Transcription antitermination in vitro by lambda N gene product: requirement for a phage nut site and the products of host nusA, nusB, and nusE genes.

Employing specifically engineered plasmids in which the expression of E. coli galK cistron is regulated by transcription termination, we have analyzed the antitermination function of phage lambda N gene product in S30 extracts. Antitermination by N, dependent on its site of action, nutL, is defective in the extracts prepared from nusA, nusB, and nusE mutants. By complementation analysis, we demonstrate that none of the these nus mutations affects the synthesis of N or the other nus gene products to cause a defect in antitermination. Rather, these mutations have inactivated a set of specific host components, the Nus factors, which are essential for N activity. Curiously, an appreciable portion of N and Nus complementation activities of an S30 extract is ribosome-associated. The significance of this finding remains to be uncovered.