Thermophilic DNA ligase. Purification and properties of the enzyme from Thermus thermophilus HB8.

Thermophilic and thermostable DNA ligase was purified to near homogeneity from the extract of Thermus thermophilus HB8. The purified enzyme has an isoelectric point at pH 6.6 and consists of a single polypeptide of about 79,000 in molecular weight on the bases of sodium dodecyl sulfate-polyacrylamide gel electrophoresis data and an equilibrium sedimentation method. The enzyme requires divalent cations, Mg2+ or Mn2+, and the optimum concentration of these ions being 5-9 X 10(-3) M and 3-6 X 10(-3) M, respectively. The enzyme also requires NAD as a cofactor. The apparent Km for NAD is 1.85 X 10(-8) M and that of (dT)10 is 1.4 X 10(-4) M. The pH optimum is 7.4-7.6 in Tris-HCl and 8.0 in collidine/HCl buffer. The joining reaction is activated by K+ and NH+4 at a concentration of 2-100 mM and inhibited by Na+ above 25 mM. The optimum temperatures of the joining of thymidylate oligomers in the presence of poly(dA) as a template are 27.5 degrees C for p(dT)s, 34.5 degrees C for p(dT)10, and 37 degrees C for p(dT)12-18 and that of cohesive-end DNA restriction fragments is 24-37 degrees C. The nick-closing activity of the enzyme was observed over a wide range of the temperature from 15 to 85 degrees C and the optimum temperature is 65-72 degrees C. The temperature dependency of ligation with HB8 DNA ligase for various substrates was found to shift to a region of 7-10 degrees C higher than that of T4 DNA ligase and the activity of HB8 DNA ligase decreased remarkably below 4 degrees C. The enzyme was stable for 1 week at 37 degrees C, its activity dropped by 50% within 2 days at 65 degrees C.