STRUCTURE OF THE REPLICATION ORIGINS OF PLASMIDS RK2 AND R6K AND THE REGULATION OF PLASMID DNA REPLICATION

Regions of the plasmid genome essential for plasmid replication and maintenance in Escherichia coli have been identified for two antibiotic resistance Plasmids, RK2 and R6K. In both cases a small segment of the plasmid, containing an origin of replication, has been maintained as an extrachromosomal element when another essential region(s) of the plasmid that specifies a trans-acting product is present in the host cell. Nucleotide sequence analysis of the origin regions of plasmids RK2 and R6K revealed seven 22 base pair (bp) direct repeats in tandem for the R6K origin region and eight 17 bp direct repeats in clusters of three and five for RK2. A model is presented involving the interacting of the π protein, the trans-acting product of the R6K genome that is required for the initiation of R6K replication, with the 22 bp direct repeats as a necessary step in the initiation of plasmid R6K replication.