Escherichia coli nusG mutations that block transcription termination by coliphage HK022 Nun protein

The Escherichia coli nusG gene product is required for transcription termin ation by phage HK022 Nun protein at the lambda nutR site in vivo. We show t hat it is also essential for Nun termination at lambda nutL. Three recessiv e missense nusG mutations have been isolated that inhibit termination by Nu n at lambda nutR. The mutations are ineffective in a lambda pL nutL fusion, even when lambda nutR replaces lambda nutL. The mutant strains support lam bda growth, indicating that lambda N antitermination activity is not impair ed. Transcription arrest by Nun in vitro is stimulated by NusG protein at b oth lambda nutR and lambda nutL. Mutant NusG protein fails to enhance trans criptional arrest by Nun at either site. The mutant protein, like the wild- type protein, suppresses transcriptional pausing by RNA polymerase and stim ulates Rho-dependent termination. These results imply that the role of NusG in Nun termination may be distinct from its roles in other transcription r eactions.