Molecular Chaperones and Quality Control in Noncoding RNA Biogenesis

Since detection of an RNA molecule is the major criterion to define transcriptional activity, the fraction of the genome that is expressed is generally considered to parallel the complexity of the transcriptome. We show here that several supposedly silent intergenic regions in the genome of S. cerevisiae are actually transcribed by RNA polymerase II, suggesting that the expressed fraction of the genome is higher than anticipated. Surprisingly, however, RNAs originating from these regions are rapidly degraded by the combined action of the exosome and a new poly(A) polymerase activity that is defined by the Trf4 protein and one of two RNA binding proteins, Air1p or Air2p. We show that such a polyadenylation-assisted degradation mechanism is also responsible for the degradation of several Pol I and Pol III transcripts. Our data strongly support the existence of a posttranscriptional quality control mechanism limiting inappropriate expression of genetic information.

The exosome complex of 3'-5' exonucleases participates in RNA maturation and quality control and can rapidly degrade RNA-protein complexes in vivo. However, the purified exosome showed weak in vitro activity, indicating that rapid RNA degradation requires activating cofactors. This work identifies a nuclear polyadenylation complex containing a known exosome cofactor, the RNA helicase Mtr4p; a poly(A) polymerase, Trf4p; and a zinc knuckle protein, Air2p. In vitro, the Trf4p/Air2p/Mtr4p polyadenylation complex (TRAMP) showed distributive RNA polyadenylation activity. The presence of the exosome suppressed poly(A) tail addition, while TRAMP stimulated exosome degradation through structured RNA substrates. In vivo analyses showed that TRAMP is required for polyadenylation and degradation of rRNA and snoRNA precursors that are characterized exosome substrates. Poly(A) tails stimulate RNA degradation in bacteria, suggesting that this is their ancestral function. We speculate that this function was maintained in eukaryotic nuclei, while cytoplasmic mRNA poly(A) tails acquired different roles in translation.

The biological role of many nonessential tRNA modifications outside of the anticodon remains elusive despite their evolutionary conservation. We show here that m7G46 methyltransferase Trm8p/Trm82p acts as a hub of synthetic interactions with several tRNA modification enzymes, resulting in temperature-sensitive growth. Analysis of three double mutants indicates reduced levels of tRNA(Val(AAC)), consistent with a role of the corresponding modifications in maintenance of tRNA levels. Detailed examination of a trm8-delta trm4-delta double mutant demonstrates rapid degradation of preexisting tRNA(Val(AAC)) accompanied by its de-aminoacylation. Multiple copies of tRNA(Val(AAC)) suppress the trm8-delta trm4-delta growth defect, directly implicating this tRNA in the phenotype. These results define a rapid tRNA degradation (RTD) pathway that is independent of the TRF4/RRP6-dependent nuclear surveillance pathway. The degradation of an endogenous tRNA species at a rate typical of mRNA decay demonstrates a critical role of nonessential modifications for tRNA stability and cell survival.