Figure 1.

Behavior of RM systems and other selfish genetic elements. (A) Parasitic behavior. Once established in a cell clone, the RM gene complex is difficult to eliminate because its loss leads to cell death (post-segregational cell killing). (B) Competitive advantage of post-segregational cell killing. A selfish genetic element (an RM gene complex) (red circle) is maintained in a cell clone. Invasion by a competing genetic element (such as an incompatible plasmid or allelic DNA) (blue circle) leads to competitive exclusion between the two elements. The cell where the resident genetic element is disturbed by the incoming element dies by post-segregational killing (see Fig. 2C). The incoming genetic element is thus eliminated together with the host cell. Thus, the resident genetic element maintains its frequency in the clonal cell population. [This scenario also operates in the super-infection exclusion between RM elements (Fig. 5).]