The functional boundaries of the Q-utilization site required for antitermination of late transcription in bacteriophage lambda.

Expression of the late genes of bacteriophage lambda requires, in addition to the host functions, the lambda p'R promoter, the antiterminator sequence qut, and the product of gene Q which interacts with the Q utilization (qut) site. In the absence of the Q function or qut site, the p'R-initiated transcription is blocked by the t'R terminator at the 194th nucleotide downstream of the start point, s'R, producing a short 6 S mRNA. In this study the position and boundaries of the qut site were deduced by constructing plasmids containing various portions of the p'R-qut region, the t'R1 terminator, and the reporter gene galK. We measured galK gene expression in response to the gamma Q gene product supplied in trans by a prophage or Q-expression plasmid. We show that among the lambda proteins, the Q gene product alone is necessary and sufficient for complete qut-mediated transcription antitermination in vivo. These antitermination experiments, employing plasmids that contain different lengths of lambda p'R-qut sequence, identified the right boundary of the qut site, which is located between +4 and +18 (for s'R = +1). The functional left boundary of qut does not extend upstream from the -26th nucleotide of the p'R promoter, as based on the following experiments. The promoter function of the truncated (-26)p'R-s'R-(+18) sequence can be restored by fusion to the complete but qut-less p'R, pp, or PLac promoter; however, no antitermination was observed for such a p-(-26)p'R-s'R-(+18)-t'R-galK plasmid. Thus we conclude that the qut site partially overlaps with the p'R promoter sequence. However, promoters that contain the -10 region of p'R, s'R, and the +1 to +18 qut sequence did mediate Q-dependent antitermination when properly fused to the homologous or heterologous -35 promoter regions. Only those transcripts that start at s'R (+1 or very near to it) and also contain at least the first 18 nucleotides (actually greater than 4 and less than or equal to 18) of 6 S RNA appear to be a target for the Q-qut-mediated transcription antitermination, which acts not only at t'R but also at other Rho-independent or Rho-dependent terminators.