We found the 2',5'-oligoadenylate synthetase-like (OASL) gene to be significantly elevated by high virus loads In human liver infected with hepatitis C virus (HCV) Here, we determined whether OASL inhibited HCV replication using an in vitro system. We constructed three expression vectors of OASL to produce isoform a (OASLa), isoform b (OASLb). and the C-terminal ubiquitin-like domain of isoform a (Ub) When Huh7 JFH-1 HCV replicon cells were separately transfected with these three vectors, colony formation of HCV-replicating cells was inhibited by 95%, 94%, and 65%, respectively Both OASLa and OASLb were also inhibitory for cells as well as the virus because colony formation of OASL-producing cells was reduced to 41% and 8%, respectively. Stable Huh7 clones producing each of the three OASLs were established and assessed for their inhibition of HCV replication using luciferase reporter gene-containing JFH-1 I replicon RNA. HCV replication was inhibited by 50-90% in several stable OASL clones Association analysis in six Ub clones expressing different levels of Ub mRNA showed that the degree of Inhibition of HCV replication was significantly associated with the amount of Ub present In conclusion. OASL possesses two domains with HCV inhibitory activity. The N-terminal OAS-homology domain Without OAS activity is inhibitory for cell growth as well as HCV replication. whereas C-terminal Ub is inhibitory only for HCV replication. Therefore, OASLa, a major isoform of this molecule induced in human liver. may mediate anti-HCV activity through two different domains (C) 2010 Elsevier Inc All rights reserved