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) (By similarity). Homodimer. Interacts with CSTF2 (By similarity). 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 (By similarity). Belongs to the transcriptional coactivator PC4 family.