Structure of the genes of two homologous intracellularly heterotopic isoenzymes

Abstract

The genes of mitochondrial and cytosolic aspartate aminotransferase of chicken were cloned and sequenced. In both genes nine exons encode the mature enzyme. The additional exon for the N-terminal presequence that directs mitochondrial aspartate aminotransferase into the mitochondria is separated by the largest intron from the rest of the gene. A comparison of the two genes of chicken with the aspartate aminotransferase genes of mouse [Tsuzuki, T., Obaru, K., Setoyama, C. & Shimada, K. (1987) J . Mol. Biol. 198, 21 -31; Obaru, K., Tsuzuki, T., Setoyama, C. & Shimada, K. (1988) J . Mol. Biol. 200,[13][14][15][16][17][18][19][20][21][22] reveals closely similar structures: in the gene of both the mitochondrial and the cytosolic isoenzyme all but one intron positions are conserved in the two species and five introns out of nine are placed at the same positions in all four genes indicating that the introns were in place before the genes of the two isoenzymes diverged.The variant consensus sequence (T/C),,T(C/T)AG at the 3' splice site of the introns of the genes for nuclearencoded mitochondrial proteins, which had been deduced from a total of 34 introns [Juretik, N., Jaussi, R., Mattes, U. & Christen, P. (1987) Nucleic Acids Res. 15, 10083-100861, was confirmed by including an additional 22 introns into the comparison. The position -4 at the 3' splice site is occupied by base T in 43% of the total 56 introns and appears to be subject to a special evolutionary constraint in this particular group of genes.The following course of evolution of the aspartate aminotransferase genes is proposed. Originating from a common ancestor, the genes of the two isoenzymes intermediarily evolved in separate lineages, i.e. the ancestor eukaryotic and ancestor endosymbiontic cells. When endosymbiosis was established, part of the endosymbiontic genome, including the aspartate aminotransferase gene, was transferred to the nucleus. This process probably led to the conservation of certain splicing factors specific for nuclear-encoded mitochondrial proteins. The presequence for the mitochondrial isoenzyme was acquired by DNA rearrangement. In the eukaryotic lineage, the mitochondrial isoenzyme evolved more slowly than its cytosolic counterpart.Aspartate aminotransferase (AspAT) is the most extensively investigated pyridoxal-5'-phosphate-dependent enzyme functioning in amino acid metabolism (for a review, see [l]). Two distinct isoenzymes of AspAT exist in all higher eukaryotes, one being located in the cytosol (cAspAT) and the other in the mitochondria (mAspAT). The isoenzymes are homologous proteins and are both coded for by nuclear DNA. The complete amino acid sequences of the two isoenzymes from pig [2-41, chicken [5, 61 and horse [7] have been determined. The nucleotide sequences of the cDNAs for both isoenzymes are known for chicken [8, 91, mouse [lo], rat [I 1, 121 and pig [13,14]. Mitochondria1 AspAT is one ofthe typical nuclear-encoded mitochondrial proteins that are synthesized as larger precursors on free ribosomes in the cytoso...

Isolation, characterisation and expression of a cDNA clone encoding plastid aspartate aminotransferase from Arabidopsis thaliana

“…The deduced amino acid sequence from pcAAT1 has been compared with those of other AATs from bacterial, vertebrate and plant species and (plastid-located, msaat2) [8], soybean AAT5 (plastid-located, soyaspam) [21], carrot AAT (cytosolic,Dcaat) [18], chicken mitochondrial (chickm) [13] and cytosolic (chickc) [13] AATs and E. coli AAT (Ecaat) [7]. Residue numbering is with respect to the N-terminal amino acid ofArabidopsis Ataatl.…”

Structure of genes that encode isozymes of aspartate aminotransferase in Panicum miliaceum L., a C4 plant

“…Five of these introns are at the identical codon positions in the genes for cAspAT and mAspAT [16,29]. From these observations it was concluded that the introns were already in place before the divergence of the genes for cAspAT and mAspAT.…”

“…The dendrogram constructed with the neighbor-joining method [33] does not show antecedent divergence of plant cAspAT genes. Since the comparison of amino acid sequences of AspAT suggests that mAspAT genes have evolved more slowly than cAspAT genes [ 16,41], the low evolution rate of mAspAT genes could shift their place in the tree. It is thought that most aminotransferase proteins originate from a single universal ancestor protein and have diverged by early gene duplication and subsequent specialization for substrate specificity [27].…”

“…Phylogenetic relationship between AspAT and tyrosine aminotransferase. The references for these genes are: Panicum miliaceum cAspAT and mAspAT [41], alfalfa cAspAT [44] and alfalfa pAspAT [8], lupin cAspAT [47], mouse cAspAT [29] and mAspAT [42], chicken cAspAT and mAspAT [ 16], Escherichia coil AspAT [19], yeast ( Saccharomyces cerevisiae) cApAT [5], Rhizobium meliloti AspATA [30] and AspATB [1], Bacillus sp. strain YM-2 [39], Sulfolobus solfataricus AspAT [ 6 ], and human TAT (tyrosine aminotransferase) [ 31 ].…”

First cloning and characterization of aspartate aminotransferase from river buffalo (Bubalus bubalis)

“…Mammalian mitochondrial AST has recently been reported to have kynurenine aminotransferase activity and play a role in the biosynthesis of kynurenic acid in rat, mouse and human brains (Han et al 2011). Studies on animal AST have shown that the cytosolic and mitochondrial isozymes are encoded by different but structurally related genes (Obaru et al 1988;Juretic et al 1990).…”