Abstract
Transcription during the lytic cycle of phage Mu occurs in three phases: early, middle, and late. Late transcription requires the Mu C protein and initiates at four promoters: Plys, PI, PP, and Pmom. Northern blot analysis of total RNA isolated 30 min after heat induction of Mu cts lysogens demonstrated that the full-length lys and P transcripts were approximately 7.6 and 6.3 kb long, respectively. The 3' ends of the lys and P transcripts were further localized by S1 nuclease mapping to intergenic regions between G and I and between U and U' in both the G(+) and G(-) orientations of the invertible G segment, respectively. As expected, when DNA fragments containing these termination regions were cloned into plasmids between Pgal and the galK gene, they showed efficient termination activity, even in a Rho-deficient background. Deletion analysis indicated that efficient termination required the presence of potential RNA stem-loop structures immediately preceding the RNA 3' ends. For the P transcript from phage with the G(-) orientation, full termination activity required both the region containing the stem-loop structure and upstream sequences. Taken together, these results suggest that the transcription termination sites of the lys and P transcripts are Rho-independent terminators.