||The human PC4 is a non-TAF transcription coactivator that mediates activator-dependent transcription by RNA polymerase II in vitro through most tested activators. The function of PC4 is apparently through interactions with transcriptional activators and the basal transcription machinery. It is negatively regulated by casein kinase II phosphorylation both in vivo and in vitro. PC4 strongly binds single stranded DNA and regulates HSSB (RPA)-dependent SV40 DNA replication. Recent studies indicated that PC4 can be acetylated by several histone acetyltransferase. Recombinant PC4 protein (wild type, 127 amino acids) is isolated from an E. coli strain that carries the coding sequence of human PC4 under the control of T7 promoter and purified by conventional chromatography. Recombinant PC4 has been utilized for in vitro function studies, including transcription, DNA replication,in vitrophosphorylation, gel mobility shift assay, protein-protein interactions, and as a substrate for in vitro acetylation. Protein is greater than 95% homogeneous and contains no detectable protease, DNase, and RNase activity.