Low-Resolution Structural Characterization of the Arginine Repressor/Activator from Bacillus subtilis: a Combined X-ray Crystallographic and Electron Microscopical Approach

Attempts to determine the X‐ray crystal structure of the intact homohexameric arginine repressor/activator from B. subtilis have so far been unsuccessful. The major problem appears to be the lack of an isomorphous heavy‐atom derivative with a manageable number of substitution sites. Here it is shown how electron microscopy of thin three‐dimensional crystals, the same as those used for the X‐ray crystallographic studies, made it possible (i) to obtain experimental support for some conclusions drawn on the basis of X‐ray data alone, (ii) to determine the low‐resolution distribution of electron density in several different crystallographic projections, and (iii) to obtain a tentative low‐resolution model of the whole hexamer.