Plasmids containing cos ends inhibit the replication of phage φCTX in Pseudomonas aeruginosa

In bacteriophage φCTX, the cohesive end sequences cos, the integrase gene int, the attachment site attP (the target site for int) and the gene ctx encoding a pore-forming cytotoxin CTX, are clustered. φCTX can infect some Pseudomonas aeruginosa strains with a subsequent induction of CTX expression. The 41 and 477 bp fragments containing cos ends of φCTX DNA were cloned into the high copy number plasmid pHA10. After pretransformation with the cos ends containing plasmids, plaque formation of φCTX and cytotoxin production in φCTX-infected Pseudomonas aeruginosa cells decreased by 100- and 10-fold respectively. Twelve hours after φCTX infection proteins binding with cooperativity to cos sequence containing cleavable ends and the 10 bp flanking sequences were detected by gel electrophoretic mobility retardation of [32P]cos DNA. The results suggest that the cos binding proteins of φCTX are involved in φCTX replication.