Dependence of Initiation Factor IF-2 Activity on Proteins L7 and L12 from Escherichia coli 50 S Ribosomes

The activity of initiation factor IF-2 from Escherichia coli strain MRE600 has been studied as a function of the presence or absence of the 50 S ribosomal proteins L7 and L12. Three specific aspects of initiation factor IF-2 function were examined: (a) the binding of radioactive initiation factor IF-2 to 70 S ribosomes; (b) the ribosomal binding of N-formyl-methionyl transfer RNA which is dependent upon initiation factor IF-2; (c) 5'-guanosine triphosphate hydrolysis catalyzed by initiation factor IF-2 in the presence of 70 S ribosomes. The results show that the binding of initiation factor IF-2 and of N-formylmethionyl transfer RNA are both reduced 50 to 75% when 50 S particles deficient in L7 and L12 are employed. The 5'-guanosine triphosphate hydrolysis activity is far more drastically reduced with particles deficient in L7 and L12. The latter effect is similar to results found with the 5'-guanosine triphosphate hydrolysis activities associated with elongation factors EF-G and EF-Tu. The results suggest that proteins L7 and L12 are part of the binding site for initiation factor IF-2, but that they are more intimately involved in the 5'-guanosine triphosphate hydrolysis catalyzed by the factor. Furthermore, the results are consistent with a model in which the binding sites for all three factors, IF-2, EF-G, and EF-Tu, share certain common determinants, including the L7 and L12 proteins.