Mutational analysis of the SDD sequence motif of a PRRSV RNA-dependent RNA polymerase

Zhou, Yan; Zheng, HaiHong; Gao, Fei; Tian, DeBin; Yuan, ShiShan

It is said that perhaps the GDD motif plays a central role in RNA synthesis by binding metal ions [9, 20, 32] , which may be determined by the characteristics of amino acids (Table 1) . To investigate the effect of different charge and polarity of amino acids substituted in the SDD motif on the activity of RdRp, we introduced various amino acid substitutions (Table 1) . First, the aspartate residue was substituted by the simplest amino acid, i.e., alanine, which has no polarity; however, it was lethal to RdRp. Methionine also has no polarity, but mutant pMDD did not produce virus either. The serine residue has no charge; therefore, we substituted the serine residue with glycine residue, which also has no charge. Perhaps the substitution of Ser by Gly produced a functional mutant because of the less polar no charge similarities between Ser and Gly. Aspartate is negatively charged; therefore, we substituted the aspartate residues with glutamate residues, which are also negative charge. However, neither mutant D3051E nor D3052E produced virus, which was similar to a previous study [1] . Replacement of the aspartate residues with uncharged asparagines residues (D3051N or D3052N) also did not produce virus. It can be concluded that the strict requirement of DD residues meant that no substitutions could be tolerated, probably because the DD residues are required for metal ion binding [9, 33] .