Structure and Expression of Mitochondrial Citrate Synthases from Higher Plants

Mitochondrial citrate synthase (EC 4.1.3.7 EC] ) represents the firstenzyme of the tricarboxylic acid cycle, catalyzing the condensationof acetyl-CoA and oxaloacetate, finally yielding citrate andCoA. We report here the isolation of cDNA clones encoding citratesynthase from Nicotiana tabacum, Beta vulgaris and Populus.Nucleotide and deduced amino acid sequences were compared withpreviously published sequences of mitochondrial citrate synthasesfrom Arabidopsis thaliana and potato, as well as with the sequenceof glyoxysomal citrate synthase from pumpkin. Homologies betweenthe various plant mitochondrial enzymes were in the range from77.2% (potato vs. Arabidopsis) to 94.2% (potato vs. tobacco)on the nucleotide level (coding regions only), and in the rangefrom 70.1% to 90.4% (potato vs. Arabidopsis, and potato vs.tobacco, respectively) on the amino acid level. Identities ofthe mitochondrial isozymes to the pumpkin glyoxysomal enzymewere below 30% on the nucleotide and amino acid level. In Northernblot experiments citrate synthase mRNA was detected in all tissuesanalyzed. However, levels of expression showed tissue dependencydespite the fact that citrate synthase is usually considereda house-keeping enzyme. Whether these different levels of expressionreflect tissuespecifc variations with respect to basic metabolismawaits further analysis. (Received May 20, 1996; Accepted August 20, 1996)