Host factor requirements for processive antitermination of transcription and suppression of pausing by the N protein of bacteriophage lambda.

The N protein of phage lambda prevents termination of transcription by Escherichia coli RNA polymerase at Rho-dependent and -independent terminators in the lambda early operons. The modification of RNA polymerase by N requires an N-utilization (nut) site, present in each lambda early operon, and involves the E. coli factors NusA, NusB, NusG, and ribosomal protein S10. We show that, in the presence of NusA, N inhibits pausing by RNA polymerase and Rho-dependent termination in vitro at three sites in the lambda terminator tR1 which are located less than 100 base pairs downstream from nutR. NusA is also sufficient for partial antitermination at sites located farther downstream from nutL and nutR if there is a high concentration of N in the reaction. At low concentrations of N, the additional factors NusB, S10, and NusG are essential for antitermination at distal sites. In these conditions, the presence of NusA, NusB, S10, and NusG in the reaction enables N-modified RNA polymerase to elongate efficiently and processively through Rho-dependent and -independent terminators over distances as great as 7 kilobases downstream from the lambda nut sites. This substantial processivity of antitermination in vitro also occurs in vivo and probably reflects the stable association of N, NusA, NusB, S10, and NusG with RNA polymerase and nut site RNA in elongation complexes transcribing the lambda chromosome.