Control of Pyrimidine Biosynthesis in Synchronously Dividing Cells of Helianthus tuberosus.

Factors with potential for regulating pyrimidine biosynthesis in plant tissue have been explored in quiescent cells of Helianthus tuberosus induced to divide by auxin addition. Investigations confined to the first highly synchronous cell cycle of the tuber explants revealed that the relative activity of asparate carbamoyltransferase (ACTase) to ornithinecarbamoyltransferase (OCTase) (enzymes competing for carbamoyl phosphate for the pyrimidine and arginine pathways, respectively) changes from 0.5 in quiescent cells to 3.0 by the end of the first cell cycle. This was interpreted as a change in the state of cell function from accumulation of storage arginine to cell division with a concomitant demand for pyrimidine nucleotides for nucleic acid synthesis. The rise in ACTase activity began at the same time as the initiation of DNA synthesis and was dependent on continued DNA synthesis. OCTase activity declined whether or not auxin was added to the medium, whereas ACTase activity was observed to decline only in the absence of DNA synthesis.The low cellular concentration of the shared substrate, carbamoyl phosphate (2 micromolar), favored utilization of this substrate by the pyrimidine pathway over the arginine pathway because of the low K(m) (0.08 80 micromolar) for this substrate by ACTase compared to that for OCTase (9.0 millimolar). Unexpectedly, the total concentration of the feedback inhibitor for the pyrimidine pathway, UMP, was found to have more than doubled in dividing tissue at a time when pyrimidine nucleotide demand had increased. It is concluded that compartmentation decreased UMP in the vicinity of ACTase and/or that the extra UMP stabilizes newly synthesized ACTase in preparation for an even greater demand for nucleic acid synthesis in the second and subsequent cell cycles.