Nature of the Complementation Products Formed by a Complementing Mutant of Neurospora crassa

The mutantam-14 produces no active nicotinamide adenine dinucleotide phosphate-linked glutamate dehydrogenase (GDH) and no protein showing immunological cross-reaction with the enzyme. Nevertheless, it shows complementation with several otheram mutants in heterokaryons. Active GDH can be extracted from heterokaryons formed fromam-14 and other mutants which, by themselves, produce more or less inactive varieties of the enzyme. The enzyme fromam-14 +am-3 heterokaryons can be partially separated fromam-3 mutant GDH on a diethylaminoethyl cellulose column. It is characterized by abnormally high thermolability and by a capacity for activation by glutamate. By the same procedure as brings about hybridization between mutant GDH proteins, it has been possible to recover enzyme with the properties of puream-3 GDH from a partially purifiedam-14 +am-3 GDH preparation which was initially substantially free of unhybridizedam-3 enzyme. This is interpreted as evidence that the active complementation product is a hybrid oligomer containingam-3 monomers and alsoam-14 monomers, the latter being unable to aggregate by themselves. Heterokaryons formed fromam-14 and wild type produce GDH of abnormally high thermolability, presumably due to the formation ofam-14 +am + hybrids.