CRYSTAL-STRUCTURE OF PROKARYOTIC RIBOSOMAL-PROTEIN L9 - A BILOBED RNA-BINDING PROTEIN

The crystal structure of protein L9 from the Bacillus stearothermophil us ribosome has been determined at 2.8 Angstrom resolution using X-ray diffraction methods. This primary RNA-binding protein has a highly el ongated and unusual structure consisting of two separated domains join ed by a long exposed alpha-helix. Conserved, positively charged and ar omatic amino acids on the surfaces of both domains probably represent the sites of specific interactions with 23S rRNA. Comparisons with oth er prokaryotic L9 sequences show that while the length of the connecti ng alpha-helix is invariant, the sequence within the exposed central r egion is not conserved. This suggests that the alpha-helix has an arch itectural role and serves to fix the relative separation and orientati on of the N- and C-terminal domains within the ribosome. The N-termina l domain has structural homology to the smaller ribosomal proteins L7/ L12 and L30, and the eukaryotic RNA recognition motif (RRM).