Structural study of mutants of Escherichia coli ribonuclease HI with enhanced thermostability.

Systematic replacement of the amino acid residues in Escherichia coli ribonuclease HI with those in the thermophilic counterpart has revealed that two mutations, His62→Pro (H62P) and Lys95→Gly (K95G), increased the thermostability of the protein. These single-site mutant proteins, together with the mutant proteins His62→Ala (H62A), Lys95→Asn (K95N) and Lys95→Ala (K95A), were crystallized and their structures were determined at 1.8 A resolution. The crystal structures of these mutant proteins reveal that only the local structure around each mutation site is essential for the increase in thermostability