Mechanism of suppression in Drosophila: evidence for a macromolecule produced by the su(s)+ locus that inhibits sepiapterin synthase.

Genetic suppression was studied in the purple mutant of Drosophila melanogaster and in suppressed purple by measurement of sepiapterin synthase activity. The addition of ammonium sulfate fractions from adult Drosophila that contain one, two, three or four doses of su(s)+ to the suppressed purple sepiapterin synthase resulted in an inhibition that increased progressively as the dosage of su(s)+ increased; the wild-type sepiapterin synthase was not inhibited. This inhibition is caused by a heat-labile macromolecule. We suggest that the mechanism of suppression is neither transcriptional nor translational but is the result of decreased amounts, or altered properties, of the normal product of the su(s)+ locus when su(s)+ is replaced by su(s)2 or su(s)e6.