RIBOSOMAL-PROTEINS S5 AND L6 - HIGH-RESOLUTION CRYSTAL-STRUCTURES ANDROLES IN PROTEIN-SYNTHESIS AND ANTIBIOTIC-RESISTANCE

Antibiotic resistance is rapidly becoming a major medical problem. Man y antibiotics are directed against bacterial ribosomes, and mutations within both the RNA and protein components can render them ineffective . It is well known that the majority of these antibiotics act by bindi ng to the ribosomal RNA, and it is of interest to understand how mutat ions in the ribosomal proteins can produce resistance. Translational a ccuracy is one important target of antibiotics, and a number of riboso mal protein mutations in Escherichia coli are known to modulate the pr oofreading mechanism of the ribosome. Here we describe the high-resolu tion structures of two such ribosomal proteins and characterize these mutations. The S5 protein, from the small ribosomal unit, is associate d with two types of mutations: those that reduce translational fidelit y and others that produce resistance to the antibiotic spectinomycin. The L6 protein, from the large subunit, has mutations that cause resis tance to several aminoglycoside antibiotics, notably gentamicin. Ln bo th proteins, the mutations occur within their putative RNA-binding sit es. The L6 mutations are particularly drastic because they result in l arge deletions of an RNA-binding region. These results support the hyp othesis that the mutations create local distortions of the catalytic R NA component. When combined with a variety of structural and biochemic al data, these mutations also become important probes of the architect ure and function of the translational machinery. We propose that the C -terminal half of S5, which contains the accuracy mutations, organizes RNA structures associated with the decoding region, and the N-termina l half, which contains the spectinomycin-resistance mutations, directl y interacts with an RNA helix that binds tl-Lis antibiotic. As regards L6, we suggest that the mutations indirectly affect proofreading by l ocally distorting the EF-Tu.GTP.aminoacyl tRNA binding site on the lar ge subunit. (C) 1998 Academic Press Limited.