Mapping of the Q-utilization site (qut) required for antitermination of late transcription in bacteriophage lambda.

To locate the site required for transcription antitermination by the gene Q product, we constructed a plasmid containing the p'R promoter, the t'R1 terminator, and gene galK. We measured the galK expression in response to the lambda Q product supplied in trans, while deleting various portions of lambda DNA adjacent to p'R. The presence of the lambda p'R promoter together with the downstream DNA coding for only a 34-bp segment of 5'-proximal 6S RNA permits antitermination to occur, whereas deletions removing this segment abolish antitermination, as measured by galK expression, but do not affect the p'R promoter. Thus the 34-bp segment must contain the p'R-distal (right) boundary of the Q-specific recognition site qut (Fig. 1). The Q-mediated antitermination appears to be p'R-qut specific but not t'R1 specific, since it does not operate with the pp-t'R1 assembly, but is also effective with terminators other than t'R1, e.g., with the combination of the p'R-qut-tL3 modules.