Phage HK022 Nun protein arrests transcription on phage λ DNA in vitro and competes with the phage λ N antitermination protein

Phage HK022 Nun protein excludes phage λ by terminating transcription near the λ nut sites. We have established a purified in vitro system that reproduces the in vivo sequence and factor requirements of Nun. Nun arrests transcription by E. coli RNA polymerase at or near elongation pause sites distal to the nut sites. The boxB sequence of nut is required for optimal Nun activity; boxA plays a lesser role. The efficiency of transcription arrest is strongly enhanced by the four E. coli Nus factors. The factors increase the specific activity of Nun, and allow it to act at higher ribonucleoside triphosphate concentrations. A wild-type boxA is required for stimulation by Nus factors. Nun and the λ N antitermination protein compete for their opposing reactions. This competition may be at the level of binding of boxB RNA.