EFFECT OF THE AMINO-ACID ATTACHED TO ESCHERICHIA-COLI INITIATOR TRANSFER-RNA ON ITS AFFINITY FOR THE INITIATION-FACTOR IF2 AND ON THE IF2 DEPENDENCE OF ITS BINDING TO THE RIBOSOME

We show that the nature of the amino acid in the formylaminoacyl-tRNA influences initiation factor (IF) 2 dependence of its ribosome binding and that this IF2 dependence reflects the relative affinity of the fo rmylaminoacyl-tRNA for the initiation factor IF2. We compared the temp late-dependent ribosome binding activities, in the presence of initiat ion factors, of wild type and anticodon sequence mutants of Escherichi a coli initiator tRNAs that carry formylmethionine (fMet), formylgluta mine (fGln), or formylvaline (fVal). The fGln-tRNA bound less well tha n fMet-tRNA whereas the fVal-tRNA bound as well as flMet-tRNA. The rat e and extent of binding of fGln-tRNA to the ribosome was significantly increased by further addition of purified initiation factor IF2. In c ontrast, the binding of fVal-tRNA or fMet-tRNA was not affected much b y the addition of IF2. Using gel mobility shift assay, we have measure d the apparent K-d values of the IF2 . formylaminoacyl-tRNA binary com plexes. These are 1.8, 3.5, and 10.5 mu M for fMet-tRNA, fVal-tRNA, an d fGln-tRNA, respectively.