(Gallus gallus) causing an acute immunosuppressive disease that imposes severe loses 48 Supernatants were collected and subjected to metal-affinity chromatography (IMAC) 156 purification by using a HisTrap HP Ni 2+ affinity column (GE Healthcare, Spain). Resin-9 RNA duplex preparation was performed by annealing complementary synthetic 167 single stranded RNA oligonucleotide pairs where one of the oligonucleotides from each 168 pair was biotinylated at its 5'-end. Both oligonucleotide synthesis and annealing was 169 performed by the manufacturer (Biomers.net, Germany). Oligonucleotide sequences 170 are described in Supplemental Table 2 . 171 Surface Plasmon Resonance Analysis 172 SPR experiments were performed using a biosensor Biacore 3000 (Biacore, GE 173 Healthcare). dsRNA ligands bearing a biotin at the 5`end of one the strands were 174 immobilized on the surface of streptavidin-coated sensor chips (SA). RNA duplexes were 175 loaded at a flow rate of 10 µl/min, using HBS-P (10 mM HEPES [pH 7.4], 0.2 M NaCl, 3 176 mM EDTA, 0.005% Surfactant P20) running buffer. Reference surfaces, used as control 177 for these assays, were blank flow cells. 178 Binding assays, performed in duplicate within each experiment, were carried out 179 at 25°C at a flow rate of 30 µl/min. Proteins under study were serially diluted in running 180 buffer to reach the indicated concentrations. The protein fraction that remained bound 181 after the dissociation phase was removed by injecting 800 mM NaCl. Data were collected 182 for the association and dissociation periods as indicated in each specific experiment. SPR 183 sensograms recorded with each tested protein concentration were overlaid, aligned and 184 analysed using the BIAevaluation Software 4.1 (GE Healthcare). All data sets were 185 processed using a double-referencing method (30). Collected data were fit to a 1:1 186 Langmuir model with a correction for mass transport (31). 187 IBDV reverse genetics analysis 188 The reverse genetics analysis was performed as previously described (32) using 189 an approach based on the co-transfection of QM7 cells with two plasmids containing 190 cDNAs corresponding to the positive strand sequence of the IBDV segments A and B, 191 respectively. Plasmid pT7-SA-Rz harboring the cDNA of segment A was subjected to site 192 directed mutagenesis to introduce mutations VP3K99D or VP3K106D within the context 193 of the IBDV polyprotein open reading frame. Mutagenesis was performed as described 194 above using primer pairs K99DFW and K99DREV or K106DFW and K106DREV 195 (Supplemental Table 1 ). The resulting plasmids, pT7-SAVP3K99D-Rz and pT7- In silico VP3-dsRNA interaction model 248 As described above, our attempts to solve the crystal structure of the VP3-dsRNA 249 complex have not yet been successful. However, the previously described atomic and dissociation phases are fast and highly dependent on protein concentration. 308 Recoded information was used to calculate kinetics data using a simple 1:1 Langmuir 309 model including a term for mass transport deficiency. Noteworthy, as shown in Suppl. 