Required for rolling circle plasmid replication, has a site-specific endonuclease/ligase activity (nicking and closing). In vitro (no in vivo system yet exists) cleaves the double-stranded origin of replication (dso) site, on an ssDNA template, remaining covalently linked to the 5' end. Religates the appropriate substrates. Has nucleotidyltransferase activity, adding ATP or dATP to the 3' end of the nicked ssDNA site. Both activities require a G nucleotide at the 3' end of the nicking site. Also has topoisomerase activity, nicking and relaxing negatively supercoiled plasmids in a narrow concentration range (25-50 molar ratio of protein:DNA). Topoisomerase is not dependent on a dso sequence. Has no topoisomerase activity on slightly positively supercoiled plasmid. a 2'-deoxyribonucleoside 5'-triphosphate + DNA(n) = diphosphate + DNA(n+1) ATP + (deoxyribonucleotide)n-3'-hydroxyl + 5'-phospho-(deoxyribonucleotide)m = (deoxyribonucleotide)n+m + AMP + diphosphate. DNA nicking and nucleotidyltransferase are strictly dependent on Mn(2+); Mg(2+) is able to substitute in DNA nicking 5X less efficiently. ATP and dATP inhibit nicking and closing while stimulating nucleotidyltransferase. Optimum temperature is 105 degrees Celsius for ssDNA nicking, 75 degrees Celsius for nucleotidyltransferase activity on a ssDNA substrate. Optimum temperature for ssDNA closing is substrate dependent, being 55 and 75 degrees Celsius for the 2 substrates tested. Topoisomerase activity is only seen between 55 and 75 degrees Celsius. The N-terminus is blocked when overexpressed in E.coli. Plasmids from hyperthermophilic archaea are relaxed to positively supercoiled at physiological temperatures. Belongs to the Gram-positive plasmids replication protein type 1 family.