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

Expression of the late genes of bacteriophage λ requires, in addition to the host functions, the λ p′R, promoter, the antiterminator sequence qut, and the product of gene Q which interacts with the Q utilization (quo site. In the absence of the Q function or qut site, the p′R-initiated transcription is blocked by the t′Rterminator at the 194th nucleotide downstream of the start point, S′Rproducing 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 pR′-qut region, the t′R1terminator, and the reporter gene galK. We measured galK gene expression in response to the λ Q gene product supplied in trans by a prophage or Q-expression plasmid. We show that among the λ proteins, the Q gene product alone is necessary and sufficient for complete quit-mediated transcription antitermination in vivo. These antitermination experiments, employing plasmids that contain different lengths of λ 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 >4 and ⩽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.