Catalyzes the covalent attachment of ubiquitin to other proteins. Plays a role in transcription regulation by catalyzing the monoubiquitination of histone H2B to form H2BK123ub1. H2BK123ub1 gives a specific tag for epigenetic transcriptional activation and is also a prerequisite for H3K4me and H3K79me formation. Also involved in postreplication repair of UV-damaged DNA, in N-end rule-dependent protein degradation and in sporulation