GENETICAL AND BIOCHEMICAL EVIDENCE FOR FURTHER INTER-RELATIONSHIPS BETWEEN THE POLYAROMATIC SYNTHETIC AND THE QUINATE-SHIKIMATE CATABOLIC PATHWAYS IN NEUROSPORA CRASSA 1
Studies have been performed with a special class of revertants occurring with a high spontaneous frequency as suppressor mutations in mutants of the arom-1+ gene, which encodes the enzyme dehydroshikimate reductase in the constitutive, polyaromatic, biosynthetic pathway in Neurospora crassa. Combined genetical and biochemical evidence indicates that these reversions to prototrophy result from mutations in the unlinked qa-4+ gene, which encodes the enzyme dehydroshikimate dehydrase in the inducible, quinate-shikimate catabolic pathway. Additional evidence indicates that such revertants can occur only if a qa-3+ gene (which encodes shikimate, and quinate, dehydrogenase) is present. These results support the hypothesis that prototrophic arom-1 qa-4 double mutants accumulate high levels of dehydroshikimate which lead to the induction of high levels of shikimate dehydrogenase (the enzyme normally degrading shikimate to dehydroshikimate) and by mass action cause a net conversion of dehydroshikimate to shikimate. Thus, shikimate dehydrogenase effectively replaces dehydroshikimate reductase and supplies the aromatic amino acid requirements of arom-1 mutants. In this situation, a mutation leading to the loss of a biochemically distinct second enzyme activity acts as a special type of suppressor mutation——not by restoring the first activity as is true of nonsense suppressor mutations——but by causing the enzymic activity originally missing to be replaced by a catalytically related but otherwise physically distinct enzyme encoded in an unlinked gene.