ON LOSS OF FEEDBACK INHIBITION OF YEAST ASPARTATE TRANSCARBAMYLASE DURING DEREPRESSION OF PYRIMIDINE BIOSYNTHESIS

During the growth of a uracil-requiring mutant of S. cerevisiae under conditions of limiting uracil concentration in the growth medium, derepression of aspartate transcarbamylase (ATCase) was accompanied by a progressive loss of its sensitivity to inhibition by UTP, one of the final products in the pyrimidine biosynthetic chain. The loss in feedback inhibition could be prevented or reversed by adding excess uracil to the medium in which the cells were growing. If fully repressed and fully derepressed cells were mixed prior to extracting the enzyme, the feedback inhibition of the resulting enzyme preparation was considerably higher than that predicted on the basis of the sensitivity to UTP of the repressed and derepressed enzymes alone. No significant enhancement of feedback inhibition was noted if the crude extracts themselves were mixed (i.e., after extraction from the repressed and derepressed cells). Virtually the same effect was noted if the derepressed cells were mixed with a mutant strain whose ATCase lacked the regulatory site; cells of this mutant strain possess very high intracellular levels of UTP. The hypothesis that UTP was the intracellular factor responsible for stabilizing the feedback site of the repressed enzyme was confirmed by the finding that UTP, alone among all the pyrimidines, pyrimidine nucleotides, and purine nucleotides tested, could stabilize the feedback site during extraction. On the other hand, UTP had no effect in stabilizing the feedback of a semipurified ATCase preparation against inactivation by heat. It thus appears that UTP has three distinct roles in regulation of pyrimidine biosynthesis in the yeast cell: it is the agent both of feedback inhibition of ATCase activity and of repression of its biosynthesis and it stabilizes the regulatory site of the enzyme. © 1969.