Genetical and Biochemical Studies of Histidine-requiring Mutants of Neurospora Crassa. 3. Correspondence between Biochemical Characteristics and Complementation Map Position of Hist-3 Mutants.

NTERALLELIC complementation has been demonstrated for mutants at variIous loci (see review by YANOFSKY and ST. LAWRENCE 1960), including the hist-3 mutants of Neurospora (WEBBER 1959, 1960; CATCHESIDE 1960). Allelic mutants are tested in various combinations or in all possible combinations of pairs in heterokaryon tests, and the interaction matrix resulting from these tests is used to develop a complementation map. The map thus produced is divided into complementation subunits or complons (DE SERRES 1963). The demonstration of in vitro complementation by a number of workers (e.g., WOODWARD 1959; review by CRICK and ORGEL 1964) suggests that complementation results from interaction between defective protein molecules. One might expect that mutants exhibiting different extents or locations on a complementation map would have suffered different types of genetically determined protein alterations. The hist-3 mutants of Neurospora were considered allelic on the basis of two criteria: (1 ) they are closely linked, with a conventional map distance of about 0.03 within the region; (2) when evaluated by heterokaryon tests, all hist-3 mutants are noncomplementing with a group of hist-3 mutants which are recovered frequently in forward-mutation experiments. However, on the basis of enzyme assays and tests for accumulation of metabolic intermediates, hist-3 mutants were assigned (WEBBER 1960) to the following three subgroups: Subgroup 1-mutants which are deficient for histidinol dehydrogenase activity; Subgroup 9-mutants which are deficient for a reaction early in the biosynthetic pathway for histidine; Subgroup 3-mutants which are deficient for the early reaction and histidinol dehydrogenase. Further work by A. AHMED (unpublished) has shown that mutation in the hist-3 region may affect any or all of three different biochemical reactions in histidine biosynthesis (including histidinol dehydrogenase and two early reactions). The present paper reports the preparation of a complementation map for some hist-3 mutants and demonstrates that the classification of mutants on the basis of complementation map position corresponds rather precisely with the biochemical subgrouping of these mutants.