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Background  Double stranded RNA (dsRNA) is widely accepted as an RNA motif recognized as a danger signal by the cellular sentries. However, the biology of non-segmented negative strand RNA viruses, or  Mononegavirales , is hardly compatible with the production of such dsRNA.    Methodology and Principal Findings  During measles virus infection, the IFN-β gene transcription was found to be paralleled by the virus transcription, but not by the virus replication. Since the expression of every individual viral mRNA failed to activate the IFN-β gene, we postulated the involvement of the leader RNA, which is a small not capped and not polyadenylated RNA firstly transcribed by  Mononegavirales . The measles virus leader RNA, synthesized both  in vitro  and  in vivo , was efficient in inducing the IFN-β expression, provided that it was delivered into the cytosol as a 5′-trisphosphate ended RNA. The use of a human cell line expressing a debilitated RIG-I molecule, together with overexpression studies of wild type RIG-I, showed that the IFN-β induction by virus infection or by leader RNA required RIG-I to be functional. RIG-I binds to leader RNA independently from being 5-trisphosphate ended; while a point mutant, Q299A, predicted to establish contacts with the RNA, fails to bind to leader RNA. Since the 5′-triphosphate is required for optimal RIG-I activation but not for leader RNA binding, our data support that RIG-I is activated upon recognition of the 5′-triphosphate RNA end.    Conclusions/Significance  RIG-I is proposed to recognize  Mononegavirales  transcription, which occurs in the cytosol, while scanning cytosolic RNAs, and to trigger an IFN response when encountering a free 5′-triphosphate RNA resulting from a mislocated transcription activity, which is therefore considered as the hallmark of a foreign invader.

Cytosolic 5′-Triphosphate Ended Viral Leader Transcript of Measles Virus as Activator of the RIG I-Mediated Interferon Response