Presence of two transcribed malate synthase genes in an n-alkane-utilizing yeast, Candida tropicalis

The presence of two genomic DNA regions encoding malate synthase (MS) was shown by Southern blot analysis of the genomic DNA from an n-alkane-assimilating yeast, Candida tropicalis, using a partial MS cDNA probe, in accordance with the fact that two types of partial MS cDNAS have previously been isolated. This was also confirmed by the restriction mapping of the two genes screened from the yeast lambda-EMBL library. Nucleotide sequence analysis of the respective genomic DNAs, named MS-1 gene and MS-2 gene, revealed that both regions encoding MS had the same length of 1,653 base pairs, corresponding to 551 amino acids (molecular mass of MS-1, 62,448 Da; MS-2, 62,421 Da). Although 29 nucleotide pairs differed in the sequences of the coding regions, the number of amino acid replacements was only one: (159)Asn(MS-1) leads to (159)Ser(MS-2). In the 5'-flanking regions, there were replacements of four nucleotide pairs, deletion of one pair, and insertion of four pairs. In spite of the fact that two genomic genes were present and transcribed, RNA blot analysis demonstrated that only one band (about 2 kb) was observable even when the carbon sources in the cultivation medium were changed. A comparison of the amino acid sequences was made with MSs of rape (Brassica napus L.), cucumber seed, pumpkin seed, Escherichia coli, and Hansenula polymorpha. A high homology was observed among these enzymes, the results indicating that the protein structure was relatively well conserved through the evolution of the molecule. The replacement of only one amino acid residue seems to have little effect on the structure, even if there is some effect on the enzymatic activity. Since MS-1 and MS-2 genes were very similar and TATA-box sequences were also detected in the respective 5'-flanking regions, the mechanism regulating their biosynthesis was suggested to be present in the respective 5'-flanking regions.