FIG 6.

N protein promotes virus replication by inhibiting TRIM25-mediated IFN-β production. (A) A549 cells were transfected with TRIM25 siRNA or scramble siRNA. At 24 h posttransfection, the cells were infected with recombinant SARS-CoV at an MOI of 0.05 PFU per cell for 24 h. The virus particles released into the cell culture supernatants were determined by viral genomic RNA level as assayed by qPCR. The TRIM25 mRNA expression level was determined by RT-PCR (right). (B) A549 cells transfected with Flag-TRIM25 or Flag vector were infected with recombinant SARS-CoV as described for panel A. The virus particles released into the cell culture supernatants were quantified by viral genomic RNA level as assayed by qPCR. (C) A549 cells transfected with GFP vector, GFP-N, or GFP-N(1–361) were infected with SARS-CoV as described for panel A. The virus particles released into the cell culture supernatants were quantified by viral genomic RNA level as assayed by qPCR. (D) Calu-3 cells were transfected with full-length N protein or N(1–361), and 24 h after transfection, the cells were infected with mouse-adapted SARS-CoV at an MOI of 0.05 PFU per cell in the presence or absence of IFN-β-specific neutralizing antibody. Viral RNA from viral particles in the supernatant was quantified by qPCR. The results are expressed as the means ± SD from three independent experiments (ns, nonsignificant; *, P < 0.05; **, P < 0.01; ***, P < 0.001). The significance of results for cells not treated with the antibody from panel D was analyzed using the t test.