SUPPRESSOR GENE ACTION IN THE TRYPTOPHAN SYNTHETASE SYSTEM OF NEUROSPORA CRASSA. I. GENETIC STUDIES

EXTRAGENIC suppressors which alleviate nutritional requirements of specific microbial auxotrophs appear to function by a variety of mechanisms. The suppressor may act indirectly in one of several ways to make available the required compound. In Neurospora crassa these mechanisms include the utilization of separate pathways of metabolism ( LEIN and LEIN 1952; STRAUSS and PIEROG 1954), decompartmentalization of a metabolite which has accumulated before a partial block and is required in a separate pathway (DAVIS 1962; DAVIS and WOODWARD 1962) and the removal of an inhibitor to which the mutant enzyme is sensitive with resultant partial restoration of the biosynthetic step lacking in the mutant ( SUSKIND and KUREK 1959; SUSKIND, LIGON and CARSIOTIS 1962). Other suppressors appear to act by repair of the primary structure of the catalytically defective mutant enzyme. The existence of this type of suppressor mechanism in N . crassa was suggested initially by the work of YANOFSKY with one suppressed tryptophan synthetase mutant (1952). A number of theories involving effects on protein biosynthesis have been proposed to explain the mechanism of action of such suppressors. These include: (1) the “missense to sense” ( YANOFSKY and ST. LAWRENCE 1960; YANOFSKY, HELINSKI and MALING 1961) and the “nonsense to sense” theories (GAREN and SIDDIQI 1962; BENZER and CHAMPE 1962), which invoke changes in the specificity of protein biosynthesis at the level of the activating enzymes and transfer RNAs, and (2) modifications in the ribosomal reading of the messenger RNA code (GORINI and KATAJA 1964; DAVIES, GILBERT and GORINI 1964). Wild-type tryptophan synthetase (TSase) of N . crassa catalyzes the following reactions (YANOFSKY 1960; GARRICK, ELBERFELD and SUSKIND 1964) : 1. Indoleglycerolphosphate + 1.-serine -+ L-tryptophan + D-glyceraldehyde 3-phosphate f H,O 2 indole L-serine --t L-tryptophan + H,O 3. indoleglycerolphosphate indole + D-glyceraldshyde 3-phosphate r