ADAR1, an interferon inducible adenosine deaminase acting on RNA, catalyzes the C6 deamination of adenosine (A) to produce inosine (I) in RNA substrates with double-stranded character, leading to genetic recoding and altered RNA structure.  Adar1 expression occurs from multiple promoters and encodes either an IFN-inducible p150 cytoplasmic protein or a constitutively expressed N-terminally truncated p110 nuclear protein.  The roles of ADAR1 p150 and p110 in uninfected cells and during virus infection were examined using human cell clones made stably deficient in ADAR1, mouse cells genetically deficient in ADAR1, and by complementation strategies.  The effects of ADAR1 deficiency on virus growth, IFNb induction, and stress responses including formation of stress granules (SG) and cell death were opposite to those established for dsRNA-dependent PKR kinase deficiency.  Exemplified by measles virus (MV) infection, ADAR1 suppressed activation of PKR and IRF3 and protected cells from SG formation and cytopathic damage. MV-induced SG formation was PKR-dependent and impaired by catalytically active p150 but not p110 ADAR1.  C^KO mutant MV was an effective inducer of IFNb by IPS-1-dependent signaling that was amplified by PKR. RNA expression profiles were characterized by qPCR and RNA deep sequencing, with differences found between C^KO mutant and parental MV, and untreated and IFN-treated cells.  Significant amounts of dsRNA accumulate during C^KO mutant infection.  Results obtained with mouse cells genetically deficient in either adar1 or the related deaminase adar2, or stat1 or stat2, suggest a central role of ADAR1 in overall A-to-I editing and p150 in the IFN-induced editing. In summary, ADAR1 behaved as an anti-apoptotic host factor that, in some instances, suppressed innate immune activities and was pro-viral.  Our findings furthermore implicate a balanced interplay between PKR and ADAR1 in modulating IFNb protein production and stress responses following virus infection. (Supported by NIAID, NIH).