Termination and antitermination of transcription in temperate bacteriophages

The expression of many genes is controlled at the level of transcript elongation. One of the best understood examples is found in bacteriophage λ, where interaction of the viral N protein and nut sites with host nus proteins and RNA polymerase suppresses termination of early phage transcription. Bacteriophage HK022, although closely related to λ, regulates elongation in a surprisingly different way. HK022 expresses a function, Nun, that is related to the λ N antitermination protein. However Nun has no effect on HK022 transcription, but, instead, specifically provokes termination of λ transcripts. The HK022 chromosome contains transcription termination signals that separate functional genes from their cognate promoters. As in λ, these terminators are suppressed, but no HK022 gene is required for antitermination. Both the termination activity of Nun and the suppression of HK022 terminators can be demonstrated in purified in-vitro systems.