Protein S1 counteracts the inhibitory effect of the extended Shine-Dalgarno sequence on translation.
There are two major components of Escherichia coli ribosomes directly involved in selection and binding of mRNA during initiation of protein synthesis-the highly conserved 3' end of 16S rRNA (aSD) complementary to the Shine-Dalgarno (SD) domain of mRNA, and the ribosomal protein S1. A contribution of the SD-aSD and S1-mRNA interactions to translation yield in vivo has been evaluated in a genetic system developed to compare efficiencies of various ribosome-binding sites (RBS) in driving beta-galactosidase synthesis from the single-copy (chromosomal) lacZ gene. The in vivo experiments have been supplemented by in vitro toeprinting and gel-mobility shift assays. A shortening of a potential SD-aSD duplex from 10 to 8 and to 6 bp increased the beta-galactosidase yield (four- and sixfold, respectively) suggesting that an extended SD-aSD duplex adversely affects translation, most likely due to its redundant stability causing ribosome stalling at the initiation step. Translation yields were significantly increased upon insertion of the A/U-rich S1 binding targets upstream of the SD region, but the longest SD remained relatively less efficient. In contrast to complete 30S ribosomes, the S1-depleted 30S particles have been able to form an extended SD-aSD duplex, but not the true ternary initiation complex. Taken together, the in vivo and in vitro data allow us to conclude that S1 plays two roles in translation initiation: It forms an essential part of the mRNA-binding track even when mRNA bears a long SD sequence, and through the binding to the 5' untranslated region, it can ensure a substantial enhancing effect on translation.
D020121:5' Untranslated Regions; D001483:Base Sequence; D001665:Binding Sites / Q000235:genetics; D004742:Enhancer Elements, Genetic; D004926:Escherichia coli / Q000235:genetics* / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000235:genetics / Q000378:metabolism*; D007763:Lac Operon; D014176:Protein Biosynthesis; D012329:RNA, Bacterial / Q000235:genetics* / Q000378:metabolism*; D012333:RNA, Messenger / Q000235:genetics* / Q000378:metabolism*; D012269:Ribosomal Proteins / Q000378:metabolism*; D012270:Ribosomes / Q000378:metabolism; D001616:beta-Galactosidase / Q000096:biosynthesis / Q000235:genetics
D020121:5' Untranslated Regions; D029968:Escherichia coli Proteins; D012329:RNA, Bacterial; D012333:RNA, Messenger; D012269:Ribosomal Proteins; C015494:ribosomal protein S1; D001616:beta-Galactosidase