Following UV irradiation, cellular mRNA levels are transiently decreased due to the inhibition of transcription and mRNA polyadenylation. The UV-induced inhibition of 3’ processing reflects the interaction of the polyadenylation factor CstF-50 with the tumor suppressors BRCA1/BARD1 as well as the BRCA1/BARD1-mediated degradation of RNA polymerase II (RNAP II). As CstF-50 can interact with BRCA1/BARD1 and RNAP II inhibiting or activating polyadenylation, respectively, and cells with reduced levels of CstF show an enhanced sensitivity to UV and reduced ability to ubiquitinate RNAP II and repair DNA, we propose a coordinating role for this factor in the DNA damage response.

Our studies add complexity to the cellular response to UV treatment by linking RNA processing to the p53 network. Here, we show that the C-terminal domain of the tumor suppressor p53 associates not only with BARD1 but also with CstF-50. Although the p53/CstF complex is detected independently of DNA damage, we found that the tumor suppressor BARD1 can coexist in complexes with CstF and p53 only after UV treatment. Consistent with this, we found that the C-terminal domain of p53 inhibits pre-mRNA cleavage in vitro. Significantly, siRNA-mediated knockdown of p53 has different effects on the UV-induced inhibition of polyadenylation in cells expressing different levels of p53. Supporting the physiological significance of these results, a previously identified mutation in p53 (Ser241Phe) decreases the p53-BARD1 interaction and abrogates inhibition of polyadenylation, which can be restored by the expression of wild-type p53. We also found that p53 participates in the post-transcriptional regulation of endogenous transcripts of housekeeping but not p53-regulated genes.

We also determined that the interaction between CstF-50 and BARD1 involves the ankyrin-BRCT linker but not the ankyrin or BRCT domains. The structural plasticity imparted by the ANK-BRCT linker helps explain the regulated assembly of different proteins involved in DNA damage to BARD1, including BARD1-dependent stabilization of p53 and the induction of apoptosis. Furthermore, supporting the role of BARD1 in signaling in the DNA damage response, we found that UV-induced BARD1 phosphorylation is important not only for its functions associated with CstF-50 but also for its functions in regulating p53 phosphorylation and stability.