ISOLATION, EXPRESSION, AND EVOLUTION OF THE GENE ENCODING MITOCHONDRIAL ELONGATION-FACTOR TU IN ARABIDOPSIS-THALIANA

We have characterized a second nuclear gene (tufM) in Arabidopsis thal iana that encodes a eubacterial-like protein synthesis elongation fact or Tu (EF-Tu). This gene does not closely resemble the previously desc ribed Arabidopsis nuclear tufA gene, which encodes the plastid EF-Tu, and does not contain sequence elements found in all cyanobacterial and plastid tufA genes. However, the predicted amino acid sequence includ es an N-terminal extension which resembles an organellar targeting seq uence and shares three unique sequence elements with mitochondrial EF- Tu's, from Saccharomyces cerevisiae and Homo sapiens, suggesting that this gene encodes the Arabidopsis mitochondrial EF-Tu. Consistent with this interpretation, the gene is expressed at a higher level in flowe rs than in leaves. Phylogenetic analysis confirms the mitochondrial ch aracter of the sequence and indicates that the human, yeast, and Arabi dopsis tufM genes have undergone considerably more sequence divergence than their cytoplasmic counterparts, perhaps reflecting a cross-compa rtmental acceleration of gene evolution for components of the mitochon drial translation apparatus. As previously observed for tufA, the tufM gene is present in one copy in Arabidopsis but in several copies in o ther species of crucifers.