Complementation among histidine mutants of Neurospora crassa

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Abstract
Complementation between alleles occurs quite widely in Neurospora crassa. Certain pairs, when combined in a heterocaryon, manifest a particular function, although each alone lacks it. The relationships found within a set of alleles are often complex, but can be represented by an orderly complementation map. It is argued that the phenomenon is due to interaction in aggregates (polymers) of differently altered, but homologous, gene products and that the site of interaction is in the protein forming the enzyme itself. If so the phenomenon should be restricted to those genes which specify polymerous enzymes, or polymerous proteins with other functions. A sample of 389 independent histidine mutants of N. crassa, induced by ultra-violet light and isolated by the technique of filtration enrichment, represent mutations at six loci, two of which (his-5 and his-7) have not been reported previously. Complementation between alleles occurs at four loci (his-1, his-2, his-3 and his-5). The absence of complementation at his-6, amongst 95 mutant alleles tested together in all pairwise combinations, probably indicates that the phenomenon does not occur in connexion with this gene. Matrices, representing the patterns of interaction at each of the loci at which complementation occurs, have been constructed and these show that there are several physiologically distinct types of mutant at each locus. The matrix can be represented by a linear complementation map, in which each type of mutant is inactive over one particular section, in the case of three of the loci. The fourth locus (his-1) appears to represent an exception, in which at least one mutant type must be represented by two regions of inactivation in a one-dimensional map, though it could be represented by a single region of a two-dimensional map.

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