Suppression of ColE1 RNA-RNA mismatch mutations in vivo by the ColE1 Rop protein.

In the bacterial plasmid ColE1 the control of initiation of DNA replication is mediated by the interaction of two complementary RNA molecules, the replication primer and RNA1. The rate of interaction between RNA1 and the primer RNA in vitro can be increased by the product of the ColE1 rop gene, a 63-amino-acid polypeptide. We have investigated the role of the Rop protein in suppressing the incompatibility defects of 13 RNA1-mutant alleles. These RNA1 mutants are defective due to single nucleotide mismatches with their target, the primer RNA. The rop gene suppresses the defective behavior of most of the RNA1 point mismatch mutants in vivo. However, certain mutations that map in stems I and III of RNA1 are not suppressed by rop. The interaction of wild-type and mutant species of RNA1 with ColE1 replication primer transcripts was studied in vitro in the presence or absence of purified Rop protein. The Rop protein is known to increase the rate of wild-type RNA1-primer interaction about twofold in vitro. This enhancement was also observed for mutant RNA1 species having point alterations or a deletion of the 5' terminus of RNA1, which is consistent with the in vivo suppression results. The implications of these results on the mechanism of Rop activity are considered.