The E2 ubiquitin-conjugating enzymes, AtUBC1 and AtUBC2, play redundant roles and are involved in activation of FLC expression and repression of flowering in Arabidopsis thaliana.
Xu L, Ménard R, Berr A, Fuchs J, Cognat V, Meyer D, Shen WH. Plant J. 2009 Jan; 57(2):279-88. Epub 2008 Oct 14.