The plasmid RK2 initiation protein binds to the origin of replication as a monomer

The TrfA protein encoded by the broad host range bacterial plasmid RK2 specifically binds to eight direct repeats (iterons) present at the plasmid replication origin to initiate DNA replication. Purified TrfA protein is largely in the form of a dimer, and using a dimerization test system that involves the fusion of the amino-terminal domain of the lambda cI repressor protein to TrfA, we show that the TrfA protein forms dimers in vivo. Because of the high stability of the dimer form of TrfA, the formation of heterodimers between the wild-type and different sized TrfA proteins requires in vivo de novo folding of the primary protein sequence or in vitro denaturation and renaturation. The results of gel mobility shift assays using in vitro or in vivo formed heterodimers indicated that the TrfA protein binds to the iteron DNA as a monomer. Furthermore, when the monomeric and dimeric forms of TrfA are separated by gel filtration chromatography, only the protein in the chromatographic position of the monomeric form demonstrated significant DNA binding activity. These results indicate that only the monomer form of the TrfA protein is active for binding to the iterons at the RK2 replication origin.