Observations on the Behavior of Suppressors in Neurospora

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Abstract
Crosses were made between a suppressed pyrimidineless strain (pyr-s) and prolineless mutants (prol. 1, 2, 3 and 4), ornithineless mutants (orn. 1, 2 and 3) or citrullineless mutants (cit. 1 and 2). Spore pairs were isolated from ascus dissections, cultured together, and tested on minimal medium with appropriate supplements. From phenotypic classification of the spore pairs it was evident that a suppressed prol. 2, 3 and 4, but not prol. 1 and the orn. and cit. mutants. Prol. 2-s, prol. 3-s and prol. 4-s were indistinguishable from wild type in growth response on minimal medium and were not affected by added proline or arginine. Prol. 2-pyr3a-s and prol. 3-pyr3a-s did not differ from pyr3a-s. The presence of s seems to cause orn. 1, 2 and 3, and cit. 1 and 2 to have absolute requirements for citrulline or arginine. When pyr.3a was combined with orn.3 the pyrimidine requirement seemed to be completely suppressed on unsupple-mented medium. Growth was slower on media supplemented with ornithine, citrulline, or arginine than on minimal, but more rapid on media supplemented with casein. These results suggested that ornithine, citrulline, and arginine interfered with suppression of pyr3a by orn.3 as they did with suppression of pyr3a by s. Pyr3a-orn.l and pyr3a-orn.2 failed to grow on unsupplemented media, but grew slowly when supplied ornithine or arginine, until they reached the dry wt. characteristic of the single orn. mutant. A gene change in pyr.3a was considered unlikely as a cause of the suppression of the pyrimidine requirement because of the diversity of isolates inv. in the crosses and also since pyr3a with pyrimidine requirement was recovered both as a single segregant and as an isolate from a double mutant. Lysine significantly stimulated pyr3a-orn.3 when the ratio of lysine/ ornithine was 3:1. The ratio completely inhibited pyr3a-orn.l, pyr3a-orn.2, orn.1 and 2. Pyr3a has a suppressing effect on utilization of citrulline by orn.l and 2 and cit. 1, but no effect in the case of orn.3 and no effect upon ornithine or arginine utilization by orn.l, 2 or 3. Cytidine did not affect utilization of citrulline by the single orn. and cit. mutants, nor did citrulline affect utilization of cytidine by pyr.3a. 3 other non-allelic pyr. mutants, pyr.1, 2 and 4 were not suppressed by s. The behavior of the pyr. mutants, alone and in combination with other mutants requiring various supplements, is discussed. A summary is presented of the orn., prol., cit., pyr. and s single mutants and their double or triple recombinants, with respect to their growth responses on minimal media and their supplementary requirements. The action of the mutants having suppressor effects is also summarized and the postulated mechanisms of suppressor effects are discussed.