The substrates for the phosphatase reaction were 59 end labeled using [c-32 P] ATP and T4 polynucleotide kinase according to manufacturer's protocol. Free nucleotides were removed from the end labelled substrate using Sephadex G25 spun column. rNSs (1.5 mg) was incubated at 25uC for 30 min with 59 labeled ssDNA (K189A sense primer, 0.5 nM), ssRNA {poly(A), 4 nM}, dsDNA (1 nM) and dsRNA (0.2-1 nM) in 50 mM MOPS buffer pH 7.0 containing 2 mM magnesium chloride, 2 mM dithiothreitol, 0.5 mg of bovine serum albumin per ml. The reaction was stopped by addition of 100 mM EDTA (1 ml) in a total volume of 10 ml and release of Pi was measured as described above.