Structural organization of the mouse aspartate aminotransferase isoenzyme genes. Introns antedate the divergence of cytosolic and mitochondrial isoenzyme genes

We have cloned and characterized a mouse cytosolic aspartate aminotransferase (AspAT) (EC 2.6.1.1) gene, which is about 32,000 base-pairs long and is interrupted by eight introns. The 5′ and 3′-flanking regions, and the exact sizes and boundaries of the exon blocks, including the transcription-initiation sites, were determined. The 5′ end of the gene lacks the TATA and CAAT boxes characteristic of eukaryotic promoters, but contains G + C-rich sequences, three putative binding sites for a cellular transcription factor, Spl, and multiple transcription-initiation sites. The sequences around the transcription-initiation sites are compatible with the formation of a number of potentially stable stem-loop structures. We compared the structural organization of the mouse cytosolic AspAT gene with that of the mouse mitochondrial AspAT gene, which has nine introns. We found that the promoter regions share a high level of homology and five of the introns are at identical places. This close matching leads to the tentative conclusion that the introns were in place before the divergence of cytosolic and mitochondrial isoenzyme genes.