Biology Department, Brookhaven National Laboratory, Upton, New York 11973.
Although a low resolution model for the arrangement of the proteins of the small and large ribosomal subunits is known, a detailed mechanistic understanding of the function of the ribosome awaits a high resolution structure of its components. While crystals have been obtained of several ribosomal proteins from Bacillus stearothermophilus, determination of atomic resolution structures of these proteins is impeded by the difficulty of obtaining large amounts of native proteins for crystallographic or NMR studies. We describe here the cloning and overexpression in Escherichia coli of the genes for ribosomal proteins S5, L6, L9, and L18 from B. stearothermophilus. S5 is extremely toxic to E. coli when overexpressed, and we have taken advantage of a new tightly regulated expression system to obtain high yields (more than 100 mg of pure protein/liter of culture) of this protein. The B. stearothermophilus S5 produced in E. coli crystallizes, and the crystals are identical to those obtained from the native protein. The crystals diffract to 2-A resolution.