Probable ubiquitin-protein ligase involved in the degradation-related ubiquitination of histones. Contributes to the post-translational regulation of histone protein levels by polyubiquitination of excess histones for subsequent degradation. [E2 ubiquitin-conjugating enzyme]-S-ubiquitinyl-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-N(6)-ubiquitinyl-L-lysine. Protein modification; protein ubiquitination. Localizes to DNA double-strand breaks (DSBs). Members of the RBR family are atypical E3 ligases. They interact with the E2 conjugating enzyme UBE2L3 and function like HECT-type E3 enzymes: they bind E2s via the first RING domain, but require an obligate trans-thiolation step during the ubiquitin transfer, requiring a conserved cysteine residue in the second RING domain. Belongs to the RBR family.