An analysis of the role of host factors in transcription antitermination in vitro by the Q protein of coliphage lambda.

We used two different approaches to study the requirement for Escherichia coli Nus factors for the activity of bacteriophage lambda late antiterminator Q. Using an in vitro coupled transcription-translation assay, based on Q-dependent synthesis of galactokinase from a pR'-tR'-galK template, we showed that mutations in the host nusB and nusE genes do not affect Q activity. A mutation in nusA (nusA1) only partially affects Q action at all temperatures tested. Defective Q function in the nusA1 mutant extract could be restored by the addition of pure NusA but not by excess Q. In a pure transcription system, measurement of the run-off transcript produced by Q-mediated suppression of tR' revealed that NusA is greatly stimulatory to Q activity, whereas NusB and S10, in the presence or absence of NusA, have no effect. Unidentified E. coli factor(s) present in an S30 extract efficiently suppress the natural pausing by RNA polymerase at +15, +16 of pR' without affecting Q activity. These results show that NusA is the only host protein that directly participates in Q function.