STRUCTURAL ELEMENTS OF RPSO MESSENGER-RNA INVOLVED IN THE MODULATION OF TRANSLATIONAL INITIATION AND REGULATION OF ESCHERICHIA-COLI RIBOSOMAL-PROTEIN S15

Previous experiments showed that S15 inhibits its own translation by b inding to its mRNA in a region overlapping the ribosome loading site. This binding was postulated to stabilize a pseudoknot structure that e xists in equilibrium with two stem-loops and to trap the ribosome on i ts mRNA loading site in a transitory state. In this study, we investig ated the effect of mutations in the translational operator on: the bin ding of protein S15, the formation of the 30S/mRNA/tRNA(f)(Met) ternar y initiation complex, the ability of S15 to inhibit the formation of t his ternary complex. The results were compared to in vivo expression a nd repression rates. The results show that (1) the pseudoknot is requi red for S15 recognition and translational control; (2) mRNA and 16S rR NA efficiently compete for S15 binding and 16S rRNA suppresses the abi lity of S15 to inhibit the formation of the active ternary complex; (3 ) the ribosome binds more efficiently to the pseudoknot than to the st em-loop; (4) sequences located between nucleotides 12 to 47 of the S15 coding phase enhances the efficiency of ribosome binding in vitro; th is is correlated with enhanced in vivo expression and regulation rates .