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. Required for postreplication repair of UV-damaged DNA and for adapting root developmental programs to suboptimal availability of iron. S-ubiquitinyl-[E1 ubiquitin-activating enzyme]-L-cysteine + [E2 ubiquitin-conjugating enzyme]-L-cysteine = [E1 ubiquitin-activating enzyme]-L-cysteine + S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine. Protein modification; protein ubiquitination. Interacts with yeast and human Mms2, with the RING domain of RGLG2 and with UEV1A, UEV1B, UEV1C and UEV1D. Ubiquitously expressed at low level. Mainly expressed in the vasculature. Not induced by salt, abscisic acid, mannitol, H(2)O(2), low temperature, MMS or iron. No visible phenotype under normal growth conditions or in phosphate-deficient plants. Unable to form branched root hairs in response to iron-deficient conditions. Partly functionally redundant with UBC36. May be due to a competing donor splice site. May be due to an intron retention. Belongs to the ubiquitin-conjugating enzyme family.