Mutants of NEUROSPORA CRASSA Permeable to Histidinol.

I N Neurospora, L-histidinol is the immediate precursor of L-histidine, as shown by the isolation of L-histidinol dehydrogenase and the study of its catalytic functions (AMES 1957; CREASER, BENNETT and DRYSDALE 1965). The failure of histidine mutants, with the histidinol dehydrogenase function intact, to grow on histidinol, suggests that histidinol cannot enter the cell. However, CATCHESIDE ( 1960) and WEBBER, CASE and GILES (quoted in WEBBER 1960) have independently obtained variants of histidine mutants that can grow on histidinol. CATCHESIDE (1960) suggested that the mutation might involve a change in the system (permease) by which histidine is transported. The transport of histidine is connected with that of several amino acids. Growth of histidine mutants is inhibited by combinations of a neutral (especially aromatic) amino acid with arginine or lysine (HAAS, MITCHELL, AMES and MITCHELL 1952) and this was shown by MATHIESON and CATCHESIDE (1955) to be due to the prevention of histidine uptake. The present work sought to determine which genes were able to mutate to allow entry to histidinol and, by comparative physiological experiments, to discover the functions of the normal alleles of these genes. Such work should contribute to an understanding of these mutations and the mechanism of inhibition of uptake of histidine.