PHOSPHORYLATED AND UNPHOSPHORYLATED FORMS OF HUMAN SINGLE-STRANDED DNA-BINDING PROTEIN ARE EQUALLY ACTIVE IN SIMIAN-VIRUS-40 DNA-REPLICATION AND IN NUCLEOTIDE EXCISION-REPAIR

The trimeric human single-stranded DNA-binding protein (HSSB; also cal led RP-A) plays an essential role in DNA replication, nucleotide excis ion repair, and homologous DNA recombination. The p34 subunit of HSSB is phosphorylated at the G(1)/S boundary of the cell cycle or upon exp osure of cells to DNA damage-inducing agents including ionizing and UV radiation. We have previously shown that the phosphorylation of p34 i s catalyzed by both cyclin-dependent kinase-cyclin A complex and DNA-d ependent protein kinase. In this study, we investigated the effect of phosphorylation of p34 by these kinases on the replication and repair function of HSSB. We observed no significant difference with the unpho sphorylated and phosphorylated forms of HSSB in the simian virus 40 DN A replication or nucleotide excision repair systems reconstituted with purified proteins. The phosphorylation status of the p34 subunit of H SSB was unchanged during the reactions. We suggest that the phosphoryl ated HSSB has no direct effect on the basic mechanism of DNA replicati on and nucleotide excision repair reactions in vitro, although we cann ot exclude a role of p34 phosphorylation in modulating HSSB function i n vivo through a yet poorly understood control pathway in the cellular response to DNA damage and replication.