RIBOSOME-MESSENGER RECOGNITION IN THE ABSENCE OF THE SHINE-DALGARNO INTERACTIONS

In an attempt to understand how Escherichia coli ribosomes recognize t he initiator codon on mRNAs lacking the Shine-Dalgarno (SD) sequence, we have studied 30S initiation complex formation in extension inhibiti on (toeprinting) experiments using (-SD)mRNAs which are known to be re liably translated in E. coli: the plant viral messenger A1MV RNA 4 and two chimaeric mRNAs coding for beta-glucuronidase (GUS) and bearing t he 5'-untranslated sequence of TMV RNA (Omega) or the Omega-derived se quence (CAA)(n) as 5'-leaders. Ribosomal protein S1 and IF3 have been found to be indispensable for translational initiation. Protein S1 app ears to be a key recognition element. S1 binds to sequences within the leaders of (-SD)mRNAs thus providing their affinity to E. coli riboso mes.