Homologous recombination is required for the formation of chromatin bridges but not of UFBs. (A) HR promotes the formation of chromatin bridges. Dpb11-YFP bridges were counted in wild-type (ML533) and in sgs1Δ (SMG223-2C), rad51Δ (VS16-3C), rad52Δ (VS19-5A), and sgs1Δ rad51Δ (VS26-15A) cells and scored for Hoechst staining before and after treatment with 0.03% MMS for 70 min. (B) MMS-induced Dpb11 bridges are HR independent. Dpb11-YFP bridges were counted in wild-type (ML533) and in rad51Δ (VS16-3C), rad54Δ (VS17-1C), and rad52Δ (VS19-5A) cells at different time points after addition of 0.03% MMS. (C) Rad51 overexpression induces chromatin bridges. Cells expressing Dpb11-YFP (ML533) were transformed with plasmids for galactose-induced expression of wild-type RAD51 (pYES-GAL-RAD51), a catalytically inactive rad51-K191R (pYES-GAL-rad51-K191R), or the empty vector (pYES). Cells were grown in SC-Leu containing 2% raffinose and inspected by microscopy after induction with 2% galactose for 1 h and Hoechst staining. (D) Dpb11 suppresses the formation of chromatin bridges by HR. Wild-type RAD54 (ML628) and rad54Δ mutant (VS17-1C) cells were grown 12 h with or without 20 µg/ml doxycycline to repress Dpb11-YFP expression from the Tet-Off (tetO₂) promoter and stained with Hoechst before imaging. Error bars represent 95% confidence intervals. (E) Dpb11 suppresses the formation of Hoechst-positive Rfa1 anaphase bridges. Wild-type (SMG216-10A) cells coexpressing Dpb11-YFP under the control of the Tet-off promoter and Rfa1-CFP were grown in the absence or presence of 20 µg/ml doxycycline (Dox) for 12 h to repress expression of Dpb11-YFP and stained with Hoechst before imaging.