A further genetic variety of glutamic acid dehydrogenase in Neurospora crassa

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Abstract
A new Neurospora crassa mutant allele of the am locus, a chromosome segment already known to be concerned with glutamic-dehydrogenase formation, causes the formation of a distinct variety of this enzyme. The new allele, which was derived by mutation of the mutant allele am3, itself associated with apparent complete absence of giutamic dehydrogenase, is called am 3a. The giutamic dehydrogenase associated with am3a (the 3a enzyme) shows abnormally high Michaelis constants for all substrates, particularly for glutamate, triphosphopyridine nucleotide and NH4+ ion. The ratio of maximum velocity in the reverse reaction (glutamate synthesis) to that in the forward reaction is about ten times as great for the 3a enzyme as for the wild-type enzyme. Extracts of am3a strains have much higher maximum velocities in the reverse reaction than have wild-type extracts. The 3a enzyme is less stable to heat than wild-type giutamic dehydrogenase. In many preparations the 3a enzyme differs from the wild-type enzyme in not showing full activity until it has been given a mild heat treatment (e.g. 38[degree] for a few minutes). This activation can also be brought about, to some extent, by incubation of the enzyme in the presence of [alpha]-oxo-gluta-rate. Heat-activated 3a enzyme does not revert readily to the inactive form when returned to room temperature. The main differences between the 3a and wild-type enzymes are just as marked in 40-fold-purified preparations as in crude extracts, though it was occasionally found that purified 3a preparations did not require activation. The two kinds of enzyme retained their respective properties when mixed together. A heterocaryon containing both am+ (i.e. wild type at the am locus) and am3a nuclei produced giutamic dehydrogenase, which behaved as if it were a mixture of the 3a and wild-type enzymes. Two mutant alleles at the am locuse, each associated with a quite distinct abnormal type of giutamic dehydrogenase, are now known. Tests are described which distinguish the two mutant enzymes from each other, and from the wild type.