FROM STANDBY TO DECODING SITE - ADJUSTMENT OF THE MESSENGER-RNA ON THE 30S RIBOSOMAL-SUBUNIT UNDER THE INFLUENCE OF THE INITIATION-FACTORS

The hypothesis of an adjustment of the mRNA in its ribosomal channel u nder the influence of the initiation factors has been tested by site-d irected crosslinking experiments. Complexes containing 30S subunits wi th bound mRNA having 4-thio-uracil at specific positions were prepared in the presence or absence of initiation factors and/or fMet-tRNA and subjected to UV irradiation to obtain specific crosslinks of the radi oactively labeled mRNA with bases of the 16S rRNA and with ribosomal p roteins. The subsequent identification of the specific sites of both m RNA and rRNA and individual ribosomal proteins involved in the crossli nking, obtained under different conditions of complex formation, provi de direct evidence for the occurrence of a partial relocation of the m RNA on the 30S ribosomal subunits under the influence of the factors. The nature of this mRNA relocation is compatible with our previous pro posal of a shift of the template from an initial ribosomal ''stand-by site'' to a second site closer to that occupied when the initiation tr iplet of the mRNA is decoded in the P-site.