Sequence and expression pattern of the Drosophila melanogaster mitochondrial porin gene: evidence of a conserved protein domain between fly and mouse.

We have recently cloned a cDNA encoding mitochondrial porin in Drosophila melanogaster and shown its chromosomal localization (Messina et al., FEBS Lett. (1996) 384, 9-13). Such cDNA was used as a probe for screening a genomic library. We thus cloned and sequenced a 4494-bp genomic region which contained the whole gene for the mitochondrial porin or VDAC. It was found that this D. melanogaster porin gene contains five exons, numbered IA (115 bp), IB (123 bp), II (320 bp), III (228 bp) and IV (752 bp). The exons II, III and IV contain the protein coding sequence and the 3' untranslated sequence (3'-UTR). The first base in exon II precisely corresponds to the first base of the starting ATG codon. Exon IA corresponds to the 5'-UTR sequence reported in the published cDNA sequence. Exon IB corresponds to an alternative 5'-UTR sequence, demonstrated to be transcribed by 5'-RACE experiments. The exon-intron splicing borders and the length of the exon III perfectly match a homologous internal exon detected in the mouse genes. Such exon encodes a protein domain predicted by sequence transmembrane arrangement models to contain major hydrophilic loops and it is thus suspected to have a conserved distinct function. In situ hybridization experiments confirmed the localization of the genomic clone on the chromosome 2L at region 32B3-4. Together with genomic Southern blotting at various stringencies, the same experiment did not confirm the presence of a second genetic locus on D. melanogaster chromosomes. Northern blots demonstrated that the porin gene is a housekeeping one: three messages of approx. 1.2-1.6 kbp are transcribed in every fly developmental stage that was studied. They were shown to derive by an alternative usage of different promoters and polyadenylation sites.