RPAP1, a novel human RNA polymerase II-associated protein affinity purified with recombinant wild-type and mutated polymerase subunits

We have programmed human cells to express physiological levels of recombinant RNA pollymerase II (RNAPII) subunits carrying tandem affinity purification (TAP) tags. Double-affinity chromatography allowed for the simple and efficient isolation of a complex containing all 12 RNAPII subunits, the general transcription factors TFIIB and TFIIF, the RNAPII phosphatase Fcp1, and a novel 153-kDa polypeptide of unknown function that we named RNAPII-associated protein I (RPAPI). The TAP-tagged RNAPII complex is functionally active both in vitro and in vivo. A role for RPAPI in RNAPII transcription was established by shutting off the synthesis of Ydr527wp, a Saccharomyces cerevisiae protein homologous to RPAPI, and demonstrating that changes in global gene expression were similar to those caused by the loss of the yeast RNAPII subunit RpbII. We also used TAP-tagged Rpb2 with mutations in fork loop 1 and switch 3, two structural elements located strategically within the active center, to start addressing the roles of these elements in the interaction of the enzyme with the template DNA during the transcription reaction.