In vitro studies of the initiation of staphylococcal plasmid replication. Specificity of RepD for its origin (oriD) and characterization of the Rep-ori tyrosyl ester intermediate.

Several staphylococcal plasmids from different incompatibility (inc) groups which replicate by a rolling circle mechanism each specify a replication initiator protein (Rep) which is homologous with that of the inc3 tetracycline resistance plasmid pT181. The rep gene sequences of six pT181-like plasmids are known, each encoding proteins of molecular mass 38 kDa with 62% overall amino acid sequence identity. The initiation of replication in vivo by each of the Rep proteins is plasmid specific, acting in trans only at the cognate replication origin (ori) of the encoding plasmid. Previous studies in vitro of the RepC protein of pT181 demonstrated replication initiator, topoisomerase-like, and DNA binding activities, which appeared to be specific for the origin (oriC) of pT181 when compared with unrelated staphylococcal plasmids. Although RepD, specified by the inc4 chloramphenicol resistance plasmid pC221, has a range of activities similar to those noted previously for RepC, manipulation of in vitro conditions has revealed discrete steps in the overall reaction of RepD with oriD. In addition, factors have been identified which are necessary not only for sequence-dependent discrimination in vitro by Rep proteins for all pT181-like plasmids but also for the absolute specificity of RepD for its cognate pC221 replication origin (oriD), the latter occurring in vivo and a function of the topological state of the ori-containing target DNA. Here we also demonstrate the presence of a covalent phosphoryl-tyrosine linkage between the RepD protein of plasmid pC221 and an oligonucleotide substrate corresponding to its replication origin (oriD). The reactive tyrosine (Tyr-188) was identified from amino acid sequences of 32P-labeled peptide-oligonucleotide fragments. Substitution of Tyr-188 with phenylalanine confirms the importance of the tyrosyl hydroxyl group since the Y188F protein retains the sequence-specific DNA-binding capabilities of wild-type RepD but is unable to attach covalently to the replication origin or participate in the nicking-closing reaction in vitro.