Regulation of int gene expression in bacteriophage P2

The int gene of bacteriophage P2 is the only viral gene necessary for the integration of P2 into the Escherichia coli host chromosome. This gene is situated between the phage attachment site, attP, and the repressor C gene, and is cotranscribed with C from the Pc promoter, towards attP. The Pc promoter is negatively controlled by the cox gene, which is the first gene of the early operon. In vitro recombination assays have indicated that in P2 an overproduction of Int is deleterious to the integrative process. We report here that the level of int expression is affected by several different mechanisms after transcriptional initiation. First, a partial transcription termination signal located between the int and C genes reduces the the transcriptional readthrough by about 30%. Second, the ribosome binding site and AUG codon of the int gene are located in a putative stem-loop structure, which may inhibit the initiation of translation. The nip1 mutation (a G to A substitution at the 22nd coding nucleotide of int which results in an increased efficiency of excision) is shown to relieve this inhibition, possible through the formation of an alternative mRNA secondary structure. However, the third and probably most important control of int expression in P2 seems to be that of posttranscriptional autoregulation. The binding site of the Int protein on int gene mRNA is shown to extend into the ribosome binding site of int, supporting our earlier proposed model of competitive binding between Int and ribosomes.