Oligoadenylate synthetase 1 (OAS1) produces the second messenger 5’-triphosphorylated 2'-5'-oligoadenylate (2-5A), which limits viral propagation through the activation of the enzyme RNase L (reviewed by Silverman RH 2007; Hornung V et al. 2014). OAS1 produces 2-5A from ATP by transferring an AMP unit from the AMP donor substrate (ATP) to the 2′-hydroxyl group of an AMP acceptor substrate, ATP or a preformed 2-5A oligomer (Lohofener J et al. 2015). This produces a 2-5A dimer (ppp5'A(2'-5')A) or an elongated 2-5A oligomer (ppp5'A((2'-5')A)n), as well as one molecule of pyrophosphate (PPi) for each AMP residue added (Lohofener J et al. 2015). Structural studies showed that RNA-induced conformational rearrangement in OAS1 positions the active site residues D75, D77, and D148 compactly for coordination of two Mg2+ ions and for binding of ATP (Donovan J et al. 2013). The assembly of this critical active-site structure of OAS1 provides the gate that couples binding of dsRNA to the production and downstream functions of 2-5A, enabling OAS1 to function as a sensor of double-stranded RNA (dsRNA) (Donovan J et al. 2013).