RECRUITMENT OF PHOSPHORYLATED CHROMATIN ASSEMBLY FACTOR 1 TO CHROMATIN AFTER UV IRRADIATION OF HUMAN-CELLS

The subcellular distribution and posttranslational modification of hum an chromatin assembly factor 1 (CAF-1) have been investigated after UV irradiation of HeLa cells. In an asynchronous cell population only a subfraction of the two large CAF-1 subunits, p150 and p60, were found to exist in a chromatin-associated fraction. This fraction is most abu ndant during S phase in nonirradiated cells and is much reduced in G2 cells. After UV irradiation, the chromatin-associated form of CAF-1 dr amatically increased in all cells irrespective of their position in th e cell cycle. Such chromatin recruitment resembles that seen for PCNA, a DNA replication and repair factor. The chromatin-associated fractio n of p60 was predominantly hypophosphorylated in nonirradiated G2 cell s. UV irradiation resulted in the rapid recruitment to chromatin of ph osphorylated forms of the p60 subunit. Furthermore, the amount of the p60 and p150 subunits of CAF-1 associated with chromatin was a functio n of the dose of UV irradiation. Consistent with these in vivo observa tions, we found that the amount of CAF-1 required to stimulate nucleos ome assembly during the repair of UV photoproducts in vitro depended u pon both the number of lesions and the phosphorylation state of CAF-1. The recruitment of CAF-1 to chromatin in response to UV irradiation o f human cells described here supports a physiological role for CAF-1 i n linking chromatin assembly to DNA repair.