The topographical localization of IF3 on Escherichia coli 30 S ribosomal subunits as a clue to its way of functioning

A problem of considerable interest concerns the topographical localization of the translational initiation factors on the ribosome. This is essential to improve our knowledge of the ribosomal topography and to gain a better insight into the physical and mechanistic aspects of the interactions between the factors and the ribosome. Several protein-RNA and protein-protein crosslinking studies dealt so far with this subject (reviews [ 1,2]). Here, we present results obtained on the protein neighborhood of IF3 by use of very mild reaction conditions. Taking into account the functional and structural properties of IF3, its topographical localization on the ribosome and its effect on the properties of other ribosomal proteins, we also present an hypothetical model which attempts to explain how the factor might function. some experiments 70 S ‘tight couples’ and ribosomal subunits derived from them were prepared essentially as in [4]. Initiation factor IF3 was purified [5], and labelled in vitro by either reaction with N-[ 3H] ethylmaleimide (NEM) or by reductive methylation with [ 14C] formaldehyde as detailed in [ 61. Poly(U)-dependent polyphenylalanine synthetic activity test for 30 S ribosomal subunits was performed as in [7].

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