Bacteriophage P2 late promoters. II. Comparison of the four late promoter sequences.

The late genes of bacteriophage P2 are clustered into four transcription units. We have reported the transcription initiation sites for two of the late messenger RNAs, encoding genes QP and ONMLKRS. We have now located the 5' ends of the two remaining late mRNAs. The first gene in the VJHG transcription unit has been located by DNA sequence determination of the single nucleotide change in a V amber mutant. Location of the first gene in the FETUD transcription unit has been inferred from the DNA sequence. The 5' ends of the mRNAs for these two transcription units were located by protection of end-labeled restriction fragments in RNA-DNA hybrids from digestion with nuclease S1. Similar protection of hybrids using RNA that had been 5' end-labeled with [alpha-32P]GTP and guanylyl transferase confirmed that these 5' termini resulted from initiation of transcription. The DNA sequences preceding the P2 late transcription starts are different from the Escherichia coli promoter consensus sequences at -10 and -35, consistent with the apparent requirement for phage-encoded proteins in the regulation of P2 late gene expression. The four P2 late promoters do share sequence homologies in the -10 and -35 regions, however, and several additional homologies further upstream. P2 late gene expression also appears to involve negative regulation by a product of the ONMLKRS gene cluster. When cells are infected with P2 polar O amber mutants, a marked increase in the levels of proteins encoded by the other three gene clusters is observed. This increase is reflected in the amounts of late mRNAs, suggesting that RNA synthesis is normally repressed or that late mRNAs are more labile in the presence of a gene product from the ONMLKRS transcription unit. Satellite phage P4 induced P2 late gene expression without the usual requirement for P2 DNA replication. The 5' ends of the P2 late mRNAs are the same during P4 transactivation as during normal P2 late gene expression. Thus, the regulation of P2 late gene expression by P4 does not involve altered promoter selection.