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

Most citrus (Citrus L. spp.) fruits accumulate a considerable amount of cit ric acid in the vacuoles of the juice sac cells. As part of research aimed to understand the mechanism of acid accumulation, we compared the gene stru ctures and transcript levels of citrate synthase in sour lemon (high acid, C. limon (L.) Burm.), `Shamouti' orange (moderate acid, C. sinensis (L.) Os beck) and sweet lime (acidless, C. limmetioides Tan.). Southern analyses su ggested that a single gene for citrate synthase was present in the genomes of all three Citrus varieties. The gene structures seemed to be very simila r, with minor differences in Shamouti orange. Overall, the transcript level s of citrate synthase were similar in sweet lime and sour lemon, and about 2-fold lower in Shamouti orange. The enzymatic activity of citrate synthase was compared between sour lemon and sweet lime. In sour lemon, the specifi c activity of the enzyme was induced early in fruit development and, in par allel with the increase in acid content, reached a maximal level, and did n ot diminish significantly towards fruit maturation; the pattern and level o f activity detected during sweet lime fruit development were similar. These results suggest that the difference in acid accumulation between acidless and acid-containing fruits may not be attributed to changes in the activity of citrate synthase.