The regulation of RNA synthesis in yeast. V. tRNA charging studies

Allele-specific conditional destabilization of glutamine repeat mRNAs

Several late-onset neurological diseases are caused by the inheritance of an expanded CAG repeat coding for polyglutamine. To date there is no effective means of halting the progression of these diseases, and their underlying molecular mechanisms remain a mystery. Strategies designed to specifically reduce the levels of long repeat mRNA might provide an effective therapy for these diseases. An emphasis on allele specificity is necessary to avoid the potential toxicities associated with reduction of expression. The experiments described here are based on the relationship between translation and mRNA stability and the idea that translation of a repeated codon might be extremely sensitive to reductions in levels of cognate aminoacylated tRNA. Consistent with this hypothesis, we have discovered that reduced glutamine concentration destabilizes mRNAs coding for long glutamine repeats while sparing short repeat versions of the same mRNAs. These results suggest therapy might be attained with existing compounds or environmental conditions known to decrease free glutamine levels.

An aminoacylation-dependent nuclear tRNA export pathway in yeast

Yeast Los1p, the homolog of human exportin-t, mediates nuclear export of tRNA. Using fluorescence in situ hybridization, we could show that the export of some intronless tRNA species is not detectably affected by the disruption of LOS1. To find other factors that facilitate tRNA export, we performed a suppressor screen of a synthetically lethal los1 mutant and identified the essential translation elongation factor eEF-1A. Mutations in eEF-1A impaired nuclear export of all tRNAs tested, which included both spliced and intronless species. An even stronger defect in nuclear exit of tRNA was observed under conditions that inhibited tRNA aminoacylation. In all cases, inhibition of tRNA export led to nucleolar accumulation of mature tRNAs. Our data show that tRNA aminoacylation and eEF-1A are required for efficient nuclear tRNA export in yeast and suggest coordination between the protein translation and the nuclear tRNA processing and transport machineries.