The structure of brain-specific rat aldolase C mRNA and the evolution of aldolase isozyme genes

The cDNA clones for rat aldolase C mRNA having the nearly complete length were isolated from a rat brain cDNA library and sequenced. The nucleotide sequence of pRAC2-1, a cDNA clone having the largest cDNA insert, indicates that the cDNA is composed of a 105-base-pair 5'-noncoding sequence, a 1089-base-pair coding-sequence and a 382-base-pair 3'-noncoding sequence. The amino acid sequence of aldolase C deduced from a possible open reading frame was composed of 362 residues having a relative molecular mass of 39,164 excluding the initiating methionine, one amino acid shorter than aldolases A and B. The length of aldolase c mRNA was 1750 residues, somewhat longer than that of the aldolase A and B transcripts. The aldolase C mRNA was distributed mainly in the brain, some in ascites hepatoma and fetal liver. Comparison of the amino acid sequences of rat aldolase C with those for rat aldolase A and B [Joh et al. (1985) Gene 39, 17-24; Tsutsumi et al. (1984) J. Biol. Chem. 259, 14572-14575], which have been determined previously, shows the existence of highly conserved stretches of amino acid among the three isozymic forms throughout their sequences. The extent of the homology between aldolases A and C is 81%, while those between aldolases A and B, and B and C are 70%, respectively. The analysis of amino acid substitution among aldolases A, B and C from several species suggests that the isozyme genes diverged much earlier than animal species appeared and that the aldolase C gene has evolved from the aldolase A gene after aldolase A and B genes diverged.