RIBOSOMAL-PROTEIN S7 - A NEW RNA-BINDING MOTIF WITH STRUCTURAL SIMILARITIES TO A DNA ARCHITECTURAL FACTOR

Background: The ribosome is a ribonucleoprotein complex which performs the crucial function of protein biosynthesis. Its role is to decode m RNAs within the cell and to synthesize the corresponding proteins. Rib osomal protein S7 is located at the head of the small (30S) subunit of the ribosome and faces into the decoding centre. S7 is one of the pri mary 16S rRNA-binding proteins responsible for initiating the assembly of the head of the 30S subunit. In addition, S7 has been shown to be the major protein component to cross-link with tRNA molecules bound at both the aminoacyl-tRNA (A) and peptidyl-tRNA (P) sites of the riboso me. The ribosomal protein S7 clearly plays an important role in riboso me function. It was hoped that an atomic-resolution structure of this protein would aid our understanding of ribosomal mechanisms. Results: The structure of ribosomal protein S7 from Bacillus stearothermophilus has been solved at 2.5 Angstrom resolution using multiwavelength anom alous diffraction and selenomethionyl-substituted proteins. The molecu le consists of a helical hydrophobic core domain and a beta-ribbon arm extending from the hydrophobic core. The helical core domain is compo sed of a pair of entangled helix-turn-helix motifs; the fold of the co re is similar to that of a DNA architectural factor. Highly conserved basic and aromatic residues are clustered on one face of the S7 molecu le and create a 16S rRNA contact surface. Conclusions: The molecular s tructure of S7, together with the results of previous cross-linking ex periments, suggest how this ribosomal protein binds to the 3' major do main of 16S rRNA and mediates the folding of 16S rRNA to create the ri bosome decoding centre.