The BRCA1 and BARD1 association with the RNA polymerase II holoenzyme

We have previously shown that endogenous BRCA1 and overexpressed epitope-tagged BRCA1 are present in the transcription complex called the RNA polymerase II holoenzyme (holo-pol). In this study, we further characterized BRCA1 association with the holo-pol by overexpressing deletion mutants of epitope-tagged BRCA1. We found that BRCAl-associated RING domain protein (BARD1) is a component of the holo-pol complex. Deletion of the BRCA1 NH₂ terminus, which is bound by BARD1 as well as other proteins, eliminates >98% of BRCA1 association with the holo-pol. In contrast with earlier observations, deletion of the COOH terminus of BRCA1 did not affect significantly the association with holo-pol. Immunocytochemistry of expressed full-length and deletion mutants of BRCA1 showed that the NH₂ terminus of BRCA1 is important for nuclear dot formation in S-phase. An intact BRCA1 NH₂ terminus is required for the association with holo-pol and for subnuclear localization in S-phase foci. Taken together, these data support a role for BRCA1 regulation of holo-pol function.