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

phi CTX is a cytotoxin-converting phage isolated from Pseudomonas aeruginos a. In this study, we determined the complete nucleotide sequence of the phi CTX phage genome. The precise genome size was 35 538 bp with 21base 5'-ext ruding cohesive ends. Forty-seven open reading frames (ORFs) were identifie d on the phi CTX 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 phi CTX genome w as an extensive homology with the coliphage P2 and PP-related phages; more than half of the ORFs (25 ORFs) had marked homology to P2 genes with 28.9-6 5.8% 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 phi C TX genes were similar to those of the host P, aeruginosa chromosome. In add ition, phi CTX was found to share several common features with P2, includin g the morphology, non-inducibility, use of lipopolysaccharide core oligosac charide as receptor and Ca2+-dependent receptor binding. These findings ind icate that phi CTX is a P2-like phage well adapted to P, aeruginosa, and pr ovide clear evidence of the intergeneric spread and evolution of bacterioph ages. Furthermore, comparative analysis of genome structures of phi CTX, P2 and other P2 relatives revealed the presence of several hot-spots where fo reign 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.