Metal compounds as tools for the construction and the interpretation of medium-resolution maps of ribosomal particles

Procedures were developed exploiting organometallic clusters and coordinati on compounds in combination with heavy metal salts for derivatization of ri bosomal crystals. These enabled the construction of multiple isomorphous re placement (MIR) and multiple isomorphous replacement combined with anomalou s scattering medium-resolution electron density maps for the ribosomal part icles that yield the crystals diffracting to the highest resolution, 3 Angs trom, of the large subunit from Haloarcula marismortui and the small subuni t from Thermus thermophilus. The first steps in the interpretation of the 7 .3-Angstrom MIR map of the small subunit were made with the aid of a tetrai ridium cluster that was covalently attached to exposed sulfhydryls on the p article's surface prior to crystallization. The positions of these sulfhydr yls were localized in difference Fourier maps that were constructed with th e MIR phases. (C) 1999 Academic Press.