GENETIC PROFILE OF PNOB8 FROM SULFOLOBUS - THE FIRST CONJUGATIVE PLASMID FROM AN ARCHAEON

The complete nucleotide sequence of the archaeal conjugative plasmid, pNOB8, from the Sulfolobus isolate NOB8-H2, was determined. The plasmi d is 41229 bp in size and contains about 50 ORFs. Several direct seque nce repeats are present, the largest of which is a perfect 85-bp repea t and a site of intraplasmid recombination in foreign Sulfolobus hosts . This recombination event produces a major deletion variant, pNOB8-33 , which is not stably maintained. Less than 20% of the ORFs could be a ssigned putative functions after extensive database searches. Tandem O RFs 315 and 470, within the deleted 8-kb region, show significant sequ ence similarity to the protein superfamilies of ParA (whole protein) a nd ParB (N-terminal half), respectively, that are important for plasmi d and chromosome partitioning in bacteria. A putative cis-acting eleme nt is also present that exhibits six 24-mer repeats containing palindr omic sequences which are separated by 39 or 42 bp. By analogy with bac terial systems, this element may confer plasmid incompatibility and de fine a group of incompatible plasmids in Archaea. Although several ORF s can form putative trans-membrane or membrane-binding segments, only two ORFs show significant sequence similarity to bacterial conjugative proteins. ORF630b aligns with the TrbE protein superfamily, which con tributes to mating pair formation in Bacteria, while ORF1025 aligns wi th the TraG protein superfamily. We infer that the conjugative mechani sm for Sulfolobus differs considerably from known bacterial mechanisms . Finally, two transposases were detected; ORF413 is flanked by an imp erfect 32-bp inverted repeat with a 5-bp direct repeat at the ends, an d ORF406 is very similar in sequence to an insertion element identifie d in the Sulfolobus solfataricus P2 genome.