Genome Comparison of Pseudomonas aeruginosa Large Phages

Pseudomonas aeruginosa phage EL is a dsDNA phage related to the giant ϕKZ-like Myoviridae. The EL genome sequence comprises 211,215 bp and has 201 predicted open reading frames (ORFs). The EL genome does not share DNA sequence homology with other viruses and micro-organisms sequenced to date. However, one-third of the predicted EL gene products (gps) shares similarity (Blast alignments of 17–55% amino acid identity) with ϕKZ proteins. Comparative EL and ϕKZ genomics reveals that these giant phages are an example of substantially diverged genetic mosaics. Based on the position of similar EL and ϕKZ predicted gene products, five genome regions can be delineated in EL, four of which are relatively conserved between EL and ϕKZ. Region IV, a 17.7 kb genome region with 28 predicted ORFs, is unique to EL. Fourteen EL ORFs have been assigned a putative function based on protein similarity. Assigned proteins are involved in DNA replication and nucleotide metabolism (NAD+-dependent DNA ligase, ribonuclease HI, helicase, thymidylate kinase), host lysis and particle structure. EL-gp146 is the first chaperonin GroEL sequence identified in a viral genome. Besides a putative transposase, EL harbours predicted mobile endonucleases related to H–N–H and LAGLIDADG homing endonucleases associated with group I intron and intein intervening sequences.