In Vivo and in Vitro Complementation between Dhq of Neurospora Crassa Synthetase Mutants in the Arom Gene Cluster

HE arom gene cluster in Neurospora crassa consists of five structural genes Twhich encode an aggregate of five aromatic synthetic enzymes catalyzing steps two through six in the pre-chorismic acid part of the polyaromatic synthetic pathway. Allelic complementation has been detected between certain pairs of mutants within two of the five genes in the cluster-the arom-2 gene which encodes dehydroquinic acid (DHQ) synthetase and the arom-1 gene which encodes dehydroshikimic acid (DHS) reducbase ( GILES, CASE, PARTRIDGE and AHMED 1967; RINES, CASE and GILES 1969). Allelic complementation is usually considered to involve molecular hybridization between two differentially defective, inactive, multimeric enzyme proteins leading to the restoration of enzyme activity (CRICK and ORGEL 1964; FINCHAM 1966). However, the occurrence of allelic complementation between mutants within a gene encoding an enzyme which is part of a multienzyme complex (aggregate) raises interesting questions concerning the mechanism of complementation in such mutants. This paper discusses studies of both in vivo complementation in heterocaryons between arom-2 mutants and in uitro complementation produced in mixtures of partially purified arom aggregate proteins obtained from arom-2 mutants lacking DHQ synthetase activity.