ADARs (adenosine deaminases that act on double-stranded RNA) are RNA editing enzymes that catalyze a change from adenosine to inosine, which is then recognized as guanosine by translational machinery. We demonstrate here that overexpression of ADARs but not of an ADAR mutant lacking editing activity could upregulate human immunodeficiency virus type 1 (HIV-1) structural protein expression and viral production. Knockdown of ADAR1 by RNA silencing inhibited HIV-1 production. Viral RNA harvested from transfected ADAR1-knocked-down cells showed a decrease in the level of unspliced RNA transcripts. Overexpression of ADAR1 induced editing at a specific site in the env gene, and a mutant with the edited sequence was expressed more efficiently than the wild-type viral genome. These data suggested the role of ADAR in modulation of HIV-1 replication. Our data demonstrate a novel mechanism in which HIV-1 employs host RNA modification machinery for posttranscriptional regulation of viral protein expression.Human immunodeficiency virus (HIV) uses many host cellular machineries for its replication. Posttranscriptional modifications of the HIV type 1 (HIV-1) RNA, i.e., alternative splicing, capping, and poly(A) synthesis of the viral pre-mRNA, have been well described (15). However, the process of RNA editing, another subtle type of pre-mRNA modification, is still not well defined, and its role in HIV-1 replication is in doubt.RNA editing was found to be an important process in regulating gene expression in many eukaryotes and viruses. In hepatitis D virus, the process of RNA editing has been well studied (reviewed in reference 3). A-to-I editing converts a stop codon into a Trp codon, providing envelope protein for virion assembly (4, 25). In the hepatitis C replicon, ADAR1 was shown to abolish replication through editing (33). Hypermutation by RNA editing was found in measles virus mRNA. It is speculated that this process leads to persistence of the virus (5). The editing has also been found in other viruses, such as parainfluenza virus 3 (9), mumps virus (22), and Ebola virus (28).Evidence of RNA editing in HIV-1 has also been demonstrated. Edited adenosine in the (transactivating response region) (TAR) when the virus is injected into Xenopus oocytes has been reported (31), though the functional consequences of this conversion are not known. It is hypothesized that the phenomenon caused a change in the secondary structure of TAR, which may affect transcription and translation of the viral RNA. Base modification of A to G and C to U has also been shown in the HIV-1 transcript (1). The modification was proposed to play a role in modulation of viral gene expression.There are many data suggesting that RNA editing might be involved in HIV-1 gene expression. ADARs are the RNA editing enzymes of interested that may be involved in modulation of HIV-1 expression. It was demonstrated that mouse ADAR1 but not ADAR2 is upregulated in lymphocytes in response to inflammation (34). In this study, we demonstrated that ADARs could enhance H...