SOK-ANTISENSE RNA FROM PLASMID-R1 IS FUNCTIONALLY INACTIVATED BY RNASE-E AND POLYADENYLATED BY POLY(A)-POLYMERASE-I

The hok/sok system of plasmid R1, which mediates plasmid stabilization by the killing of plasmid-free cells, codes for two RNA species, Sok antisense RNA and hok mRNA. Sok RNA, which is unstable, inhibits trans lation of the stable hok mRNA. The 64 nt Sok RNA folds into a single s tem-loop domain with an 11 nt unstructured 5' domain. The initial reco gnition reaction between Sok RNA and hok mRNA takes place between the 5' domain and the complementary region in hok mRNA. In this communicat ion we examine the metabolism of Sok antisense RNA. We find that RNase E cleaves the RNA 6 nt from its 5' end and that this cleavage initiat es Sok RNA decay. The RNase E cleavage occurs in the part of Sok RNA t hat is responsible for the initial recognition of the target loop in h ok mRNA and thus leads to functional inactivation of the antisense. Th e major RNase E cleavage product (denoted pSok(-6)) is rapidly degrade d by polynucleotide phosphorylase (PNPase). Thus, the RNase E cleavage tags pSok(-6) for further rapid degradation by PNPase from its 3' end . We also show that Sok RNA is polyadenylated by poly(A) polymerase I (PAP I), and that the poly(A)-tailing is prerequisite for the rapid 3' -exonucleolytic degradation by PNPase.