Unusual structure and splicing pattern of the vertebrate mitochondrial solute carrier SLC25A3 gene

The DNA sequence corresponding to the second exon of the SLC25A3 gene is duplicated in vertebrates. The second exon codes for the first transmembrane segment and parts of the immediately adjoining intermembrane and mitochondrial matrix segments. The two genomic exon 2 sequences are 84% similar in zebrafish (slc25a3b gene), 70% in chicken, 66% in mouse and 67% in human. The amino acid identity is 86% in zebrafish, 77% in chicken and 70% in mouse and human. The two copies of exon 2 are separated by an intronic interval. Translation of both exon 2 sequences would alter the reading frame of the downstream sequence, generating a modified aa sequence which would soon be truncated by a stop codon. As a matter of fact the splicing machinery is tuned in such a way that in some species only one of the two copies is expressed and the other is spliced out, while in other species both copies are expressed but only one at a time, generating two alternative protein products.