This is the largely alpha-helical, C-terminal half of 2'-5'-oligoadenylate synthetase 1, being described as domain 2 of the enzyme and homologous to a tandem ubiquitin repeat. It carries the region of enzymic activity between 320 and 344 at the extreme C-terminal end. Oligoadenylate synthetases are antiviral enzymes that counteract vial attack by degrading viral RNA. The enzyme uses ATP in 2'-specific nucleotidyl transfer reactions to synthesize 2'.5'-oligoadenylates, which activate latent ribonuclease, resulting in degradation of viral RNA and inhibition of virus replication. This domain is often associated with NTP_transf_2 pfam01909.