Microvirus of Chlamydia psittaci strain Guinea pig inclusion conjunctivitis: Isolation and molecular characterization

The authors report the isolation and molecular characterization of a bacteriophage, ϕCPG1, which infects Chlamydia psittaci strain Guinea pig Inclusion Conjunctivitis. Purified virion preparations contained isometric particles of 25 nm diameter, superficially similar to spike-less members of the ϕX174 family of bacteriophages. The single-stranded circular DNA genome of ϕCPG1 included five large ORFs, which were similar to ORFs in the genome of a previously described Chlamydia bacteriophage (Chp1) that infects avian C. psittaci. Three of the ORFs encoded polypeptides that were similar to those in a phage infecting the mollicute Spiroplasma melliferum, a pathogen of honeybees. Lesser sequence similarities were seen between two ORF products and the major capsid protein of the ϕX174 coliphage family and proteins mediating rolling circle replication initiation in phages, phagemids and plasmids. Phage ϕCPG1 is the second member of the genus Chlamydiamicrovirus, the first to infect a member of a Chlamydia species infecting mammals. Similarity searches of the nucleotide sequence further revealed a highly conserved (75% identity) 375 base sequence integrated into the genome of the human pathogen Chlamydia pneumoniae. This genomic segment encodes a truncated 113 residue polypeptide, the sequence of which is 72% identical to the amino-terminal end of the putative replication initiation protein of ϕCPG1. This finding suggests that C. pneumoniae has been infected by a phage related to ϕCPG1 and that infection resulted in integration of some of the phage genome into the C. pneumoniae genome.