Fig. 8: PARP1-dependent recruitment of USP3 into chromatin during BER in XRCC1-defective cells.

a, Chromatin retention of USP3–GFP in WT and XRCC1^−/− U2OS cells transiently expressing H2B–mCherry and USP3–GFP following mock treatment or treatment with 0.1 mg ml⁻¹ MMS in the presence or absence of 10 μM PARPi as indicated. The cells were either immediately fixed with paraformaldehyde (PFA) to detect total protein or first extracted with detergent to remove the soluble proteins (pre-extracted), as indicated. Data are the mean ±s.e.m. of four independent experiments. Statistical significance was determined using a two-way ANOVA with Sidak’s multiple comparisons test (significantly different P values are indicated). Representative scanR galleries of individual cells (left) and quantification (right) are shown. Scale bar, 10 μm. b, Model for prolonged PARP1/USP3-dependent suppression of transcription during BER. In WT cells, RNAP pausing at BER intermediates is accompanied by PARP1 activation, completion of BER by XRCC1 protein complexes and transcription resumption. In XRCC1-defective cells, persistent PARP1 activity at unrepaired BER intermediates leads to aberrant USP3 recruitment, extensive/excessive protein deubiquitination (including histones) at damaged and proximal/nearby undamaged sites, and prolonged transcriptional suppression. Note that PARP1 inhibition prevents USP3 recruitment and rescues global transcription recovery, most probably reflecting the eventual bypass of unrepaired SSBs by RNAP and/or transcription resumption at proximal/nearby undamaged sites.