Expression of phage P4 integrase is regulated negatively by both Int and Vis

Dipartimento di Scienze Biomolecolari e Biotecnologie, Università degli Studi di Milano, Via Celoria 26, 20133 Milano, Italy Correspondence D. Ghisotti Daniela.Ghisotti{at}unimi.it Phage P4 int gene encodes the integrase responsible for phage integration into and excision from the Escherichia coli chromosome. Here, the data showing that P4 int expression is regulated in a complex manner at different levels are presented. First of all, the P int promoter is regulated negatively by both Int and Vis, the P4 excisionase. The N-terminal portion of Int appears to be sufficient for such a negative autoregulation, suggesting that the Int N terminus is implicated in DNA binding. Second, full-length transcripts covering the entire int gene could be detected only upon P4 infection, whereas in P4 lysogens only short 5'-end covering transcripts were detectable. On the other hand, transcripts covering the 5'-end of int were also very abundant upon infection. It thus appears that premature transcription termination and/or mRNA degradation play a role in Int-negative regulation both on the basal prophage transcription and upon infection. Finally, comparison between P int – lacZ transcriptional and translational fusions suggests that Vis regulates Int expression post-transcriptionally. The findings that Vis is also an RNA-binding protein and that Int may be translated from two different start codons have implications on possible regulation models of Int expression. Present address: Max F. Perutz Laboratories at the Vienna Biocenter, Department of Microbiology and Genetics, University of Vienna, 1030 Vienna, Austria. Present address: Keryos, Via della Filanda 5, 20060 Gessate (MI), Italy. Present address: Molmed SpA, Via Olgettina 58, 20132 Milano, Italy.