E3 ubiquitin-protein ligase that mediates monoubiquitination of 'Lys-117' of histone H2B. H2B 'Lys-117' ubiquitination gives a specific tag for epigenetic transcriptional activation and is also prerequisite for histone H3 'Lys-4' and 'Lys-79' methylation. It thereby plays a central role in histone code and gene regulation. Required for the expression of Notch target genes in development by affecting the levels of Su(H) in imaginal disk cells and stimulating the Su(H)-mediated transcription of Notch-specific genes. 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. Belongs to the BRE1 family.