Structural organization of the mouse mitochondrial aspartate aminotransferase gene

Structural organization of the entire mouse mitochondrial aspartate aminotransferase (EC 2.6.1.1) gene was determined by analyzing the overlapping genomic clones obtained from a Charon 4A DNA library. The gene is 25 × 10³ base-pairs long and contains ten exons interrupted by nine introns of various sizes. The 5′ and 3′-flanking regions, the exact sizes and boundaries of the exon blocks including the transcription-initiation sites were determined. The 5′ end of the gene lacks the prototypical 5′ transcriptional regulatory sequence elements, such as TATA and CAAT boxes, but contains G + C-rich sequences, two putative binding sites for a cellular transcription factor, Sp1, and multiple transcription-initiation sites. Moreover, the sequences around the transcription-initiation sites are compatible with the formation of a number of potentially stable stem-loop structures. The leader sequence, which is essential for the transport of the protein into the mitochondria, is coded by the first exon and is separated from the mature protein by the first intron. The pyridoxal 5′-phosphate-binding domain, consisting of seven alternating β-sheets and α-helical polypeptide strands, is separated by four introns present at the ends of α-helices. These genomic DNA structures suggest that the introns were not inserted into a previously uninterrupted coding sequence, but rather are products of evolution of the ancestral gene. However, a further correlation between the positions of introns relative to the well-defined structural domains of the mature protein was not obvious.