Proteins from thermophilic organisms are able to function under conditions that render a typical mesophilic protein inactive. Pairwise comparisons of homologous mesophilic and thermophilic proteins can help to identify the energetic features of a protein's energy landscape that lead to such thermostability. Previous studies of bacterial ribonucleases H (RNases H) from the thermophile Thermus thermophilus and the mesophile Escherichia coli revealed that the thermostability arises in part from an unusually low change in heat capacity upon unfolding (DeltaC(p)) for the thermophilic protein [Hollien, J., and Marqusee, S. (1999) Biochemistry 38, 3831-3836]. Here, we have further examined how nearly identical proteins can adapt to different thermal constraints by adding a moderately thermophilic homologue to the previously characterized mesophilic and thermophilic pair. We identified a putative RNase H from Chlorobium. tepidum and demonstrated that it is an active RNase H and adopts the RNase H fold. The moderately thermophilic protein has a melting temperature (T(m)) similar to that of the mesophilic homologue yet also has a surprisingly low DeltaC(p), like the thermophilic homologue. This new RNase H folds through a pathway similar to that of the previously studied RNases H. These results suggest that lowering the DeltaC(p) may be a general strategy for achieving thermophilicity for some protein families and implicate the folding core as the major contributor to this effect. It should now be possible to design RNases H that display the desired thermophilic or mesophilic properties, as defined by their DeltaC(p) values, and therefore fine-tune the energy landscape in a predictable fashion.