General coactivator that functions cooperatively with TAFs and mediates functional interactions between upstream activators and the general transcriptional machinery. May be involved in stabilizing the multiprotein transcription complex. Binds single-stranded DNA. Also binds, in vitro, non-specifically to double-stranded DNA (ds DNA). Homodimer. Interacts with CSTF2. Activity is controlled by protein kinases that target the regulatory region. Phosphorylation inactivates both ds DNA-binding and cofactor function, but does not affect binding to ssDNA. Seems to be phosphorylated in vivo by CK2 in at least 7 sites in the N-terminal Ser-rich region. Belongs to the transcriptional coactivator PC4 family.