Temporal translational control by a metastable RNA structure

Programmed cell death by the hok/sok locus of plasmid R1 relies on a comple x translational control mechanism. The highly stable hok mRNA is activated by 3 ' -end exonucleolytical processing. Removal of the mRNA 3 ' end releas es a 5 ' -end sequence that triggers refolding of the mRNA. The refolded ho k mRNA is translatable but can also bind the inhibitory Sok antisense RNA. Binding of Sok RNA leads to irreversible mRNA inactivation by an RNase III- dependent mechanism. A coherent model predicts that during transcription ho k mRNA must be refractory to translation and antisense RNA binding. Here we provide genetic evidence for the existence of a 5 ' metastable structure i n kok mRNA that locks the nascent transcript in an inactive configuration i n vivo. Consistently, the metastable structure reduces the rate of Sok RNA binding and completely blocks hok translation in vitro. Structural analyses of native RNAs strongly support that the 5 ' metastable structure exists i n the nascent transcript. Further structural analyses reveal that the mRNA Wend triggers refolding of the mRNA 5 ' end into the more stable tac-stem c onformation. These results provide a profound understanding of an unusual a nd intricate posttranscriptional control mechanism.