Microvirus of Chlamydia psittaci strain Guinea pig Inclusion Conjunctivitis: isolation and molecular characterization

The authors report the isolation and molecular characterization of a bacter iophage, phi CPG1, which infects Chlamydia psittaci strain Guinea pig Inclu sion Conjunctivitis. Purified virion preparations contained isometric parti cles of 25 nm diameter, superficially similar to spike-less members of the phi X174 family of bacteriophages. The single-stranded circular DNA genome of phi CPG1 included five large ORFs, which were similar to ORFs in the gen ome of a previously described Chlamydia bacteriophage (Chp1) that infects a vian C. psittaci. Three of the ORFs encoded polypeptides that were similar to those in a phage infecting the mollicute Spiroplasma melliferum, a patho gen of honeybees. Lesser sequence similarities were seen between two ORF pr oducts and the major capsid protein of the phi X174 coliphage family and pr oteins mediating rolling circle replication initiation in phages, phagemids and plasmids. Phage phi CPG1 is the second member of the genus Chlamydiami crovirus, the first to infect a member of a Chlamydia species infecting mam mals. Similarity searches of the nucleotide sequence further revealed a hig hly conserved (75% identity) 375 base sequence integrated into the genome o f the human pathogen Chlamydia pneumoniae. This genomic segment encodes a t runcated 113 residue polypeptide, the sequence of which is 72% identical to the amino-terminal end of the putative replication initiation protein of p hi CPG1. This finding suggests that C. pneumoniae has been infected by a ph age related to phi CPG1 and that infection resulted in integration of some of the phage genome into the C. pneumoniae genome.