STRUCTURE AND EXPRESSION OF MITOCHONDRIAL CITRATE SYNTHASES FROM HIGHER-PLANTS

Mitochondrial citrate synthase (EC 4.1.3.7) represents the first enzym e of the tricarboxylic acid cycle, catalyzing the condensation of acet yl-CoA and oxaloacetate, finally yielding citrate and CoA, We report h ere the isolation of cDNA clones encoding citrate synthase from Nicoti ana tabacum, Beta vulgaris and Populus. Nucleotide and deduced amino a cid sequences were compared with previously published sequences of mit ochondrial citrate synthases from Arabidopsis thaliana and potato, as well as with the sequence of glyoxysomal citrate synthase from pumpkin . Homologies between the various plant mitochondrial enzymes were in t he range from 77.2% (potato vs. Arabidopsis) to 94.2% (potato vs, toba cco) on the nucleotide level (coding regions only), and in the range f rom 70.1% to 90.4% (potato vs. Arabidopsis, and potato vs. tobacco, re spectively) on the amino acid level. Identities of the mitochondrial i sozymes to the pumpkin glyoxysomal enzyme were below 30% on the nucleo tide and amino acid level. In Northern blot experiments citrate syntha se mRNA was detected in all tissues analyzed. However, levels of expre ssion showed tissue dependency despite the fact that citrate synthase is usually considered a house-keeping enzyme, Whether these different levels of expression reflect tissue-specifc variations with respect to basic metabolism awaits further analysis.