Role of E. coli Transcription-Repair Coupling Factor Mfd in Nun-mediated Transcription Termination

Phage HK022 Nun protein excludes phage λ by binding nascent λ-nut RNA and inducing termination and transcript release. In contrast, in a purified in vitro system, Nun arrests transcription on λDNA templates without dissociation of the transcription elongation complex (TEC). Our evidence indicates that transcription-repair coupling factor (Mfd) frees Nun-arrested RNA polymerase. The activity of Nun is enhanced in an mfd-null mutant, consistent with prolonged association of Nun with the TEC. Furthermore, expression of λ nut RNA in the mfd mutant titrates Nun, allowing superinfecting λ to form plaques. Finally, addition of Mfd releases a Nun-arrested transcription complex in vitro.