One protein, called 2'5'Oligo A synthetase, is an enzyme that converts ATP into a unique polymer (2'5' Oligo A) containing 2'- 5'phophodiester bonds. Double stranded RNA is required for the activity of this enzyme. The 2'5'Oligo A in turn activates RNAse L which then breaks down viral mRNA. The second protein is an protein kinase that, in the presence of double stranded RNA, is autophosphorylated and thereby activated. The activated protein kinase in turn phosphorylates elongation factor eIF-2 and inactivates it. By the action of these two IFN-induced enzymes protein synthesis is inhibited. Although the infected cell may die as a consequence of the inhibition of host protein synthesis, the progress of the infection is stopped. Uninfected cells are not killed by IFN treatment since activation of the two enzymes requires double stranded RNA, which is not produced. Some viruses have means of inhibiting the antiviral effects of IFN. For example the adenoviruses produce an RNA which prevents the activation of the protein kinase by double stranded RNA thereby reducing the antiviral effects of IFN
