Plasmid replication initiator protein RepD increases the processivity of PcrA DNA helicase

The replication initiator protein RepD encoded by the Staphylococcus chlora mphenicol resistance plasmid pC221 stimulates the helicase activity of the Bacillus stearothermophilus PcrA DNA helicase in vitro. This stimulatory ef fect seems to be specific for PcrA and differs from the stimulatory effect of the Escherichia coil ribosomal protein L3. Whereas L3 stimulates the Pcr A helicase activity by promoting co-operative PcrA binding onto its DNA sub strate, RepD stimulates the PcrA helicase activity by increasing the proces sivity of the enzyme and enables PcrA to displace DNA from a nicked substra te. The implication of these results is that PcrA is the helicase recruited into the replisome by RepD during rolling circle replication of plasmids o f the pT181 family.