Splicing and transcription-associated proteins PSF and p54nrb/NonO bind to the RNA polymerase II CTD

The carboxyl-terminal domain (CTD) of the largest subunit of eukaryotic RNA polymerase 11 (pol 11) plays an important role in promoting steps of pre-mRNA processing. To identify proteins in human cells that bind to the CTD and that could mediate its functions in pre-mRNA processing, we used the mouse CTD expressed in bacterial cells in affinity chromatography experiments. Two proteins present in HeLa cell extract, the splicing and transcription-associated factors, PSF and p54(nrb)/NonO, bound specifically and could be purified to virtual homogeneity by chromatography on immobilized CTD matrices. Both hypo- and hyperphosphorylated CTD matrices bound these proteins with similar selectivity. PSF and p54(nrb)/NonO also copurified with a holoenzyme form of pol 11 containing hypophosphorylated CTD and could be coimmunoprecipitated with antibodies specific for this and the hyperphosphorylated form of pol II. That PSF and p54(nrb)/NonO promoted the binding of RNA to immobilized CTD matrices suggested these proteins can interact with the CTD and RNA simultaneously. PSF and p54(nrb)/NonO may therefore provide a direct physical link between the pol 11 CTD and pre-mRNA processing components, at both the initiation and elongation phases of transcription.