Plasmid diversity in Chlamydia

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Abstract
Chlamydiae exhibit low interspecies DNA homology and plasmids from different chlamydial species can be readily distinguished by Southern blot analysis and restriction enzyme profiling. In contrast, available plasmid sequence data from within the speciesChlamydia trachomatisindicate that plasmids from human isolates are highly conserved. To evaluate the nature and extent of plasmid variation, the complete nucleotide sequences were determined for novel plasmids from three diverse non-human chlamydial isolates: pCpA1 from avianChlamydia psittaci(N352); pCpnE1 from equineChlamydia pneumoniae(N16); and pMoPn fromC. trachomatismouse pneumonitis. Comparison of the sequence data did not identify an overall biological function for the plasmid but did reveal considerable sequence conservation (> 60%) and a remarkably consistent genomic arrangement comprising eight major ORFs and four 22 bp tandem repeats. The plasmid sequences were close to 7500 nucleotides in length (pCpA1, 7553 bp; pMoPn, 7502 bp) however the equineC. pneumoniaeplasmid was smaller (7362 bp) than all other chlamydial plasmids. The reduced size of this plasmid was due to a single large deletion occurring within ORF 1; this potentially generates two smaller ORFs. The disruption of ORF 1 is the only significant variation identified amongst the chlamydial plasmids and could prove important for future vector development studies.