Interaction of the radioactive translation initiation factor IF 3 with ribosomes

Abstract

This work describes a preparative procedure to obtain IF3, one of the main translation initiation factors, in a 35S labelled form. 35S IF3 thus prepared was 80 p. cent pure. The main features of 35S IF3 binding to E. coli ribosomes have been specified by means of the sucrose density sedimentation technique. At 5.10−3 M Mg++, 35S IF3 readily forms a stable complex with 30S subunits — previously washed free of their endogeneous factors — but not with 50S subunits. When the Mg++ concentration is lowered to 2.10−4 M, appreciable binding to the 50S subunits is observed. After mixing 35S IF3 with 70S ribosomes, one can detect the appearance of 30S-35S IF3 complexes as a result of the dissociating effect of IF3. The staechiometry of this binding corresponds to one molecule of factor per 30S subunit, 35S-IF3 is released from 30S subunits after or during formation of an initiation complex. In the presence of streptomycin or neomycin at doses inhibiting spontaneous 70S dissociation, formation of 30S-35S-IF3 complex cannot be detected. These antibiotics as well as kasugamycin or colicin E3 do not inhibit 35S-IF3 binding to pre-dissociated 30S at low Mg++.