A MUTANT-CELL LINE DEFECTIVE IN RESPONSE TO DOUBLE-STRANDED-RNA AND IN REGULATING BASAL EXPRESSION OF INTERFERON-STIMULATED GENES

Although much progress has been made in identifying the signaling path ways that mediate the initial responses to interferons (IFNs), much le ss is known about how IFN-stimulated genes (ISGs) are kept quiescent i n untreated cells, how the response is sustained after the initial ind uction, and how ISG expression is down-regulated, even in the continue d presence of IFN, We have used the cell sorter to isolate mutant cell s with constitutively high ISG expression. A recessive mutant, P2.1, h as higher constitutive ISG levels than the parental U4C cells, which d o not respond to any IFN. Unexpectedly, P2.1 cells also are deficient in the expression of ISGs in response to double-stranded RNA (dsRNA), Electrophoretic mobility-shift assays revealed that the defect is upst ream of the activation of the transcription factors NF kappa B and IFN regulatory factor 1, Analysis of the pivotal dsRNA-dependent serine/t hreonine kinase PKR revealed that the wild-type kinase is present and is activated normally in response to dsRNA in P2.1 cells, Together, th ese data suggest that the defect in P2.1 cells is either downstream of PKR or in a component of a distinct pathway that is involved both in activating multiple transcription factors in response to dsRNA and in regulating the basal expression of ISGs.