Plasmid incompatibility.

A bacterial plasmid is a species of nonessential extrachromosomal deoxyribonucleic acid (DNA) that replicates autonomously as a stable component of the cell's genome. Naturally occurring plasmids range in size from one to several hundred kilobases and in copy number from one to several hundred per cell. Copy number is a fixed characteristic of any plasmid under constant conditions and is controlled by a plasmid-coded system that determines the rate of initiation of replication. Because self-replication is potentially autocatalytic, plasmid replication control mechanisms must utilize inhibition as their primary regulatory strategy (67). As is true of any self-regulatory system, plasmid replication control systems must be capable of sensing and correcting stochastic fluctuations. This is accomplished by ensuring an inverse proportionality between copy number and replication rate in individual cells. Naturally occurring plasmids are almost always inherited with high fidelity despite their non-essentiality. An explicit partitioning mechanism is thus an important element in the total hereditary biology of unitor oligocopy plasmids. Active partitioning may or may not be required for multicopy plasmids; its possible importance in their hereditary biology is presently uncertain. Stable plasmid inheritance depends not only upon the precisely coordinated functioning of a complex set of plasmid-coded regulatory elements that govern replication and partitioning, but also upon effective synchrony with certain host functions, not yet well characterized, that provide the requisite cellular environment. It is presumed that plasmid instability is caused by defective functioning of any of these elements.