PLASMID DIVERSITY IN CHLAMYDIA

Chlamydiae exhibit low interspecies DNA homology and plasmids from dif ferent chlamydial species can be readily distinguished by Southern blo t analysis and restriction enzyme profiling. In contrast, available pl asmid sequence data from within the species Chlamydia trachomatis indi cate that plasmids from human isolates are highly conserved. To evalua te the nature and extent of plasmid variation, the complete nucleotide sequences were determined for novel plasmids from three diverse non-h uman chlamydial isolates: pCpA1 from avian Chlamydia psittaci (N352); pCpnE1 from equine Chlamydia pneumoniae (N16); and pMoPn from C. trach omatis mouse pneumonitis. Comparison of the sequence data did not iden tify an overall biological function for the plasmid but did reveal con siderable sequence conservation (> 60 %) and a remarkably consistent g enomic arrangement comprising eight major ORFs and four 22 bp tandem r epeats. The plasmid sequences were close to 7500 nucleotides in length (pCpA1, 7553 bp; pMoPn, 7502 bp) however the equine C. pneumoniae pla smid was smaller (7362 bp) than all other chlamydial plasmids. The red uced size of this plasmid was due to a single large deletion occurring within ORF 1; this potentially generates two smaller ORFs. The disrup tion of ORF 1 is the only significant variation identified amongst the chlamydial plasmids and could prove important for future vector devel opment studies.