Enzymatic studies with the suppressor of vermilion of Drosophila melanogaster.

UPPRESSION is the complete or partial restoration of the wild-type phenotype by one gene in the presence of another nonallelic mutant gene that alone causes a mutant phenotype. Intragenic “suppressors” have also been reported (CRICK, BARNETT, BRENNER and WATTS-TOBIN 1961; HELINSKI and YANOFSKY 1963) but the term “second site reversion” used by HELINSKI and YANOFSKY seems useful in distinguishing them from the more common, nonallelic suppressors. Suppression is a phenomenon observed and defined at the phenotypic level and naturally arises from many diverse mechanisms. The suppressor of vermilion, found by BRIDGES in 1915, was the first such gene to be discovered in any organism. Although many suppressor genes were later found in Drosophila, it has proved difficult to study their mode of action. On the other hand, a substantial number of suppressors have been carefully studied in microbial systems including Neurospora, yeast, bacteria, and phage (cf. YOURNO and SUSKIND 1964). This paper reports studies of the gene suppressor of vermilion, of Drosophila. This differs from many suppressors that occur in microbial systems because of the dominant-recessive relationships and because it concerns a nonlethal character. Several mechanisms of action for suppressors have been demonstrated. These include (1 ) the removal of a metabolic inhibitor accumulated because of the mutant block (STRAUSS and PIEROG 1954); (2) the removal of an inhibitor normally present in all strains which inhibits a mutant enzyme but not the wild-type enzyme (SUSKIND and KUREK 1959; SUSKIND, LIGON, and CARSIOTIS 1962) ; ( 3 ) the alteration or elimination of a repressor substance so that a mutant operator (0° type) no longer irreversibly binds the repressor and could therefore function (MUKAI and MARGOLIN 1963); and (4) the alteration in the primary structure of a mutant enzyme (BRODY and YANOFSKY 1963). Vermilion mutants ( U , 1-33.0) in D. melanogaster lack the brown eye pigment possessed by wild-type flies, and are completely or nearly devoid of tryptophan pyrrolase activity, the enzyme that converts tryptophan into formylkynurenine ( BAGLIONI 1960).