Has no ubiquitin ligase activity on its own. The heterodimer with UBC catalyzes the synthesis of non-canonical poly-ubiquitin chains that are linked through 'Lys-63'. This type of poly-ubiquitination does not lead to protein degradation by the proteasome. Mediates transcriptional activation of target genes. May play a role in the control of progress through the cell cycle and differentiation. Involved in the error-free DNA repair pathway and contributes to the survival of cells after DNA damage. Heterodimer with UBC35 or UBC36. Expressed in roots, shoots, leaves, stems, flowers and pollen. Detected in seedlings 6 hours and 2 days post-germination. Not induced by stresses. Plants do not display apparent morphological variations, but are sensitive to the DNA-damaging agent MMS. May be due to a competing donor splice site. May be due to a competing donor splice site. Belongs to the ubiquitin-conjugating enzyme family.