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In 1947, Houlahan and Mitchell' reported the first instance of a suppressor mutation in Neurospora although such a phenomenon had previously been observed in Drosophila. In the case which they reported, a mutation arose which resulted in suppression of a pyrimidine requirement in three mutants presumed to be allelic but not in two demonstrated as non-allelic to these.The present study deals with a spontaneously occurring suppressor mutation which has as its effect partial removal of the growth requirement for three non-allelic Neurospora mutants which will grow when supplied with acetate. In a previous study,2 it was hypothesized that acetate is synthesized through two pathways, the main one being blocked by gene mutations in the class of mutants studied, and the other pathway blocked by some product of glucose metabolism. The secondary pathway becomes operative when certain carbon sources are used and leads to the synthesis of limited quantities of acetate. In the present report, it is suggested that the suppressor gene is responsible for opening the secondary pathway of acetate formation which ordinarily is inoperative due to an inhibition phenomenon.Genetic Studies on Mutants.-Three mutants, designated S-34, S-210 and S-48, were isolated in independent mutant hunts by the method of Lein, Mitchell and Houlahan.3 These mutants behaved similary under a variety of conditions and as yet no distinguishing physiological characteristics have been found Various growth characteristics of these mutants have been reported previously.2 4 In general, the manipulative techniques used in the studies to be reported were those described by Beadle and Tatum.5 When crossed to wild type, the mutaits yielded segregations indicating that the acetate requirement was associated with a single gene mutation. Good heterocaryons resulted when each mutant was grown in combination with the other on minimal medium. In addition to this indication of non-allelism, genetic studies were also undertaken. Preliminary attempts to cross the mutants on acetate-supplemented Westergaard medium6 proved unsuccessful. After various modifications gave no satisfactory results, it was found that fertile perithecia could be obtained when heterocaryons, which were formed -on minial-medium, were inocIted on Westergaard medium and conidia of one of the members of the heterocaryotic pair applied, a procedure suggested by Beadle and Coonradt.7 The PROC. N. A. S. 44

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