This protein is probably a specific topoisomerase involved in initiating replication. This protein is specifically required and may be rate-limiting for replication of the plasmid in vivo. The nicking site of REP proteins is sequence, but not plasmid, specific. It appears that the REP protein of pTZ12 is a fusion protein of pT181 RepC and pC221 RepD proteins, formed spontaneously by a recombination event between a pT181-like plasmid and a pC221-like plasmid. Belongs to the plasmid replication initiation factor family.