Control of pMB1 replication: inhibition of primer formation by Rop requires RNA1.

The rop gene participates in the control of plasmid copy number by interfering with transcripts originating from the primer promoter. Here we show that this inhibition mechanism requires RNA1 in trans. Mutations in the RNA1 coding sequence that result in plasmids with altered incompatibility properties do not affect the ability of the molecule to participate in the Rop inhibition mechanism. Furthermore we show that the target of the Rop-RNA1 inhibitory mechanism is located, at least in part, after the 52nd nucleotide of the sequence encoding the primer transcript.