A PSEUDOKNOT IS REQUIRED FOR EFFICIENT TRANSLATIONAL INITIATION AND REGULATION OF THE ESCHERICHIA-COLI RPSO GENE CODING FOR RIBOSOMAL-PROTEIN S15

Escherichia coli ribosomal protein S15 down regulates its own synthesi s by binding to its mRNA in a region overlapping the ribosome binding site, called the translational operator. This binding stabilizes a pse udoknot structure that exists in equilibrium with two stem-loop struct ures. When synthesized in excess over 16S rRNA, S15 binds to its trans lational operator and traps the ribosome on its loading site in a tran sient state, preventing the formation of the active ternary (30S-mRNA- rRNA(f)(Met)) complex. This inhibition can be suppressed by 165 rRNA, which displaces S15 from the mRNA. An extensive mutational analysis sh owed that the pseudoknot is the structural element required for S15 re cognition and in vivo translational control. Specific sequence determi nants are located in limited regions of the structure formed by the ps eudoknot. An unexpected result is that the pseudoknot can exist in a v ariety of topologically equivalent structures recognizable and shapabl e by S15. Based on footprinting experiments and computer graphic model ling, S15 shields the two stems of the pseudoknot, sitting in the majo r groove of the coaxial stack.