The rNSs with its N-terminal histidine tag was purified by the Ni-NTA chromatography. C43 E.coli cells harboring the pRSETC-NSs clone were cultured in LB medium (2L) containing ampicillin (50 mg/ml) at 37uC. The expression of rNSs was induced, when the O.D. at 600 nm reached 0.6, by the addition of IPTG (0.3 mM) and the cells were cultured further for 4 hours at 37uC. The cells were harvested, re-suspended in buffer A and sonicated till the suspension became optically clear. The cell lysate was then spun down at 10000 rpm for 10 minutes. To the supernatant, Ni-NTA resin (500 ml/500 ml of culture) was added and left for 2 hours in an end-to-end rotor for binding. The beads were allowed to settle and the unbound supernatant discarded. The beads were washed once with buffer A and then twice with buffer A containing 20 mM imidazole. The rNSs was eluted with 5 ml buffer A containing 250 mM imidazole pH 8.0. The eluted fractions were analyzed by SDS-PAGE [19] . The fractions containing rNSs were pooled and the protein was dialyzed against 20 mM Tris-HCl pH 8.0 buffer containing 100 mM NaCl. The protein obtained by this method was reasonably pure and was suitable for further characterization. The molecular mass of the purified rNSs was determined by matrix-assisted laser desorption/ ionization (MALDI) mass spectrometer.