A connection between the ribosome and two tRNA modification mutants subject to rapid tRNA decay.

tRNA modifications are crucial in all organisms to ensure tRNA folding and stability, and accurate translation in the ribosome. In both the yeast and the evolutionarily distant yeast , mutants lacking certain tRNA body modifications (outside the anticodon loop) are temperature sensitive due to rapid tRNA decay (RTD) of a subset of hypomodified tRNAs. Here we show that for each of two mutants subject to RTD, mutations in ribosomal protein genes suppress the temperature sensitivity without altering tRNA levels. Prior work showed that . mutants, lacking 7-methylguanosine, were temperature sensitive due to RTD and that one class of suppressors had mutations in the general amino acid control (GAAC) pathway, which was activated concomitant with RTD, resulting in further tRNA loss. We now find that another class of suppressors have mutations in genes, encoding 60S subunit proteins, and that suppression occurs with minimal restoration of tRNA levels and reduced GAAC activation. Furthermore, suppression extends to other mutations in the large or small ribosomal subunit. We also find that mutants, lacking 4-acetylcytidine, are temperature sensitive due to RTD, that one class of suppressors have mutations, associated with minimal restoration of tRNA levels, and that suppression extends to other and mutations. However, although temperature sensitivity is associated with some GAAC activation, suppression by an mutation does not significantly inhibit GAAC activation. These results suggest that ribosomal protein mutations suppress the temperature sensitivity of Δ and Δ mutants due to reduced ribosome concentrations, leading to both a reduced requirement for tRNA, and reduced ribosome collisions and GAAC activation. Results with mutants are consistent with this model, and fuel speculation that similar results will apply across eukaryotes.