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?