Saccharomyces cerevisiae expresses two genes encoding isozymes of 5-aminoimidazole-4-carboxamide ribonucleotide transformylase.

We have isolated and cloned two Saccharomyces cerevisiae genes which encode isozymes of 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) transformylase, the ninth step of the de novo purine biosynthesis pathway. This reaction involves the formylation of AICAR using 10-formyltetrahydrofolate as the formyl donor. ADE16 is located on chromosome XII and encodes an open reading frame of 591 amino acids. ADE17 is located on chromosome XIII and encodes an open reading frame of 592 amino acids. The deduced amino acid sequences of the two genes are 84% identical to each other and are 60-63% identical to the chicken and human bifunctional AICAR transformylase/IMP cyclohydrolase amino acid sequences. Disruption of the two chromosomal yeast genes resulted in adenine auxotrophy, while the expression of either gene alone was sufficient to support growth without adenine. In vitro assays of AICAR transformylase activity demonstrated the lack of IMP production in the double disruptant strain. S. cerevisiae is the only organism known thus far to possess isozymes of this protein. Because it is likely that the proteins encoded by ADE16 and ADE17 also contain IMP cyclohydrolase activity, these two genes complete the set of clones and mutants for the entire de novo purine biosynthesis pathway in yeast.