FIG. 6.

Model of the regulation of par PSK. In plasmid-containing cells (A) most RNA I (lined crescent) and RNA II (hatched rectangle) are in complex, as suggested by their 1:1 ratio observed in cells carrying wild-type par. Excess RNA II may be transcribed in order to ensure translational repression of RNA I, but any RNA II not in complex is rapidly degraded by a cellular RNase (hatched Pac-Man). RNA II is more slowly removed from the RNA I-RNA II complex, perhaps by a different RNase (lined Pac-Man). In cells that retain the plasmid (B), RNA II removed from the complex is rapidly replaced by newly synthesized RNA II. However, in cells that lose the plasmid (C), RNA II cannot be replaced, RNA I is translated by ribosomes (bilobed shaded shapes), and the toxin kills the cell (rectangles in the membrane). Empty circles represent pAD1 DNA.