THE ENZYMATIC ACTIVITY ASSOCIATED WITH THE PROTEIN IMMUNOLOGICALLY RELATED TO TRYPTOPHAN SYNTHETASE

It has been well established in a variety of organisms that gene mutations affect the formation of specific enzymes. In view of the complexity of the gene in terms of numbers of distinct mutational sites,1 it might be expected that different changes within any one gene would result in different alterations of a specific enzyme or protein. Some enzyme alterations have, in fact, been associated with gene changes,2-5 and, in one case, a single amino acid substitution has been shown to distinguish the mutant from the normal form of a protein. These instances strengthen the view that the amino acid sequence of each protein may reflect the arrangement of the subunits of its determining gene. In Neurospora, certain mutants defective in the ability to form the enzyme tryptophan synthetase (TSase) are distinguishable from one another on the basis of analyses of the following types: growth in the absence of tryptophan, accumulation of intermediates, reversion frequency, response or lack of response to specific suppressor genes, enzyme activity measurements, and immunological comparisons.7-11 Since these strains represent mutations of the same gene and are distinguishable from one another, it may be assumed that in some cases the dissimilarities detected result from different alterations of TSase. In this regard the immunological findings are of particular interest because they demonstrate that certain of these mutants form a protein (designated CRM) which may be an inactive or incomplete form of TSase.'0 This protein is immunologically similar to TSase and is absent from at least one mutant which lacks TSase.0" ,l Whether the CRM's formed by distinguishable mutant types differ from one another has not been determined, but differences might be expected if the CRM's represent altered forms of TSase. Studies with the analogous system in Escherichia coli have shown that mutants lacking TSase fall into the same two groups, viz., those which form a cross-reacting protein (CRM) and those which do not.'2 In E. coli, mutants in the latter group have two additional characteristics which distinguish them from strains which form CRM; they accumulate indoleglycerol instead of indole,'2, 13 and they lack an enzyme required for the conversion of indoleglycerol phosphate to indole.'4 The absence from these mutants of both CRM and an enzyme involved in the synthesis of indole suggested a second interpretation of the nature of CRM, viz., that it is an enzyme catalyzing an earlier step in tryptophan synthesis.12 This report is concerned with a study of this possibility.