Studies on the regulation of pyrimidine nucleotide biosynthesis and its effect on ribonucleic acid accumulation during synchronous growth of Chlorella pyrenoidosa

Synchronous cultures of a high temperature strain of Chlorella pyrenoidosa have been used to measure the apparent levels of aspartate transcarbamylase and dihydroorotase, the first two enzymes on the pyrimidine nucleotide biosynthetic pathway during the cell cycle of this organism. When the apparent levels of these two enzymes were plotted on a relative increase basis, their rates of increase throughout the cell cycle were different. Dihydroorotase had the greater rate of increase through the first 0.5 of the cycle, whereas aspartate transcarbamylase had the greater rate of increase for the remainder of the cell cycle. These findings tend to support the conclusion that these t:wo enzymes are not coordinately synthesized in Chlorella.

A theoretical approach was taken to try to elucidate the mechanisms controlling the rate of RNA accumulation. Of the two enzymes studied, dihydroorotase appears to fulfill the requirements of the enzyme which rate-limits RNA synthesis in this organism.