On the Nature of the Glutamic Dehydrogenase Produced by Inter - Allele Complementation at the 600am Locus of Neurospora crassa

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Abstract
SUMMARY: A method for the 40-fold purification of glutamic dehydrogenase from Neurospora crassa is described. Strains which are homocaryotic for the alleles am1, am2or am3produce no detectable glutamic dehydrogenase, but heterocaryons of composition am1+am2or am1+am3do possess the enzyme activity. Extracts of am1+am2mycelia have 10%, or rather less, of typical wild-type activity when assayed at about 20°. Enzyme preparations from am1+am2are distinguished from wild-type preparations in: (a) their capacity for thermal activation as the temperature is raised between 20° and 35°; (b) their low stability at 60°. Extracts of am1+am3mycelia show 20–25% of typical wild-type activity. Enzyme preparations from am1+am3are distinguished from wild-type in their lower affinity for glutamate, and they tend also to be more thermolabile than wild-type enzyme, though less so than am1+am2preparations. Experiments on mixed enzymes showed no evidence for any effect of either kind of heteroearyon preparation on the properties of wild-type enzyme, or vice versa. It thus seems likely that the effects observed are due to differences in the enzyme molecules themselves. The significance of these observations for theories on the mechanism of inter-allele complementation is discussed.

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