Putative E3 ubiquitin-protein ligase that mediates monoubiquitination of 'Lys-119' of histone H2A (H2AK119ub), thereby playing a central role in histone code and gene regulation. As part of the PRC1-like complex, repress class I KNOX gene expression. PcG PRC1 complex maintains the transcriptionally repressive state of many genes, including Hox genes, throughout development. PcG PRC1 complex acts via chromatin remodeling and modification of histones, rendering chromatin heritably changed in its expressibility. 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. Homodimer or heterodimer with RING1B. Interacts with CLF. Component of the PRC1-like complex, at least composed of RING1A, RING1B and LHP1. A number of isoforms are produced. According to EST sequences. The RING-type zinc finger domain mediates binding to an E2 ubiquitin-conjugating enzyme. Normal phenotype. Drastic growth defects like ectopic meristem formation and sterility when associated with the disruption of RING1B.