E3 ubiquitin-protein ligase that functions as regulator of MED25 stability by targeting MED25 for degradation in a RING-H2-dependent way. Proteasome-dependent degradation of MED25 seems to activate its function as positive regulator of FLOWERING LOCUS T (FT) and is important to induce the expression of FT and consequently to promote flowering. 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. Interacts with MED25 and UBC11. A number of isoforms are produced. According to EST sequences. The RING-type zinc finger domain mediates binding to an E2 ubiquitin-conjugating enzyme. No visible phenotype under normal growth conditions, but the double mutant plants mbr1-1 and mbr2-1 show delayed flowering. Belongs to the RING-type zinc finger family.