The major objective of this proposal is to elucidate how the N protein of coliphage lambda controls gene expression by modulating transcription termination. Components of this transcription proteins called Nus, transcription termination signals and an antitermination signal, nut. Genetic studies coupled with physiological experiments are directed toward: 1) characterizing the structure and function of the host NusA and NusE proteins, 2) identifying a new host factor suspected of being involved in antitermination, 3) determining if a host protein binds to the boxA component of the nut RNA signal, and if an antitermination complex formed at one nut site can be donated in trans to a second nut site, 4) assessing a new and unexpected finding that the promoter influences nut activity, and 5) extending studies on the nucleic acid signals for transcription termination. In a related project, we will extend studies on a host factor, Sip, that appears to modulate the binding to DNA of the C1 transcription factor of lambdoid phage P22. This genetic approach should continue to facilitate biochemical analysis, and to point the way toward new avenues of study of antitermination. The well-characterized lambda N system should continue to serve as a useful model for studies on eukaryotic expression systems thought to be regulated by antitermination, such as TAT regulation of HIV expression and c-myc tran ription.