The results presented thus far clearly demonstrate that rNSs exhibits poly(A) stimulated NTPase/dATPase activities. As mentioned earlier, rNSs has sequence motifs (Walker A and Walker B) that are typical of NTP binding proteins (Fig. 1) . It was of interest to examine the role of these motifs in the NTP/dATPase function of rNSs. It was shown earlier that the lysine residue of Walker motif A is crucial for NTP binding whereas aspartate and glutamate of Walker motif B coordinate the Mg 2+ ion [21] . Therefore, K189 and D159 of the Walker A (GxxxxGKT) and B (DExx) motifs respectively in rNSs were mutated to alanine by site directed mutagenesis as described in the methods section. The K189A and D159A rNSs mutants were over-expressed and purified by Ni-NTA chromatography as described for the wild type rNSs. The purified mutant proteins were analysed on a 12% SDS-PAGE and the proteins were nearly homogeneous. The ATPase activities of the mutants were tested using 0.4, 0.8 and 1.2 mg of the protein along with the wild type rNSs as the control. As shown in Fig. 5A , the K189A rNSs mutant was completely inactive whereas the D159A mutant was partially active. Kinetic analysis of D159A mutant NSs (Fig. 5B ) revealed that the k cat was reduced by 28 fold (table 2) compared to wild type rNSs.