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

In an attempt to understand how Escherichia coli ribosomes recognize the initiator codon on mRNAs lacking the Shine-Dalgarno (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 beta-glucuronidase (GUS) and bearing the 5'-untranslated sequence of TMV RNA (omega) or the omega-derived sequence (CAA)n as 5'-leaders. Ribosomal protein S1 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.