The pooled active fractions from th

The pooled active fractions from the phosphocellulose step were dialyzed twice against 1 liter of Buffer A at 25° C. and applied to an 250×10 mm SynChropak AX-300 anion exchange HPLC column pre-equilibrated in Buffer A (Rainin Corp., Emeryville, Calif.) at a flow rate of 2.4 ml/minute. Samples were in Buffer A. Bound proteins were eluted with a fifty ml linear gradient from 100 mM to 700 mM NaCl in Buffer A at 2.4 ml/minute. Bst DNA polymerase activity eluted at an ionic strength corresponding to a salt concentration of between about 0.2 and 0.4 M NaCl.

In some cases, the purified full-length Bst polymerase was further treated with a protease to generate an active truncated form of the enzyme. In such cases, purified Bst polymerase (0.33 mg/ml) was treated with subtilisin in Buffer A at a 1:200 (w/w) ratio of protease to Bst polymerase at 25° C. The reaction mixture was incubated at 25° C. for 40 minutes, and the reaction was terminated with the addition of PMSF to a final concentration of 1 mM. The active proteolytic fragment of Bst DNA polymerase was purified using a 60×10 mm column of hydroxyapatite (HA) (Bio Gel-HT, BioRad Laboratories, Richmond, Calif.) according to the method of Jacobsen et al., Eur. J. Biochem. 45:623 (1974) the disclosure of which is hereby incorporated by reference herein. The HA column was equilibrated in 20 mM sodium phosphate (pH 7.0), and Bst polymerase was eluted with a linear gradient from 20 to 350 mM sodium phosphate (pH 7.0) at a flow rate of 1 ml/minute. The active protein eluted at an ionic strength corresponding to about 0.3 M sodium phosphate. The active fractions were pooled.