Genetical and cytological location of the structural parts coding for the first three steps of pyrimidine biosynthesis in Drosophila melanogaster.

The rudimentary locus (r; X--55.3) of Drosophila melanogaster is shown to contain the structural sequences for the enzymes CPSase, ATCase and DHOase. The enzyme concentration in adult flies is correlated with the number of r+ copies in the genome. The expression of the locus follows the rules of the gene dosage compensation hypothesis when extracts of newly emerged males and females are compared.