A group of isoleucine-valine requiring mutants of Neurospora crassa referred to as the group 4 mutants have been described in an earlier publication from this laboratory (WAGNER, BERGQUIST, BARBEE and KIRITANI 1964). Genetic analyses indicated that they were located in linkage amup IV. Mitochondrial fractions and supernatants from them were unable to carry out the overall synthesis of valine from pyruvate, but were able to do so from a-acetolactate. The data indicated that they were blocked at

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... he step at which pyruvate and a-ketobutyrate arc condensed with "active" acetaldehyde to form a-acetolactate and a-aceto-a-hydroxybutyrate, the two acetohydroxy acid precursors of valine and isoleucine, respectively. This present communication further characterizes these mutants genetically and biochemically, and presents data which make it clear that they should now be referred to as the iv-3 mutants according to the standard Neurospora genetic nomenclature rather than group 4 mutants. An accompanying communication by CAROLINE, HARDING, KUWANA, SATYANARAYANA and WAGNER ( 1969) describes the enzyme deficiency found in the iv-3 mutants MATERIALS A N D METHODS Strains: The wild-type strain used in the crosses and as a source of enzyme was KJT1960A. This strain was derived from a cross between EM 5256A and EM 5297a. The mutant strains were either isolated in this laboratory or obtained from the Fungal Genetics Stock Center. The iu-3 (group 4) mutants 3444, 345, 346, 349, 353, 354, 355, 357, 360, 361, 363, 364 . and 366 were all isolated after ultraviolet irradiation of the lys-I strain, 33933A, and therefore were originally obtained as lys-I, iu-3 double mutants (WAGNER et al. 1964) . Single iu-3 mutants were derived from these by outcrossing. The iu-3 mutant Y7110, and the marker strains me-5 (9666), pdx-1 (37803), ad-6 (28610), pan-1 (5531), and Ieu-2 (37501) were supplied by the Stock Center. Several iu strains not linked to the iu-3 strain were also used. These, 305 and 322, alleles at the iu-2 locus, and 3W, 31 1,321 and 326, alleles at the iu-I locus, have been described in detail by WAGNER et al. (1W) . Genetic analysis: The genetic analysis of the iu-3 locus was made by random spore isolation and the plating method of NEWMEYER (1954). The ascospores were treated with 0.1% sodium hypochlorite before heat treatment at 60°C for 30 min to induce germination. The density of the ascospore suspensions used in plating was estimated by means of a hemocytometer. Complementation studies: Conidia were washed by suspending in deionized water and cen-