Rift Valley fever virus (RVFV) is an important cause of epizootics and epidemics in

Rift Valley fever virus (RVFV) nonstructural protein NSs acts as the major determinant of virulence by antagonizing interferon β (IFN-β) gene expression. We demonstrate here that NSs interacts with the host protein SAP30, which belongs to Sin3A/NCoR/HDACs repressor complexes and interacts with the transcription factor YY1 that regulates IFN-β gene expression. Using confocal microscopy and chromatin immunoprecipitation, we show that SAP30, YY1, and Sin3A-associated corepressor factors strongly colocalize with nuclear NSs filaments and that NSs, SAP30 and Sin3A-associated factors are recruited on the IFN-β promoter through YY1, inhibiting CBP recruitment, histone acetylation, and transcriptional activation. To ascertain the role of SAP30, we produced, by reverse genetics, a recombinant RVFV in which the interacting domain in NSs was deleted. The virus was unable to inhibit the IFN response and was avirulent for mice. We discuss here the strategy developed by the highly pathogenic RVFV to evade the host antiviral response, affecting nuclear organization and IFN-β promoter chromatin structure.

The Rift Valley fever virus (RVFV) is the causative agent of fatal hemorrhagic fever in humans and acute hepatitis in ruminants. We found that infection by RVFV leads to a rapid and drastic suppression of host cellular RNA synthesis that parallels a decrease of the TFIIH transcription factor cellular concentration. Using yeast two hybrid system, recombinant technology, and confocal microscopy, we further demonstrated that the nonstructural viral NSs protein interacts with the p44 component of TFIIH to form nuclear filamentous structures that also contain XPB subunit of TFIIH. By competing with XPD, the natural partner of p44 within TFIIH, and sequestering p44 and XPB subunits, NSs prevents the assembly of TFIIH subunits, thus destabilizing the normal host cell life. These observations shed light on the mechanism utilized by RVFV to evade the host response.

. This defect is correlated with a large in-frame deletion in the NSs coding region of the S segment of the tripartite genome. Here, we show that the truncated NSs protein of clone 13 is expressed and remains in the cytoplasm, where it is degraded rapidly by the proteasome. Through the analysis of reassortants between clone 13 and a virulent strain, we localized the marker(s) of attenuation in the S segment of this attenuated virus. This result raises questions regarding the role of NSs in pathogenesis and highlights, for the first time in the Bunyaviridae family, a major role of the S segment in virulence and attenuation, possibly associated with a defect in the nonstructural protein.Rift Valley fever (RVF) virus is an arthropod-borne virus which periodically causes epidemics and epizootics in sub-Saharan countries of Africa and in Egypt (for a review, see reference 17). The most recent outbreaks occurred in 1997 and 1998 in eastern (Kenya, Somalia, and Uganda) and western (Mauritania) Africa (references 1 and 21 and references therein). In humans, infection provokes a wide range of clinical symptoms from benign fever to encephalitis, retinitis, and fatal hepatitis associated with hemorrhages. Among young animals, lambs, calves, and kids are severely affected and die from acute hepatitis. In adults, the symptoms are less pronounced but teratogenic and abortogenic effects are frequent in pregnant animals. Mice, hamsters, and some strains of rats are laboratory animal models for the study of RVF pathogenesis since they are highly sensitive to virulent strains and develop hepatitis or encephalitis when inoculated by peripheral routes (17).Rift Valley fever virus belongs to the Phlebovirus genus of the Bunyaviridae family. Like all the members of the family, it possesses a single-stranded tripartite RNA genome composed of large (L), medium (M), and small (S) segments (for a review, see reference 23). The L and M segments are of negative polarity and code, respectively, for the L RNA-dependent RNA polymerase and for a polyprotein precursor cleaved to generate the envelope glycoproteins G1 and G2 and two nonstructural proteins, 14K and 78K. The S segment utilizes an ambisense strategy and codes for two proteins: the nucleoprotein N and the nonstructural protein NSs. These proteins are translated from two individual mRNAs of opposite polarities. The mRNA synthesizing the N protein is complementary to the genomic sense molecule, whereas the mRNA synthesizing the NSs protein is of genomic polarity. The role of the nonstructural proteins, for any member of the family, is still undetermined. Except in clone 13, the NSs proteins of all the RVF strains analyzed so far form filamentous structures in the nuclei of infected cells (16). The absence of nuclear NSsassociated filament in cells infected with clone 13 was correlated with a large internal deletion of the NSs open reading frame. This defect, which affects 70% of the coding sequence (i.e., 549 nucleotides) and conserves in frame the N and C termini of the protein, was ...

Rift Valley fever virus (RVFV, Bunyaviridae, Phlebovirus) is a mosquito-transmitted arbovirus that causes human and animal disease in sub-Saharan Africa and was introduced into the Arabian Peninsula in 2000. Here, we describe a method of reverse genetics to recover infectious RVFV from transfected plasmids based on the use of the cellular RNA polymerase I promoter to synthesize viral transcripts. We compared its efficiency with a system using T7 RNA polymerase and found that both are equally efficient for the rescue of RVFV generating titers of approx 107 to 108 pfu per ml. We used the RNA polymerase I-based system to rescue both attenuated MP12 and virulent ZH548 strains as well as chimeric MP12-ZH548 viruses, and in addition RVFV expressing reporter proteins.