Biochemical characterization and messenger specificity of polypeptide chain initiation factors from Escherichia coli

Abstract

Three protein factors are required for maximum poly(U, G)- or AUG-directed binding of fMet-tRNA to ribosomes. The same three factors are both necessary and sufficient for “natural” mRNA-directed binding of fMet-tRNA to ribosomes. Bound fMet-tRNA cosediments with the 70S ribosome as does bound mRNA. All three factors are required for the fMet-tRNA and GTP-dependent binding of mRNA to the 70S initiation complex. The physiochemical characteristics of the initiation factors are described. Purified IF1 and IF3 are electrophoretically homogeneous. Purified IF2 preparations exhibit 1–4 bands on gel electrophoresis. IF1 and IF3 are heat stable, basic proteins, each consisting of a single polypeptide chain of molecular weight 9000 and 21,000, respectively. IF2 is a heat-labile acidic protein. Its sedimentation coefficient of 3.9 corresponds to a mol wt of 55,000 but SDS-gel electrophoresis suggests that the actual molecular weight may be somewhat higher. Both IF1 and IF3 are insensitive to inhibition by p-hydroxymercuribenzoate while IF2 is sensitive. The factor-promoted binding of fMet-tRNA to ribosomes has been characterized with respect to various requirements. Optimum conditions are similar to those for polypeptide chain elongation. In the absence of high salt treatment, all three factors reside on native 30S ribosomal particles. They are not found on native 70S particles or 50S particles, nor in the soluble fraction of the cell extract.