Replication control of plasmid pLS1: efficient regulation of plasmid copy number is exerted by the combined action of two plasmid components, CopG and RNA II

Two elements, the products of genes copG and rnaII, are involved in the copy-number control of plasmid pLS1. RNA II is synthesized in a dosage-dependent manner. Mutations in both components have been characterized. To determine the regulatory role of the two genes, we have cloned copG, rnaII or both elements at various gene dosages into pLS1-compatible plasmids. Assays of incompatibility towards wild-type or mutant pLS1 plasmids showed that: (i) the rnaII gene product, rather than the DNA sequence encoding it, is responsible for the incompatibility, and (ii) CopG and RNA II act in trans and are able to correct up fluctuations in pLS1 copy number. A correlation between the gene dosage at which the regulatory elements were supplied and the incompatibility effect on the resident plasmid was observed. The entire copG-rnaII circuit has a synergistic effect when compared with any of its components in the correction of pLS1 copy-number fluctuations, indicating that, in the homoplasmid steady-state situation, the control of pLS1 replication is exerted by the co-ordinate action of CopG and RNA II.