Host Enzymes Required for Hdv Rna Editing

ADAR edits adenosines in double-stranded RNA (dsRNA); this activity is present in nuclear extracts from numerous metazoan species (Bass). As mentioned above, it was shown that ADAR from Xenopus laevis can edit the amber/W site in the HDV antigenome with considerable specificity in vitro (Polson et al. 1996). While only one ADAR has been identified in Xenopus, mammalian cells contain two related genes, ADAR1 and ADAR2, that encode proteins capable of editing adenosine in dsRNA (Melcher et al. 1996; O'Con-nell et al. 1995; Patterson and Samuel 1995; Yang et al. 1997). These proteins contain a catalytic deaminase domain along with three or two, respectively, copies of dsRNA binding motifs (DRBMs). Both genes are essential for viability in mice (Brusa et al. 1995; Wang et al. 2000). Both ADAR1 and ADAR2 can edit HDV RNA at the amber/W site in transfected cultured cells (Jayan and Casey 2002a; Sato et al. 2001; Wong et al. 2001). However, because the level of ADAR1 mRNA expression is considerably higher than ADAR2 in liver, it seems likely that ADAR1 is responsible for editing during HDV infection of hepatocytes. Consistent with this idea, knockdown experiments using small interfering RNA (siRNA) have shown that the short form of ADAR1, which is localized in the nucleus, is responsible for amber/W site editing during HDV replication in Huh-7 cells (Jayan and Casey 2002b; Wong and Lazinski 2002). Because the HDV amber/W site was edited with high specificity in vitro using just purified Xenopus ADAR, no additional factors aside from HDV RNA and ADAR are required for amber/W site editing to occur (Polson et al. 1996). Nevertheless, it is possible that additional factors, such as HDAg, can contribute to the efficiency and specificity of editing (see Sect. 3).

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