CRYSTAL-STRUCTURE OF RIBOSOMAL-PROTEIN S8 FROM THERMUS-THERMOPHILUS REVEALS A HIGH-DEGREE OF STRUCTURAL CONSERVATION OF A SPECIFIC RNA-BINDING SITE

S8 is one of the core ribosomal proteins. It binds to 16 S RNA with hi gh affinity and independently of other ribosomal proteins. It also act s as a translational repressor in Escherichia coli by binding to its o wn mRNA. The structure of Thermus thermophilus S8 has been determined by the method of multiple isomorphous replacement at 2.9 Angstrom reso lution and refined to a crystallographic R-factor of 16.2% (R-free 27. 5%). The two domains of the structure have an alpha/beta fold and are connected by a long protruding loop. The two molecules in the asymmetr ic unit of the crystal interact through an extensive hydrophobic core and form a tightly associated dimer, while symmetry-related molecules form a joint beta-sheet of mixed type. This type of protein-protein in teraction could be realized within the ribosomal assembly. A compariso n of the structures of T. thermophilus and Bacillus stearothermophilus S8 shows that the interdomain loop is eight residues longer in the fo rmer and reveals high structural conservation of an extensive region, located in the C-terminal domain. From mutational studies this region was proposed earlier to be involved in specific interaction with RNA. On the basis of these data and on the comparison of the two structures of S8, it is proposed that the three-dimensional structure of specifi c RNA binding sites in ribosomal proteins is highly conserved among di fferent species. (C) 1998 Academic Press Limited.