RNA editing produces mRNA isoforms that differ in stability and may broaden thermal tolerance range of mRNA and proteins of mussels Mytilus

By acquiring multiple variants (isoforms) of a protein that differ in thermal sensitivities, ectothermic organisms may broaden their thermal tolerance ranges. Genetically, isoforms with adaptive differences in function and stability arise from allelic variation at a single locus (allozymes) and duplication of genes that allows evolution of paralogs with different thermal responses. Here, we examined a post-transcriptional mechanism for generating mRNA variants: editing of mRNA. The study system was cytosolic malate dehydrogenase (cMDH) from foot tissue of two marine molluscs (congeners of mussels of the genus Mytilus) with different optimal temperatures. Editing events occurred in all species in mRNA coding sequence region. The editing sites varied with species, and no overlap of the editing site was found between these two species. The editing events occurred at 117, 123, 135, 190, 195, 204, 279, and 444 for M. coruscus, while these at 216 and 597 for M. galloprovincialis. Single species had several edited variants of cMDH mRNA. Likewise, analyses showed that the mRNA variants differed in protein expression. Thus, variation arising from mRNA editing may complement the proposed roles of other sources of variation. These families of mRNA isoforms, which are generated from a single DNA coding sequence, may foster eurythermal biochemical function and buffer individuals against thermal stress. The finding that multiple edited forms of mRNA for a protein occur in single individuals also raises important caveats about methods for deducing amino acid sequences and for estimating adaptive molecular variation in natural populations.