TRANSCRIPTION-POSITIVE COFACTOR 4 FORMS COMPLEXES WITH HSSB (RPA) ON SINGLE-STRANDED-DNA AND INFLUENCES HSSB-DEPENDENT ENZYMATIC-SYNTHESIS OF SIMIAN-VIRUS-40 DNA

The replication of simian virus 40 (SV40) DNA in vitro requires a trim eric single-stranded DNA (ssDNA)-binding protein called HSSB or RPA. H SSB supports the unwinding of DNA containing the SV40 origin in the pr esence of the viral-encoded T antigen and is required for the initiati on of RNA primer synthesis as well as processive elongation of DNA cat alyzed by the DNA polymerase delta holoenzyme. In this report we show that the transcription positive cofactor 4 (PC4), a ssDNA-binding prot ein, forms complexes with HSSB on ssDNA and markedly affects the repli cation functions of HSSB. PC4 supports T antigen-catalyzed unwinding o f SV40 origins in lieu of HSSB but inhibits both RNA primer synthesis and polymerase delta-catalyzed DNA chain elongation reactions. These i nhibitory effects can be reversed by the addition of excess HSSB. Depe nding on the concentration of HSSB, PC4 is capable of either inhibitin g or activating SV40 DNA replication measured in both mono- and dipoly merase systems. The possible role of PC4 in the initiation of DNA repl ication is discussed.