We could all use a little more stability in our lives, and now researchers in Germany say that tRNAs get more stable when they are methylated on specific cytosines. This stability helps ensure that cells keep making proteins for proper development and differentiation.
The role of cytosine-C5 methylation on tRNAs has been more mysterious than crop circles. In a previous paper, this German group saw hints that Dnmt2 protects tRNAs from being degraded under stressful conditions in flies. Mutants were viable and fertile at normal temperatures, but had shorter lifespans at higher temperatures.
In addition, Dnmt2 associated with stress-related subcellular compartments after heat shock. In other experiments, they showed that Dnmt2 methylates tRNAs not only for aspartic acid (which was known), but also for valine and glycine. They also saw that Dnmt2 methylates only at C-38, but there was methylation at other cytosines, too, in these tRNAs.
These data made the researchers suspect that something else was methylating tRNAs. NSun2 is the only other known cytosine C-5 methyltransferase in higher eukaryotes, so they wanted to know what would happen if they got rid of both Dnmt2 and NSun2 in mice.
It turns out that Dnmt2 and NSun2 are synthetically lethal; that is, either mutation alone is viable, but lose both and very few, if any, offspring with the double loss-of-function mutations survive.
The German team looked at the double-mutant embryos and saw that they were underdeveloped, suggesting that both Dmnt2 and NSun2 are important for development. Those that were born didn’t last long and were smaller than their normal littermates. The genes also were necessary for proper cellular differentiation.
tRNA methylation also was affected. C-38 methylation was lost in Dnmt2-/- mice, and methylation elsewhere (but not at C-38) was lost in NSun2-/- mice. Cytosine C-5 methylation was greatly reduced in the livers of the double mutants, and the substrate tRNAs were around at very low levels. Protein synthesis was greatly reduced.
The researchers say these data suggest that Dnmt2 and NSun2 stabilize tRNAs, which promotes proper protein synthesis, especially under stressful conditions. They also say their “findings provide further support for the concept of RNA epigenetics.”
Read all the data at Nature Structural & Molecular Biology, September 2012.