of the other Nus factors. Nun deleted for its C terminus fails to bind RNA polymerase (RNAP) (RNAP) or NusA in vitro or to arrest transcription in vivo or in vitro. Our findings are consistent with the idea that NusA inhibits transcription arrest by binding to the Nun C terminus, thus blocking the interaction between Nun and RNAP. NusG, NusB, and NusE factors restore transcription arrest, presumably by promoting transfer of Nun from NusA to RNAP.