COMPLEMENTATION AT THE AD-4 LOCUS IN NEUROSPORA CRASSA

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Abstract
Of Neurospora crassa 123 mutants at the ad-4 locus were tested for complementation in heterocaryons. Of the mutants 51 gave at least one positive response. Although these mutants lacks adenylosuccinase activity, partial enzyme activity is restored when complementing mutants are combined in heterocaryons. The relationships of the complementing mutants at the ad-4 locus are such that these mutants can be arranged in a unique linear sequence which may be designated a complementation map of this locus. This arrangment is possible because of the apparent presence in certain alleles of continuous multiple non-functional regions which overlap non-functional regions of other alleles. A significant correlation exists between distance on the complementation map and the strength of the heterocaryon as judged by enzyme activity, growth rate, or time required for growth to commence from mixed conidial inocula. Thus heterocaryons between mutants damaged in adjacent cistrons have low adenylosuccinase activities, and activities increase with increasing distance to the maximum value of 25% of wild type for mutants separated by 2-5 cistrons. Possible interpretations of these results are discussed and the hypothesis is proposed that interallelic complementation involves the formation of defective gene products by different mutant nuclei in a heterocaryon.