COMPLEX-FORMATION BETWEEN P53 AND REPLICATION PROTEIN-A INHIBITS THE SEQUENCE-SPECIFIC DNA-BINDING OF P53 AND IS REGULATED BY SINGLE-STRANDED-DNA

Human replication protein A (RP-A) (also known as human single-strande d DNA binding protein, or HSSB) is a multisubunit complex involved in both DNA replication and repair, Potentially important to both these f unctions, it is also capable of complex formation with the tumor suppr essor protein p53, Here we show that although p53 is unable to prevent RP-A from associating with a range of single-stranded DNAs in solutio n, RP-A is able to strongly inhibit p53 from functioning as a sequence -specific DNA binding protein when the two proteins are complexed, Thi s inhibition, in turn, can be regulated by the presence of various len gths of single-stranded DNAs, as RP-A, when bound to these single-stra nded DNAs, is unable to interact with p53, Interestingly, the lengths of single-stranded DNA capable of relieving complex formation between the two proteins represent forms that might be introduced through repa ir and replicative events. Increasing p53 concentrations can also over come the inhibition by steady-state levels of RP-A, potentially mimick ing cellular points of balance. Finally, it has been shown previously that p53 can itself be stimulated for site-specific DNA binding when c omplexed through the C terminus with short single strands of DNA, and here we show that p53 stays bound to these short strands even after bi nding a physiologically relevant site, These results identify a potent ial dual role for single-stranded DNA in the regulation of DNA binding by p53 and give insights into the p53 response to DNA damage.