Escherichia coli nusB mutations that suppress nusA1 exhibit lambda N specificity.

The bacteriophage lambda N protein regulates phage development by selectively suppressing transcription termination in its host, Escherichia coli. The E. coli nus mutants prevent N activity. To provide additional information on transcription termination, we have isolated pseudo-revertants of the nusA1 mutation that restore lambda N function. One series of pseudo-revertants lie in the E. coli nusB gene, whose product is normally required for lambda N activity. These mutations are N-specific: mutations that restore lambda N activity do not restore the activity of the analogous N protein of phage 21. Similarly, nusB mutations that restore phage 21 N function are deficient for lambda N function. Mapping of the two classes of mutation is consistent with their location in two distinct domains in the nusB protein. We discuss whether nusB is specific for N protein or for some other component of this regulation system, e.g. the phage site (nut) required for N action.