Fig. 23. Uracil salvage pathway, by which many exogenously supplied pyrimidines are cycled via uracil to uridine 5'-phosphate and, thus, into the latter part of the pyrimidine biosynthetic pathway. The figure shows the sites of action of the salvage pathway uc genes, the ud and uc-5 genes, which control uptake of exogenous uridine and uracil, and udk, which controls uridine kinase (126, 420, 616, 977, 1141). For endogenous pyrimidine synthesis, see Fig. 20.