Molecular characterization of the mitochondrial citrate synthase gene of an acidless pummelo (Citrus maxima)

Pummelo (Citrus maxima [Burm.] Merrill) cDNAs encoding mitochondrial citrate synthase (mCS) were cloned by reverse transcription of juice-sac poly(A)(+) mRNA, followed by Taq Polymerase-mediated amplification. The nucleotide sequence of the citrus gene (citl) is 77% conserved relative to plant mRNAs for mCS. The encoded polypeptide includes a mitochondrial targeting signal at its amino terminus; all 20 putative active-site residues of the citrus enzyme are conserved. Southern hybridization showed that citrus citl is a single-copy gene. A polymorphism associated with citl did not cosegregate with fruit acidity indicating that acitric, the gene causing the acidless phenotype of pummelo 2240, is not an allele of citl locus. Quantitative detection of cirl mRNA showed that transcript levels are not developmentally regulated in juice sacs; no differences were observed between high- and low-acid genotypes.