In September 2010, autochthonous transmission of chikungunya virus was recorded in southeastern France, where the Aedes albopictus mosquito vector is present. Sequence analysis of the viral genomes of imported and autochthonous isolates indicated new features for the potential emergence and spread of the virus in Europe.

BackgroundDouble 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 FindingsDuring 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/SignificanceRIG-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.

We propose a reference model of the kinetics of a viral RNA-dependent RNA polymerase (vRdRp) activities and its regulation during infection of eucaryotic cells. After measles virus infects a cell, mRNAs from all genes immediately start to accumulate linearly over the first 5 to 6 h and then exponentially until ϳ24 h. The change from a linear to an exponential accumulation correlates with de novo synthesis of vRdRp from the incoming template. Expression of the virus nucleoprotein (N) prior to infection shifts the balance in favor of replication. Conversely, inhibition of protein synthesis by cycloheximide favors the latter. The in vivo elongation speed of the viral polymerase is ϳ3 nucleotides/s. A similar profile with fivefold-slower kinetics can be obtained using a recombinant virus expressing a structurally altered polymerase. Finally, virions contain only encapsidated genomic, antigenomic, and 5-end abortive replication fragment RNAs.Viruses are obligate intracellular parasites. To understand the complexity of virus replication and to develop potent antiviral drugs, there is a growing need for a systematic analysis of the underlying molecular mechanisms. Although the kinetics of protein expression has been established for many viruses and a model of the dynamics of virus transcription and replication for bacteriophage Q has been proposed (see reference 10 for a review), knowledge in this area is very limited for all eucaryotic viruses. We report the first kinetics model of the transcription and replication of a negative-sense, singlestranded RNA virus.Measles virus (MV) belongs to the order Mononegavirales (33). The replication strategy involves a viral RNA-dependent RNA polymerase (vRdRp), which uses as a template a nucleocapsid (NC) made of a single strand of RNA in tight complex with the nucleoprotein (N). The negative-strand genome contains six transcription units encoding the N, phospho (P), matrix (M), fusion (F), hemagglutinin (H), and large (L) or polymerase protein, in that order (Fig. 1). The transcription units are flanked by short leader (Le) and trailer (Tr) sequences containing the genomic promoter (on the minus strand) and the antigenomic promoter (on the plus strand), respectively. The P gene also encodes the nonstructural V and C proteins by RNA editing and alternative overlapping open reading frame, respectively.Every N protein binds to 6 nucleotides so that the N polymer entirely covers the 15,894-nucleotide genome (5) and makes it resistant to nucleases (33). NC comprises 2,649 N, ϳ300 P, and ϳ20 to 50 L proteins (33) and is enclosed within a membrane envelope containing H and F glycoproteins. After fusion with the plasma membrane at neutral pH, intact NCs are released into the cytoplasm (24), where they serve as a template for both primary transcription from and replication of the genomic RNA (Fig. 1). Transcription is initiated from a single promoter at the 3Ј end of the genome (for a review, see reference 23) by vRdRp made of L and P proteins. Immediately prior to an intergenic junction...