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The RNA polymerase II (Pol II) holoenzyme in yeast is an essential transcriptional regulatory complex which has been defined by genetic and biochemical approaches. The mammalian counterpart to this complex, however, is less well defined. Experiments herein demonstrate that, along with Pol II and SRB proteins, proteins associated with transcriptional regulation as cofactors are associated with the Pol II holoenzyme. Earlier experiments have demonstrated that the breast cancer-associated tumor suppressor BRCA1 and the CREB binding protein (CBP) were associated with the holoenzyme complex. The protein related to CBP, the E1A-associated p300 protein, is shown in these experiments to be associated with the holoenzyme complex as well as the BRG1 subunit of the chromatin remodeling SWI/SNF complex. Importantly, the Pol II holoenzyme complex does not contain some factors previously reported as stoichiometric components of the holoenzyme complex, most notably the proteins which function in repair of damaged DNA, such as PCNA, RFC and RPA. The presence of the p300 coactivator and the chromatin-modifying BRG1 protein support a role for the Pol II holoenzyme as a key target for regulation by enhancer binding proteins.