Genetic and biochemical properties of revertants at the rudimentary locus in Drosophila melanogaster.

The rudimentary (r) locus of Drosophila melanogaster encodes the first three enzymes for the de novo synthesis of pyrimidines: carbamyl phosphate synthase (CPSase), aspartyl transcarbamylase (ATCase), and dihydroorotase (DHOase). Revertants of leaky r mutants deficient in CPSase have been recovered. Of those examined, most (28/30) appear to result from reversion of the original mutation or a second site mutation in the mutant gene: They map to the r locus, have levels of CPSase and Km's for glutamine similar to wild type, and have comparatively little change in ATCase and DHOase activity. Two of the revertants appear to involve regulatory mutations that lead to an increase in defective CPSase molecules: They map to the r locus, have Km's for glutamine like that of the CPSase in the progenitor stock, and lead to relatively small increases in CPSase activity that are roughly paralleled by increases in ATCase and DHOase. The recovery of these putative regulatory mutations increasing the activity of the three enzymes supports the conclusion that the three activities are part of a trifunctional polypeptide or that their genes are transcribed together as parts of a multicistronic transcript.