THE SOLUTION STRUCTURE OF RIBOSOMAL-PROTEIN S4 DELTA-41 REVEALS 2 SUBDOMAINS AND A POSITIVELY CHARGED SURFACE THAT MAY INTERACT WITH RNA

S4 is one of the first proteins to bind to 16S RNA during assembly of the prokaryotic ribosome, Residues 43-200 of S4 from Bacillus stearoth ermophilus (S4 Delta 41) bind specifically to both 16S rRNA and to a p seudoknot within the a operon mRNA, As a first step toward understandi ng how S4 recognizes and organizes RNA, we have solved the structure o f S4 Delta 41 in solution by multidimensional heteronuclear nuclear ma gnetic resonance spectroscopy. The fold consists of two globular subdo mains, one comprised of four helices and the other comprised of a five -stranded antiparallel P-sheet and three helices, Although cross-linki ng studies suggest that residues between helices alpha 2 and alpha 3 a re close to RNA, the concentration of positive charge along the crevic e between the two subdomains suggests that this could be an RNA-bindin g site, In contrast to the L11 RNA-binding domain studied previously, S4 Delta 41 shows no fast local motions, suggesting that it has less c apacity for refolding to fit RNA. The independently determined crystal structure of S4 Delta 41 shows similar features, although there is sm all rotation of the subdomains compared with the solution structure. T he relative orientation of the subdomains in solution will be verified with further study.