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)nas 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. © 1994.