Nonallelic suppressors of a Neurospora tryptophan synthetase mutant.

TSase mutant tdzol forms elevated levels of a mutationally altered enzyme ( CRM) which retains only reaction (2) activity, and the mutant requires either indole or L-tryptophan for growth. It has been reported previously that seven independently isolated tdZo1 specific suppressors, hereafter designated sul (201 ) suppressors, are all allelic or closely linked and map in the right arm of linkage group VI1 (YOURNO and SUSKIND 1963 1964a). The suppressors restore a low level of wild-type-like TSase activity to the mutant while causing no gross deleterious effects on generalized cell function. The restored activity was associated with a protein which was wild-type-like by several enzymic and physical criteria (YOURNO and SUSKIND 1964b). Among four successfully characterized new suppressed mutant isolates, three were found to have the heretofore observed slow-growth characteristics on minimal medium and to contain suppressors mapping in the sul (201) region of linkage group VIIR. The remaining suppressed mutant however, su3 (201) -1, grew much more slowly in the absence of tryptophan or indole and was not linked to the others. These phenotypically different suppressors have been compared with respect to sitespecificity, location, general effects on cell growth, and properties of the functional TSase produced. ~ ~ ~ ( 2 0 1 ) and ~ ~ ~ ( 2 0 1 ) will be symbolized by the abbreviated symbols suI and suI throughout the paper. suf will indicate the wild-type allele of the suppressor gene which is inactiue, and su the mutant allele which is active in suppression.