Myoviridae bacteriophages of Pseudomonas aeruginosa: a long and complex evolutionary pathway.

Recently we have accomplished the entire DNA sequence of bacteriophage ΦKZ, a giant virus infecting Pseudomonas aeruginosa. The 280334-bp of ΦKZ genome is a linear, circularly permutated and terminally redundant, AT-rich dsDNA molecule that contains no sites for NotI, PstI, SacI, SmaI, XhoI and XmaIII endonucleases. Limited homology to other bacteriophages on the DNA and protein levels indicated that ΦKZ represents a distinct branch of the Myoviridae family. In this work, we analyzed a group of six P. aeruginosa phages (Lin68, Lin21, PTB80, NN, EL, and RU), which are morphologically similar to ΦKZ, have similar genome size and low G+C content. All phages have a broad host range among P. aeruginosa strains, and they are resistant to the inhibitory action of many P. aeruginosa plasmids. The analysis of the genomic DNA by restriction enzymes and DNA–DNA hybridization shows that phages are representative of three ΦKZ-like species: ΦKZ-type (ΦKZ, Lin21, NN and PTB80), EL-type (EL and RU) and Lin68 which has a shorter tail than other phages. Except for related phages EL and RU, all ΦKZ-like phages have identical N-terminal amino acid sequences of the major capsid protein. Random genome sequencing shows that the EL and RU phages have no homology to the ΦKZ-like phages on DNA level. We propose that the ΦKZ, Lin21, NN, PTB80 and Lin68 phages can be included in a new ΦKZ genus, and that the EL and RU phages belong to a separate genus within the Myoviridae family. Based on the resistance to many restriction enzymes and the transduction ability, there are indications that over the long pathway of evolution, the ΦKZ-like phages probably inherited the capacity to infect different bacterial species.