Effect of thiostrepton on recycling of Escherichia coli initiation factor 2

The recycling of initiation factor 2, which requires both GTP and the 50S subunit and involves GTP hydrolysis, appears to be blocked by thiostrepton. However, this antibiotic has little or no effect on the AUG-directed ribosomal binding promoted by initiation factor 2 of fMet-tRNA under conditions that do not permit optimal recycling of the factor. Evidence is presented suggesting that the 50S ribosomal proteins L7 or L12, which are required for the function of the chain elongation factors Tu and G and the accompanying hydrolysis of GTP, are also involved in recycling of initiation factor 2. Although both 5'-guanylylmethylene diphosphonate and thiostrepton seem to block recycling of initiation factor 2, fMet-tRNA bound to the ribosomes in the presence of GTP and thiostrepton can react with puromycin, whereas that bound in the presence of 5'-guanylylmethylene diphosphonate and the antibiotic cannot. It is proposed that initiation factor 2 may interact with two nonidentical ribosomal sites during polypeptide chain initiation.