The Saccharomyces cerevisiae ADE5,7 protein is homologous to overlapping Drosophila melanogaster Gart polypeptides

Abstract

The Drosophila melanogaster Gart locus encodes two polypeptides specified by overlapping alternative transcripts. One transcript encodes only glycinamide ribotide synthetase (GARSase) on a 45,000 M r polypeptide, while the other encodes GARSase aminoimidazole ribotide synthetase (AIRSase), and glycinamide ribotide transformylase (GARTase) on a 145,000 M r polypeptide. In Saccharomyces cerevisiae, glycinamide ribotide synthetase and aminoimidazole ribotide synthetase are encoded at the ADE5,7 locus. Here I report the cloning, sequencing, and determination of the transcriptional organization of the yeast ADE5,7 gene. There is sufficient homology to align the predicted 802 amino acid ADE5,7 polypeptide with its Drosophila counterpart. These results, together with the sequence of the S. cerevisiae ADE8 gene encoding glycinamide ribotide transformylase, show that the entire Drosophila large polypeptide can be accounted for by the three enzymatic activities. A novel finding is that successive Drosophila domains are each homologous to the aminoimidazole ribotide synthetase portion of the yeast ADE5,7 gene, such that regions of homology with yeast aminoimidazole ribotide synthetase alternate from one of the Drosophila repeats to the other. Such a relationship suggests that the two Drosophila aminoimidazole ribotide synthetase domains together participate in catalysis. This model is consistent with a 20-year-old explanation of complex interallelic complementation such as that characterizing the gene segment encoding yeast aminoimidazole ribotide synthetase.