E3 ubiquitin-protein ligase that mediates monoubiquitination of histone H2B to form H2BK123ub1 in association with the E2 enzyme RAD6/UBC2. H2BK123ub1 gives a specific tag for epigenetic transcriptional activation, elongation by RNA polymerase II, telomeric silencing, and is also a prerequisite for H3K4me and H3K79me formation. It thereby plays a central role in histone code and gene regulation. Also modulates the formation of double-strand breaks during meiosis. S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein ubiquitination. Forms a complex with the E2 enzyme RAD6/UBC2. May interact with LGE1. Present with 2980 molecules/cell in log phase SD medium. Belongs to the BRE1 family.