PROGRAMMED CELL-DEATH IN BACTERIA - TRANSLATIONAL REPRESSION BY MESSENGER-RNA END-PAIRING

The hok/sok and pnd systems of plasmids R1 and R483 mediate plasmid ma intenance by killing plasmid-free cells. Translation of the exceptiona lly stable hok and pnd mRNAs is repressed by unstable antisense RNAs. The different stabilities of the killer mRNAs and their cognate repres sors explain the onset of translation in plasmid-free cells. The full- length hok and pnd mRNAs are inert with respect to translation and ant isense RNA binding. We have previously shown that the mRNAs contain tw o negative translational control elements. Thus, the mRNAs contain ups tream anti-Shine-Dalgarno elements that repress translation by shieldi ng the Shine-Dalgarno elements. The mRNAs also contain fold-back-inhib ition elements (fbi) at their 3' ends that are required to maintain th e inert mRNA configuration. Using genetic complementation, we show tha t the 3' fbi elements pair with the very 5' ends of the mRNAs. This pa iring sets the low rate of 3' exonucleolytical processing, which is re quired for the accumulation of an activatable pool of mRNA. Unexpected ly, the hok and pnd mRNAs were found to contain translational activato rs at their 5' ends (termed tac). Thus, the fbi elements inhibit trans lation of the full-length mRNAs by sequestration of the tao elements. The fbi elements are removed by 3' exonucleolytical processing. Mutati onal analyses indicate that the 3' processing triggers refolding of th e mRNA 5' ends into translatable configurations in which the 5' tac el ements base pair with the anti-Shine-Dalgarno sequences.