Characterization of a third, cII-dependent, coordinately activated promoter on phage lambda involved in lysogenic development.

Lysogenic development by bacteriophage lambda is known to require the coordinate expression of two phage operons. Coordinate control is achieved by a positive regulatory mechanism which activates transcription from the promoters of these operons, PRE and PI. The positive effector is the phage regulatory protein cII. We now identify and characterize a third cII-dependent transcription unit on phage lambda, which is positioned in the middle of the Q regulatory gene and has an anti-Q orientation. We demonstrate the cII-dependent function of this promoter and precisely map its 5' transcription start-site both in vitro and in vivo. Most importantly, we show that cII binding and transcription activation at PaQ occur at essentially the same cII levels as those required for PRE and PI activation, and that all three promoters respond to cII at the same time following phage infection. In addition, DNase protection studies demonstrate that cII selectively interacts with the same TTGCN6TTGC DNA sequence repeat in the -35 region of PaQ which cII interacts with at both PRE and PI. We find that cII also binds other TTGCN6TTGC repeat sequences on lambda but binding at these sites does not lead to productive transcription. We conclude that PaQ functions in concert with PRE and PI to regulate the lysogenic growth response of phage lambda. We presume that the PaQ directed anti-sense Q RNA transcript functions to down-regulate the expression of the Q gene, which is needed for the expression of all phage late genes during lytic growth.