Two genes, pemK and pemI, responsible for stable maintenance of resistance plasmid R100.

Plasmid R100 was found to have two genes, designated pemK and pemI, that were responsible for its stable inheritance during cell division. They are located near the region that is essential for autonomous replication. Under conditions that inhibit replication of R100 derivatives, the plasmid containing these pem genes gave only a few segregants in viable cells and increased the number of nonviable cells in the population, suggesting that a product from the pem region stabilized the plasmid by killing plasmid-free segregants. Inactivation of one of the two translational open reading frames in the pem region caused the loss of the killing function, and thus, the open reading frame is a gene designated pemK, which encodes the killing factor. The coexistence of the pem+ plasmid with a high-copy-number plasmid carrying the other open reading frame inhibited stabilization, and thus, the second open reading frame is a gene designated pemI, which encodes the inhibitor which might control the killing function of pemK. It is likely that the two open reading frames were transcribed from a promoter. There were no significant homologies in DNA sequences between the pem gene of R100 and the genes previously shown to be responsible for the stable inheritance of the other plasmids.