Ribosomes cannot interact simultaneously with elongation factors EF Tu and EF G.

Prior binding of EF G and GDP to 70S ribosomes from Escherichia coli prevents the subsequent binding of aminoacyl-tRNA, mediated by EF Tu. However, the interaction of EF Tu.GTP.aminoacyl-tRNA with the 30S subunit, which results in aminoacyl-tRNA binding without GTP hydrolysis, appears to be unaffected by EF G, GDP, and fusidic acid. We conclude that elongation factors Tu and G cannot interact simultaneously with the ribosome. The simplest interpretation of these and earlier data is that EF G and EF Tu interact with the same, or overlapping, 50S ribosomal sites in the course of GTP hydrolysis associated with translocation and aminoacyl-tRNA binding, respectively. In any event, these factors must alternate in binding to the ribosome in the course of each elongation cycle.