REGULATION OF HUMAN DNA-POLYMERASE-DELTA DURING THE CELL-CYCLE

The expression of polymerase delta (pol delta) during the cell cycle was studied in Molt 4 cells separated by counterflow centrifugal elutriation. Northern blotting showed that pol delta mRNA levels increased by 3-fold at the G(1)/S border. Levels of pol delta protein determined by Western blotting also peaked at the G(1)/S border with qualitatively similar changes as the mRNA levels, Thus, pol delta gene expression appears to be regulated during the cell cycle at the transcriptional level. The mRNA half-life for pol delta was determined to be about 8 h and the protein half-life about 10 h. Parallel examination of the expression of proliferating cell nuclear antigen showed that the mRNA levels also increased about 2-fold at the G(1)/S border, while the protein levels of proliferating cell nuclear antigen increased steadily through the whole cell cycle period, and remained high at G(2)/M. Analysis of pol or expression showed qualitatively similar behavior as pol delta, but the magnitude of the changes were higher. Pulse labeling of cells metabolically arrested in G(1), S, or G(2)/M with P-32(i) showed that pol delta is a phosphoprotein and that it is most actively phosphorylated during the S phase.