Overproduction of the first three enzymes of pyrimidine nucleotide biosynthesis in Drosophila cells resistant to N-phosphonacetyl-L-aspartate.

Drosophila cells were treated in vitro with N-phosphonacetyl-L-aspartate (PALA) which is a specific inhibitor of aspartate transcarbamylase, the second enzyme of the pyrimidine biosynthetic pathway. By stepwise selection using increasing amounts of this inhibitor, PALA-resistant (PALAr) stable clones have been isolated. Enzymatic activities of aspartate transcarbamylase, carbamyl phosphate synthetase and dihydro-orotase, borne by the same multifunctional protein, CAD, are increased 6-12-fold in these resistant clones compared with parental cells. The aspartate transcarbamylase in PALAr cells is shown by physical, kinetic and immunological criteria to be normal. The data from immunotitration and immunoblotting experiments indicate that the increased enzyme activities result from the overproduction of CAD.