The hyperthermophilic bacterium Thermotoga species strain RQ7 harbors an 846-bp plasmid, pRQ7, with a single open reading frame. Previously published analyses of the DNA sequence of pRQ7 suggested that it may replicate by a rolling-circle (RC) replication mechanism, and this report provides experimental evidence supporting this hypothesis. Single-stranded pRQ7 DNA accumulates in strain RQ7, as evidenced by the facts that this DNA bound to nitrocellulose membranes under nondenaturing conditions, was sensitive to S1 nuclease digestion, and hybridized to only one of two homologous DNA probes specific for each strand of the plasmid. The DNA encoding the open reading frame was cloned and expressed in Escherichia coli and gave a protein with a molecular mass of 26 kDa, similar to that deduced by sequence analysis. This protein bound to a fragment of pRQ7 that contains a putative double-stranded replication region in a magnesium-dependent reaction and made this fragment sensitive to S1 nuclease activity. It did not cause this same S1 nuclease sensitivity in the remainder of pRQ7. This activity on pRQ7 DNA suggests that this protein plays a role in plasmid replication
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