Effects of mutations in the repA gene of plasmid Rts1 on plasmid replication and autorepressor function

We constructed a system in which wild-type RepA or RepAcop1 protein was supplied in trans in various amounts to coexisting mini-Rts1 plasmids by clones of the repA or repAcop1 gene under the control of the native promoter with or without its operator sequence. RepAcop1 protein which contains a single amino acid substitution (Arg-142 to Lys) within its 288 amino acids could initiate the replication of the mini-Rts1 plasmid efficiently at both 37 and 42°C even if it was supplied in excess. In contrast, excess wild-type RepA inhibited plasmid replication at 37°C but supported replication at 42°C. Therefore, it appears that the initiator activity of RepA is not related to the incompatibility phenotype associated with an excess of RepA protein. An immunoblot analysis revealed that neither RepA nor RepAcop1 synthesis was temperature sensitive and that both were autogenously regulated to a similar extent because of the presence of an operator located immediately upstream of the promoter. Two mutant RepA proteins, each of which contains a 4-amino-acid insertion in the middle of the protein, maintained the autorepressor and incompatibility activities but lost the ori(Rts1)-activating function.