Molecular morphology of ribosomes: structural alteration of 50S subunits following the removal of proteins L7 and L12.

Ribosomal proteins L7 and L12 are involved in the initiation, elongation and termination of polypeptide synthesis [l-6]. These acidic proteins do not function independently, but act in concert with the ribosome to perform the above steps. These observations suggest that L7 and L12 are necessary to maintain the conformational integrity of the ribosome, and that loss of ribosomal activity following the removal of L7 and L12 results from a structural alteration of the large subunit. The present investigation is an attempt to determine whether L7 and L12 are involved in preserving the structural conformation of the 50s subunit. Lactoperoxidase-catalyzed iodination of ribosomal proteins has been utilized as a probe to study ribosome structure [7-lo]. Although this probe was originally designed to investigate the surface topography of ribosomes, it has recently been employed to detect conformational alterations of ribosomal subunits accompanying changes in ionic strength, temperature and isolation conditions [ 1 1 -131. To determine whether structural alterations of the 50s subunit occur following the removal of L7 and L12 from the ribosome, we have again used the lactoperoxidase-catalyzed iodination system. Investiga. tions have been designed to detect changes in reactivity of individual ribosomal proteins following the removal and readdition of L7 and L12 as measured by enzymic iodination. 2. Materials and methods