Mismatch DNA recognition protein from an extremely thermophilic bacterium, Thermus thermophilus HB8

Abstract

The mutS gene, implicated in DNA mismatch repair, was cloned from an extremely thermophilic bacterium, Thermus thermophilus HB8. Its nucleotide sequence encoded a 819-amino acid protein with a molecular mass of 91.4 kDa. Its predicted amino acid sequence showed 56 and 39% homology with Escherichia coli MutS and human hMsh2 proteins, respectively. The T.thermophilus mutS gene complemented the hypermutability of the E.coli mutS mutant, suggesting that T.thermophilus MutS protein was active in E.coli and could interact with E.coli MutL and/or MutH proteins. The T.thermophilus mutS gene product was overproduced in E.coli and then purified to homogeneity. Its molecular mass was estimated to be 91 kDa by SDS-PAGE but ~ 330 kDa by size-exclusion chromatography, suggesting that T.thermophilus MutS protein was a tetramer in its native state. Circular dichroic measurements indicated that this protein had an α-helical content of ~ 50%, and that it was stable between pH 1.5 and 12 at 25°C and was stable up to 80°C at neutral pH. Thermus thermophilus MutS protein hydrolyzed ATP to ADP and Pi, and its activity was maximal at 80°C. The kinetic parameters of the ATPase activity at 65°C were K(m) = 130 μM and k(cat) = 0.11 s-1. Thermus thermophilus MutS protein bound specifically with G-T mismatched DNA even at 60°C.