LOW-RESOLUTION STRUCTURAL CHARACTERIZATION OF THE ARGININE REPRESSOR ACTIVATOR FROM BACILLUS-SUBTILIS - A COMBINED X-RAY CRYSTALLOGRAPHIC AND ELECTRON-MICROSCOPIC APPROACH/

Attempts to determine the X-ray crystal structure of the intact homohe xameric arginine repressor/activator from B. subtilis have so far been unsuccessful. The major problem appears to be the lack of an isomorph ous heavy-atom derivative with a manageable number of substitution sit es. Here it is shown how electron microscopy of thin three-dimensional crystals, the same as those used for the X-ray crystallographic studi es, made it possible (i) to obtain experimental support for some concl usions drawn on the basis of X-ray data alone, (ii) to determine the l ow-resolution distribution of electron density in several different cr ystallographic projections, and (iii) to obtain a tentative low-resolu tion model of the whole hexamer.