A high frequency of plasmids was detected in the rumen bacterium Selenomonas ruminantium. Plasmids 0.9-20 kb in size were detected in more than 50% tested strains. Densitometric analysis indicated that plasmid DNA could represents more than 25% of total cellular DNA. Up to six plasmids were detected in strain S. ruminantium 18. Two smallest cryptic plasmids pSRD181 and pSRD182 from this strain were cloned into Escherichia coli vector pBluescriptSK+ and partially characterized. The plasmid pSRD181 is 1.4 kb and pSRD182 is 2.0 kb. While computer analysis of pSRD181 sequence data showed high homology with replication protein of Staphylococcus aureus plasmids, the pSRD182 sequence showed no significant homology in GenBank data. Strain S. ruminantium 28 was successfully transformed with pJW1 derived plasmid pJ1B1 using ampicillin resistance gene as marker. This is the first report on transformation of selenomonads with foreign DNA.

Institute of Animal Physiology Slovak Academy of Sciences 040 01 Kosice Slovakia

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