Structure and Expression of Mitochondrial Citrate Synthases from Higher Plants

Abstract

Mitochondrial citrate synthase (EC 4.1.3.7) represents the first enzyme of the tricarboxylic acid cycle, catalyzing the condensation of acetyl-CoA and oxaloacetate, finally yielding citrate and CoA. We report here the isolation of cDNA clones encoding citrate synthase from Nicotiana tabacum, Beta vulgaris and Populus. Nucleotide and deduced amino acid sequences were compared with previously published sequences of mitochondrial 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 the range from 77.2% (potato vs. Arabidopsis) to 94.2% (potato vs. tobacco) on the nucleotide level (coding regions only), and in the range from 70.1% to 90.4% (potato vs. Arabidopsis, and potato vs. tobacco, respectively) on the amino acid level. Identities of the mitochondrial isozymes to the pumpkin glyoxysomal enzyme were below 30% on the nucleotide and amino acid level. In Northern blot experiments citrate synthase mRNA was detected in all tissues analyzed. However, levels of expression showed tissue dependency despite the fact that citrate synthase is usually considered a house-keeping enzyme. Whether these different levels of expression reflect tissue-specific variations with respect to basic metabolism awaits further analysis.

Comparative analysis of mitochondrial citrate synthase gene structure, transcript level and enzymatic activity in acidless and acid-containing Citrus varieties

“…The results also suggest that early in lemon fruit development, the enzyme is regulated, at least partially, at the mRNA level. A similar phenomenon, induction in citrate synthase activity and expression, has been reported in young leaves of a few plant species (Landschutze et al 1995a;La Cognata et al 1996). However, in other citrus fruits, and even in sour lemon fruit, as they approach maturity, the situation might be quite different.…”

“…First, the mRNA level was about 10 times higher in flowers (which do not accumulate acid) than in the juice sac cells. High levels of expression in buds or flowers have been reported in potato and in other plant species (Landschutze et al 1995a;La Cognata et al 1996), and it has been suggested that, along with other TCA cycle enzymes, citrate synthase plays an important role in the transition from vegetative to reproductive meristem and in the early stages of flower development (Landschutze et al 1995b). This might be the case also in Citrus.…”

“…Several forms of citrate synthase have been described in higher plants: mitochondrial, glyoxisomal and peroxisomal (Kato et al 1995;La Cognata et al 1996; Papke and Gerhardt 1996). In yeast, mitochondrial and peroxisomal genes were described (Lewin et al 1990;Jia et al 1997).…”

“…The physiological role of the peroxisomal enzyme is not known yet (Papke and Gerhardt 1996). The cDNAs for mitochondrial enzymes were cloned from Arabidopsis, potato, pummelo, tobacco, poplar tree, sugar beet and carrot (Unger et al 1989;Landschutze et al 1995a;Canel et al 1996;La Cognata et al 1996;Takita et al 1999). The glyoxisomal enzyme has so far been cloned only from pumpkin, and it shows a greater homology with the prokaryotic citrate synthases than with the plant mitochondrial enzymes (Kato et al 1995;Stevens et al 1997).…”

Lack of mitochondrial citrate synthase discloses a new meiotic checkpoint in a strict aerobe

“…Unfortunately, all our attempts to find this gene have failed so far. This may be explained by the fact that a small number of pCS have been described to date and that they show low similarities to mCS, with the exception of Cit1p and Cit2p from S.cerevisiae (see La Cognata, 1996).…”