UV light‐induced DNA synthesis arrest in HeLa cells is associated with changes in phosphorylation of human single‐stranded DNA‐binding protein

We show that DNA replication activity in extracts of human HeLa cells decreases following UV irradiation. Alterations in replication activity in vitro parallel the UV‐induced block in cell cycle progression of these cells in culture. UV irradiation also induces specific changes in the pattern of phosphorylation of the 34 kDa subunit of a DNA replication protein, human single‐stranded DNA‐binding protein (hSSB). The appearance of a hyperphosphorylated form of hSSB correlates with reduced in vitro DNA replication activity in extracts of UV‐irradiated cells. Replication activity can be restored to these extracts in vitro by addition of purified hSSB. These results suggest that UV‐induced DNA synthesis arrest may be mediated in part through phosphorylation‐related alterations in the activity of hSSB, an essential component of the DNA replication apparatus.