Identification of the region required for monomerization of the rolling circle plasmid pKYM.

Plasmid pKYM, isolated from the gram-negative bacterium Shigella sonnei, is a small multicopy plasmid which replicates by a rolling circle mechanism. The formation of multimers has been observed in a derivative of pKYM which lost a part of the origin region, and the loss of the monomerization mechanism would have led to these multimers. By analyzing the constructs of several mutants, we discovered that a DNA region required for monomerization was present upstream of the RepK binding site in the replication origin. As either of the T-rich sequence or the inverted repeat sequences which were seen in that region have been lost in the multimer-forming plasmids, these sequences may be necessary for monomerization.