Control of effective expression of the phage phiCTX-encoded ctx gene in Pseudomonas aeruginosa by a promoter upstream of the cos site.

In the DNA of bacteriophage phiCTX, the attachment site (attP), the cohesive end site (cos), and the gene (ctx) encoding the pore-forming cytotoxin, are clustered. Deletion variants and PCR-generated fragments of the DNA were cloned into the Pseudomonas aeruginosa and Escherichia coli vector pHA10. Recombinant plasmids carrying the chloramphenicol acetyltransferase gene, cat, were used for promoter studies. Two promoters responsible for ctx expression are located between attP and cos and between cos and ctx, the promoter upstream of cos being stronger than the one downstream. The promoters and the ribosomal binding site are recognized by both the P. aeruginosa and E. coli transcriptional systems. The results indicate that chromosomal integration of phiCTX DNA, which requires cos site ligation, is necessary for high ctx expression in phiCTX-infected P. aeruginosa strains.