The complete nucleotide sequence of ΦCTX, a cytotoxin-converting phage of Pseudomonas aeruginosa: Implications for phage evolution and horizontal gene transfer via bacteriophages

ΦCTX is a cytotoxin-converting phage isolated from Pseudomonas aeruginosa. In this study, we determined the complete nucleotide sequence of the ΦCTX phage genome. The precise genome size was 35,538 bp with 21 base 5'-extruding cohesive ends, Forty-seven open reading frames (ORFs) were identified on the ΦCTX genome, including two previously identified genes, ctx and int. Among them, 15 gene products were identified in the phage particle by protein microsequencing. The most striking feature of the ΦCTX genome was an extensive homology with the coliphage P2 and P2-related phages; more than half of the ORFs (25 ORFs) had marked homology to P2 genes with 28.9-65.8% identity. The gene arrangement on the genome was also highly conserved for the two phages, although the G+C content and codon usage of most ΦCTX genes were similar to those of the host P. aeruginosa chromosome. In addition, ΦCTX was found to share several common features with P2, including the morphology, non-inducibility, use of lipopolysaccharide core oligosaccharide as receptor and Ca2⁺-dependent receptor binding. These findings indicate that ΦCTX is a P2-like phage well adapted to P. aeruginosa, and provide clear evidence of the intergeneric spread and evolution of bacteriophages. Furthermore, comparative analysis of genome structures of ΦCTX, P2 and other P2 relatives revealed the presence of several hot-spots where foreign DNAs, including the cytotoxin gene, were inserted. They appear to be deeply concerned in the acquisition of various genes that are horizontally transferred by bacteriophage infection.