Regulatory subunit of the dimeric ECR1-AXR1 E1 enzyme. E1 activates RUB1/NEDD8 by first adenylating its C-terminal glycine residue with ATP, thereafter linking this residue to the side chain of the catalytic cysteine, yielding a RUB1-ECR1 thioester and free AMP. E1 finally transfers RUB1 to the catalytic cysteine of RCE1. Plays an important role in auxin response (PubMed:8321287). Regulates the chromosomal localization of meiotic recombination by crossovers (COs) and subsequent synapsis, probably through the activation of a CRL4 complex (PubMed:25116939). Required for E3-mediated protein degradation in response to auxin, jasmonic acid and cold stress. Required for the COP1-COP10-CSN-mediated repression of photomorphogenesis in the dark (PubMed:12368504). May function redundantly with AXL1 in the RUB conjugating pathway (PubMed:17655650). Seems not to be functionally equivalent to AXL1 in vivo (PubMed:21311953). Protein modification; protein neddylation. Heterodimer of ECR1 and AXR1. The complex binds to RUB1/NEDD8 and RCE1. A number of isoforms are produced. According to EST sequences. Expressed in shoot, root and floral meristems, in vascular tissues of cotyledons and mature leaves, and in the stele of the root. Expressed at higher levels on the lower side of an emerging root during germination and at higher levels on the underside of the apical hook. Expressed during ovules and embryo development and very early during the formation of lateral roots. Impaired meiotic recombination characterized by a shortage in bivalent formation due to a mislocalization of class I crossovers (COs) and correlated with strong synapsis defects. Dwarf plants, highly branched, with small crinkled leaves, small flowers and short fruits, symptoms of fertility defects. Belongs to the ubiquitin-activating E1 family. ULA1 subfamily.