tRNAHis 5-methylcytidine levels increase in response to several growth arrest conditions in Saccharomyces cerevisiae

Abstract

tRNAs are highly modified, each with a unique set of modifications. Several reports suggest that tRNAs are hypomodified or, in some cases, hypermodified under different growth conditions and in certain cancers. We previously demonstrated that yeast strains depleted of tRNAHis guanylyltransferase accumulate uncharged tRNAHis lacking the G−1 residue and subsequently accumulate additional 5-methylcytidine (m5C) at residues C48 and C50 of tRNAHis, due to the activity of the m5C-methyltransferase Trm4. We show here that the increase in tRNAHis m5C levels does not require loss of Thg1, loss of G−1 of tRNAHis, or cell death but is associated with growth arrest following different stress conditions. We find substantially increased tRNAHis m5C levels after temperature-sensitive strains are grown at nonpermissive temperature, and after wild-type strains are grown to stationary phase, starved for required amino acids, or treated with rapamycin. We observe more modest accumulations of m5C in tRNAHis after starvation for glucose and after starvation for uracil. In virtually all cases examined, the additional m5C on tRNAHis occurs while cells are fully viable, and the increase is neither due to the GCN4 pathway, nor to increased Trm4 levels. Moreover, the increased m5C appears specific to tRNAHis, as tRNAVal(AAC) and tRNAGly(GCC) have much reduced additional m5C during these growth arrest conditions, although they also have C48 and C50 and are capable of having increased m5C levels. Thus, tRNAHis m5C levels are unusually responsive to yeast growth conditions, although the significance of this additional m5C remains unclear.