Expression, purification, and characterization of a recombinant ribonuclease H from Thermus thermophilus HB8

Thermus thermophilus ribonuclease H was overexpressed and purified from Escherichia coli. The determination of the complete amino acid sequence allowed modification of that predicted from the DNA sequence, and the enzyme was shown to be composed of 166 amino acid residues with a molecular weight of 18,279. The isoelectric point of the enzyme was 10.5, and the specific absorption coefficient A₂₈₀/(0.1%) was 1.69. The enzymatic and physicochemical properties as well as the thermal and conformational stabilities of the enzyme were compared with those of E. coli RNase HI, which shows 52% amino acid sequence identity. Comparison of the far and near UV circular dichroism spectra suggests that the two enzymes are similar in the main chain folding but different in the spatial environments of tyrosine and tryptophan residues. The enzymatic activities of T. thermophilus RNase H at 37 and 70 °C for the hydrolysis of either an M13 DNA/RNA hybrid or a nonanucleotide duplex were approximately 5-fold lower and 3-fold higher, respectively, as compared with E. coli RNase HI at 37 °C. The melting temperature, T(m), of T. thermophilus RNase H was 82.1 °C in the presence of 1.2 M guanidine hydrochloride, which was 33.9 °C higher than that observed for E. coli RNase HI. The free energy changes of unfolding in the absence of denaturant, ΔG[H₂O], of T. thermophilus RNase H increased by 11.79 kcal/mol at 25 °C and 14.07 kcal/mol at 50 °C, as compared with E. coli RNase HI.