Activates ubiquitin by first adenylating its C-terminal glycine residue with ATP, and thereafter linking this residue to the side chain of a cysteine residue in E1, yielding a ubiquitin-E1 thioester and free AMP. ATP + ubiquitin + [E1 ubiquitin-activating enzyme]-L-cysteine = AMP + diphosphate + S-ubiquitinyl-[E1 ubiquitin-activating enzyme]-L-cysteine. Protein modification; protein ubiquitination. Monomer. Expressed in leaves, flowers, roots and stems. Detected in germinating seeds, cotyledons, hypocotyls, vascular tissues, anthers, filaments, pollen, style, stigma, sepals, petals, ovary, developing ovules, funiculi and silique walls. Expressed over the entire range of development. No visible phenotype and no changes in disease susceptibility. Both UBA1 and UBA2 are able to activate ubiquitin and transfer it to the E2s with equal efficiency. Belongs to the ubiquitin-activating E1 family.