Transcription-dependent competition for a host factor: the function and optimal sequence of the phage lambda boxA transcription antitermination signal.

Ordered development of lambdoid phages relies on systems of transcription termination and antitermination. The phage-encoded N early regulatory proteins, acting with the Nus proteins of Escherichia coli, modify RNA polymerase to a form that overrides many transcription termination signals. These modifications require cis-acting sites, nut, located downstream of the early phage promoters. The nut sites in phages lambda, 21, and P22, which share similarities but are not identical, contain two signals, boxA and boxB. We demonstrate that although a consensus sequence for the boxA signal (boxAcon), 5'CGCTCTTTA, is found only in P22, changes to consensus in the nutR sites of lambda and 21 create more effective antitermination signals than the wild-type signals. An in vivo competition assay demonstrates that a lambda nut region with boxAcon outcompetes nut regions with wild-type, as well as other variations of the boxA sequence, for the host NusB protein. This suggests that boxA influences NusB activity in N-mediated antitermination. Successful competition by boxAcon requires transcription of the nut site as well as N activation. Nucleotide replacement further demonstrates that bases at both ends of boxA are important for antitermination.