Saccharomyces cerevisiaeExpresses Two Genes Encoding Isozymes of 5-Aminoimidazole-4-carboxamide Ribonucleotide Transformylase

We have isolated and cloned twoSaccharomyces cerevisiaegenes which encode isozymes of 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) transformylase, the ninth step of thede novopurine biosynthesis pathway. This reaction involves the formylation of AICAR using 10-formyltetrahydrofolate as the formyl donor.ADE16is located on chromosome XII and encodes an open reading frame of 591 amino acids.ADE17is 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 vitroassays of AICAR transformylase activity demonstrated the lack of IMP production in the double disruptant strain.S. cerevisiaeis the only organism known thus far to possess isozymes of this protein. Because it is likely that the proteins encoded byADE16andADE17also contain IMP cyclohydrolase activity, these two genes complete the set of clones and mutants for the entirede novopurine biosynthesis pathway in yeast.