RNA quality control: degradation of defective transfer RNA

The distinction between stable (tRNA and rRNA) and unstable (mRNA) RNA has been considered an important feature of bacterial RNA metabolism. One factor thought to contribute to the difference between these RNA populations is polyadenylation, which promotes degradation of unstable RNA. However, the recent discovery that polyadenylation also occurs on stable RNA led us to examine whether poly(A) might serve as a signal for eliminating defective stable RNAs, and thus play a role in RNA quality control. Here we show that a readily denaturable, mutant tRNA(Trp) does not accumulate to normal levels in Escherichia coli because its precursor is rapidly degraded. Degradation is largely dependent on polyadenylation of the precursor by poly(A) polymerase and on its removal by polynucleotide phosphorylase. Thus, in the absence of these two enzymes large amounts of tRNA(Trp) precursor accumulate. We propose that defective stable RNA precursors that are poorly converted to their mature forms may be polyadenylated and subsequently degraded. These data indicate that quality control of stable RNA metabolism in many ways resembles normal turnover of unstable RNA.