Gene Interactions: the Mode of Action of the Suppressor of Acetate-requiring Mutants of Neurospora crassa

Abstract

SUMMARY : Certain single-gene acetate-requiring mutants of Neurospora crassa (ac) are deficient in their ability to oxidize pyruvate directly, but can decarboxylate pyruvate to acetaldehyde and can form ethanol. The growth characteristics of the nc mutants indicate that they can form acetate from glucose or ethanol. ac Mutants are inhibited by glucose or sucrose. This glucose inhibition is relieved by the singlegene suppressor mutations sp and car which lower the activity of pyruvic carboxylase, decrease the amount of ethanol formed and permit some growth of ac sp and accar strains in absence of added acetate. The sp mutation also lowers ethanol dehydrogenase activity, but this activity can be partially restored by growth of sp strains in the presence of ethanol. As a result of the nearly complete block in their pyruvate metabolism, ac sp strains are forced to metabolize glucose by pathways which produce more of the initial respiratory CO, from the C-1 carbon of glucose.While genetic theory is reticent about the embodiment of the gene (Pontecorvo, 1952) there is little doubt that the physiological effects of genes are mediated via enzymes. Genes, individually and en masse, control the rates of the step reactions from which multi-enzyme systems (Dixon, 1949) are built up, and in so doing determine the metabolic pathways chosen by the organism. I n many instances gene mutation causes the block, complete or incomplete, of a step reaction; the consequence of such a block is the accumulation of precursors and the appearance of a growth requirement for the product of the blocked reaction (Beadle, 1945).The commonest use for genetic mutants in biochemistry is the definition of routes of biosynthesis by a study of the growth requirements induced and the identification of the precursors accumulated (Horowitz, 1950). As interesting, perhaps, are the inhibitions produced by the accumulated precursors or by ' abnormal ' products derived from these precursors (Emerson, 1949;Bonner, 1946). If genes do control the rates of step reactions, it should be possible with a second mutation to relieve an inhibition induced by an initial mutation, if the second mutation prevents the formation of the inhibitor. Such relief by a second mutation would be denoted, genetically, as a suppressor or modifier effect and would be one of the many instances in which the effects of two co-existing mutant genes are not merely additive. The gene interaction in such a situation would not be the simple interaction of two genes present in the nucleus but would, rather, express the interaction of the enzymic reaction systems present in non-genic parts of the cell; such interactions would further demonstrate the interdependence of the biochemical reactions of the organism. These investigations demonstrate an example of gene interaction in wellknown metabolic pathways and indicate that the selection of one of several alternate pathways is here under genetic control; all this would again emphasize the importance to genetical theory of inhibition by naturall...

The Formation of a New Enzymatically Active Protein as a Result of Suppression

“…Examples of suppression by both the above mechanisms are known. [2][3][4] In addition it has been suggested that certain suppressor genes might act by permitting the formation of new, effective enzymes to function for the absent or defective ones. In Neurospora crassa several cases of suppression are known in mutants which form an altered protein immunologically related to the enzyme tryptophan synthetase;' in the presence of the suppressor gene normal enzymatic activity appears.…”

Gene Interactions Affecting Methionine Biosynthesis and the Response to S-methylcysteine by Mutants of Neurospora crassa

“…We therefore investigated the characteristics of this type of suppressor to determine whether a new type of gene interaction was represented. Our results indicate that in methionine biosynthesis, these particular suppressors act by preventing the formation of an inhibitor accumulated as a result of the original mutation to auxotrophy, an action similar in nature to that of the suppressors of the acetate-requiring mutants (Strauss & Pierog, 1954). In the course of our studies, some observations were and M7 respond to either cystathionine, homocysteine or methionine, but not to cysteine, these strains are not alleles.…”

“…Such an action by the s u gene might account for the restoration of cystathionase I and I1 activities observed in m e mutants by Fischer (1957). This scheme is analogous to that devised to account for the behaviour of the suppressors of the acetate-requiring mutants of Neurospora (Strauss & Pierog, 1954). It was supposed a t that time that the acetate suppressors lowered the production of an inhibitor (acetaldehyde) formed in excess as a result of the first mutation.…”

The Nature of the Lesion in the Succinate-requiring Mutants of Neurospora crassa: Interaction between Carbohydrate and Nitrogen Metabolism

“…Pyruvic acid, AMC and C, condensation products which do not require complete oxidation accumulate in cultures of the SUC mutants (necessarily grown in the presence of nitrogen salts) because of the deficiency in the dicarboxylic acid catalyst required for C, fragment oxidation. The same accumulation products are produced in the suc mutants, where there is no immediate block to pyruvate oxidation as in the ac mutants (Strauss & Pierog, 1954), where there is a block in the oxidation of pyruvate : another illustration of the danger of using accumulations as evidence of genetic block (Adelberg, 1953). The relationship between succinate, a growth requirement, and AMC, a by-product of a true intermediate, is another case in which an end product controls the accumulation of a precursor (Strauss, 1955a, b ;Adelberg & Umbarger, 1953).…”

“…-f See (Strauss & Pierog, 1954). The at locus apparently acts only as a modifier to the SUC gene, preventing response of SUC strains to acetate.…”