Ribosome‐messenger recognition in the absence of the Shine‐Dalgarno interactions

Abstract
In an attempt to understand how Escherichia coli ribosomes recognize the initiator codon on mRNAs lacking the Shine‐Dalgamo (SD) sequence, we have studied 30S initiation complex formation in extension inhibition (toeprinting) experiments using (‐SD)mRNAs which are known to be reliably translated in E. coli: the plant viral messenger A1MV RNA 4 and two chimaeric mRNAs coding for β‐glucuronidase (GUS) and bearing the 5'‐untranslated sequence of TMV RNA Ω or the Ω‐derived sequence (CAA)n as 5'‐leaders. Ribosomal protein Sl and IF3 have been found to be indispensable for translational initiation. Protein S1 appears to be a key recognition element. S1 binds to sequences within the leaders of (‐SD)mRNAs thus providing their affinity to E. coli ribosomes.