The Arg97fiGly and Asp136fiHis mutations stabilized So-RNase HI from the psychrotrophic bacterium Shewanella oneidensis MR-1 by and C, respectively, inTm, and and kJÆmol )1 , respectively, in DG(H2O). These mutations also stabilized the So-RNase HI derivative (4·-RNase HI) with quadruple thermostabilizing mutations in an additive manner.