Structural instability of small rolling circle replication plasmids from Selenomonas ruminantium

The complete nucleotide sequence of pSRD192 plasmid from Selenomonas ruminantium 19D has been obtained and analyzed. The plasmid, 2334 bp in length, was shown to replicate by rolling circle replication mechanism. By PCR method variability of pSRD192-like plasmids was investigated and another variant of pSRD192-like plasmid; the pSRM22 plasmid 2338 bp in length; was detected and characterized. Both pSRD192 and pSRM22 plasmids share an identical rep gene and origins of replication to that of another S. ruminantium pONE429 plasmid. Other than that there are additional regions of sequence similarity between the three plasmids, interspersed with divergent regions. The sequence comparisons suggest structural instability of pSRD192-like rolling circle replication plasmids.