Using two complementary strategies for low-resolution S1 mapping, the global pattern of πC31 transcription was studied after induction of thermoinducible πC31 lysogens of A3(2). A complex pattern of early transcripts was seen, with a peak of abundance at about 10 min post-induction. Nearly all of these transcripts were from DNA located to the right of the (repressor) gene and to the left of the site: a region of about 14 kb. Early transcription was also observed immediately to the left of the gene. The gene itself was also induced, with an earlier expression peak (about 5 min post-induction). Primary late transcripts were generally relatively long, but degraded. They apparently corresponded to most of the 18 kb region to the left of the gene. Some shorter and more persistent late transcripts corresponded to DNA close to or overlapping the site. Large late transcripts from a region close to the left-hand end of the πC31 genome showed evidence of processing to more stable, smaller RNA species. A failure of older cultures (more than 12 h old) to be induced productively was correlated with a much longer period of early transcription, reduced late transcription, failure to synthesize a major virion protein, and failure to package πC31 DNA. Moreover, heat treatment of the older lysogenic cultures did not result in the πC31-dependent shut-down of host rRNA transcription previously observed for young cultures (Rodríguez (1986) 132, 1695–1701; Clayton & Bibb, (1990) 4, 2179–2185).


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