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The influence of genetic modifiers, those genes which affect the expression of other genes, has been demonstrated in a wide variety of organisms. In Neurospora crassa the influence of modifiers is suggested by quantitative differences in growth substance utilization among different reisolates of a mutant strain (Good, Heilbronner, and Mitchell, 1950), and genetic suppression (Houlahan and Mitchell, 1947) may be regarded as an extreme type of modification. In addition, Houlahan and Mitchell (1948) and Mitchell and Mitchell (1952) have demonstrated extensive interactions among genes causing pyrimidine, lysine, and arginine requirements, and Srb (private communication) has found a case in which a single gene modifier appears to control the ability of a proline- or ornithine-utilizing strain of Neurospora to utilize arginine or citrulline. The data to be presented here deal with an instance in which the modification is such that different reisolates of a tryptophane- or nicotinic acid-utilizing mutant appear tobe blocked at different steps in the pathway by which Neurospora synthesizes tryptophane and nicotinic acid. For some of the details of this pathway the reader is referred to the recent reviews of Mitchell (1950) and Horowitz and Mitchell (1951). Miss Dorothy Newmeyer, of Standford University, who has used some of the strains which were used in this work, has also encountered some of the phenomena reported here.