Mechanism of suppression in Drosophila: control of sepiapterin synthase at the purple locus.

The amounts of sepiapterin and red pteridine eye pigments (drosopterins) in Drosophila melanogaster are known to be reduced in the purple mutant and restored to normal by a suppressor mutation. We show here that sepiapterin synthase activity is 30 percent of normal in pr and prbw, two naturally occurring alleles of purple, and is restored to nearly normal levels by the suppressor su(s)2. A heterozygote of two newly induced alleles of pr has even lower enzyme activity (less than 10 percent). The sepiapterin synthase activity is proportional to the number of wild-type pr alleles in flies when one and two copies of the allele are present and is higher in three-than in two-dose flies. We hypothesize that the purple locus may be a structural gene for sepiapterin synthase in Drosophila.