The Histone Chaperones ASF1 and CAF-1 Promote MMS22L-TONSL-Mediated Rad51 Loading onto ssDNA during Homologous Recombination in Human Cells

Abstract

The access-repair-restore model for the role ofchromatin in DNA repair infers that chromatin is a mere obstacle to DNA repair. However, herewe show that blocking chromatin assembly, via knockdown of the histone chaperones ASF1 or CAF-1 or amutation that prevents ASF1A binding to histones, hinders Rad51 loading onto ssDNA during homologous recombination. This is a consequence ofreduced recruitment of the Rad51 loader MMS22L-TONSL to ssDNA, resulting in persistent RPA foci, extensive DNA end resection, persistent activation of the ATR-Chk1 pathway, and cell cycle arrest. In agreement, histones occupy ssDNA during DNA repair in yeast.We also uncovered DNA-PKcs-dependent DNA damage-induced ASF1A phosphorylation, which enhances chromatin assembly, promoting MMS22L-TONSL recruitment and, hence, Rad51 loading. We propose that transient assembly of newly synthesized histones onto ssDNA serves to recruit MMS22L-TONSL to efficiently form the Rad51 nucleofilament for strand invasion, suggesting an active role of chromatin assembly in homologous recombination.