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 counter-flow centrifugal elutriation. Northern blotting showed that pol delta mRNA levels increased by 3-fold at the G1/S border. Levels of pol delta protein determined by Western blotting also peaked at the G1/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 G1/S border, while the protein levels of proliferating cell nuclear antigen increased steadily through the whole cell cycle period, and remained high at G2/M. Analysis of pol alpha expression showed qualitatively similar behavior as pol delta, but the magnitude of the changes were higher. Pulse labeling of cells metabolically arrested in G1,S, or G2/M with 32Pi showed that pol delta is a phosphoprotein and that it is most actively phosphorylated during the S phase.