Abstract
Eukaryotes maintain separate protein translation systems for nuclear and organellar genes, including distinct sets of tRNAs and aminoacyl-tRNA synthetases (aaRSs). In animals, mitochondrial-targeted aaRSs are expressed at lower levels and are less conserved in sequence than cytosolic aaRSs involved in translation of nuclear mRNAs, likely reflecting lower translational demands in mitochondria. In plants, translation is further complicated by the presence of plastids, which share most aaRSs with mitochondria. In addition, plant mitochondrial tRNA pools have a dynamic history of gene loss and functional replacement by tRNAs from other compartments. To investigate the consequences of these distinctive features of translation in plants, we analyzed sequence evolution in angiosperm aaRSs. In contrast to previously studied eukaryotic systems, we found that plant organellar and cytosolic aaRSs exhibit only a small difference in expression levels, and organellar aaRSs are slightly more conserved than cytosolic aaRSs. We hypothesize that these patterns result from high translational demands associated with photosynthesis in mature chloroplasts. We also investigated aaRS evolution in Sileneae, an angiosperm lineage with extensive mitochondrial tRNA replacement and aaRS retargeting. We predicted positive selection for changes in aaRS sequence resulting from these recent changes in subcellular localization and tRNA substrates but found little evidence for accelerated sequence divergence. Overall, the complex tripartite translation system in plant cells appears to have imposed more constraints on the long-term evolutionary rates of organellar aaRSs compared with other eukaryotic lineages, and plant aaRS protein sequences appear largely robust to more recent perturbations in subcellular localization and tRNA interactions.