Why do the most structurally well-characterized bacterial RNA polymerases belong to thermophiles? From Lewin's Genes (11th edition, page 515): > The only bacterial RNA polymerases for which high-resolution crystal structures have been solved, however, are from two thermophilic bacterial species, _Thermus aquaticus_ and _Thermus thermophilus_. Is this just a matter of economy? I mean, as X-ray crystallography is exceedingly expensive and complex, did scientists consider enough to resolve the RNA polymerase structure of one species and deduce those belonging to the rest by homology-based folding prediction? Or is it because thermophilic proteins are inherently more stable and thus easier to crystalize?

I think both of your suggestions have some merit: (1) it appears that _Thermus_ RNAPs are more stable and easier to crystallize (2) RNAP is highly conserved and so it is not critical to crystallize it from every bacterial species.