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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?

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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.

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    $\begingroup$ It might be worth adding that knowledge about thermostable enzymes is generally economically interesting since it is often easier to use them in various experimental settings, especially after optimising/engineering them - which becomes much eaiser with structural information. $\endgroup$
    – Nicolai
    Commented Apr 7, 2019 at 9:25
  • $\begingroup$ The Nature article provided by @canadianer states that they were succesful isolating the core complex (that is, alpha dimer, beta, beta-prime and omega), but not the holoenzyme (the previous ones with sigma). I suggest that is because sigma leaves the RNA pol complex once elongation (the longest phase) begins; so researchers have a shorter window of time to isolate the holoenzyme. $\endgroup$
    – user38945
    Commented Apr 8, 2019 at 17:40

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