Transcription mapping as a tool in phage genomics: the case of the temperate Streptococcus thermophilus phage Sfi21.

For the lytic growth cycle of the temperate cos-site Streptococcus thermophilus phage Sfi21 a transcription map was developed on the basis of systematic Northern blot hybridizations. All deduced 5' ends were confirmed by primer extension analysis. Three time classes of transcripts were observed. Early transcripts were identified in four different genome regions. One prominent early mRNA of 4.8 kb length covered a group of 12 genes located between the origin of replication and the cos-site. Two short early mRNAs represented a single gene from the direct vicinity of the cos-site and the superinfection immunity gene from the lysogeny module, respectively. A fourth early transcript covered a group of four genes located between the lysin and the integrase gene. Middle transcripts of 2.1 and 5.8 kb length covered cro-like and ant-like repressor genes and the DNA replication module, respectively. Four types of late transcripts were identified. The transcripts covered the likely DNA packaging genes, the head morphogenesis module plus the major tail gene, the remainder of the tail genes, and the putative tail fiber plus lysis genes, respectively. Only the transcript from the head morphogenesis genes yielded defined late mRNA species. The transcription map concurred with most of the in silico predictions for the genome organization of phage Sfi21 except for the separation of the DNA replication module from a possible transcription regulation module. Most 5' ends of the transcripts determined in primer-extension experiments were not preceded by a consensus promoter sequence. The involvement of phage-encoded regulators for middle and late transcription was suggested by chloramphenicol-inhibition experiments.