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 (By similarity). The Y chromosome form could be involved in the survival and proliferation of differentiating spermatogonia. 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 testis in A spermatogonia and spermatids but not (or at very low levels) in pachytene spermatocytes. Also expressed in Y-bearing ovaries and at very low levels in adrenal gland. In testis, expression detected at 12.5 days post coitum (dpc) and peaking at 14.5 dpc, with expression dropping to low levels at the day of birth. After birth, levels increase 4-fold to a peak at 10 days post partum (dpp) and fall again thereafter. There are two active sites within the E1 molecule, allowing it to accommodate two ubiquitin moieties at a time, with a new ubiquitin forming an adenylate intermediate as the previous one is transferred to the thiol site. Belongs to the ubiquitin-activating E1 family.