The stringent control mechanism. Binding of uncharged tRNA and stringent factor to Escherichia coli ribosomes

The mechanism of the interaction of uncharged tRNA and stringent factor with the ribosomes during the in vitro synthesis of guanosine polypnophates (pppGpp and ppGpp) was studied. 70S ribosomes lacking proteins L7 and L12 were able to bind radioactive stringent factor but not EF-Tu or EF-G. In addition, ribosomes carrying EF-Tu, GMPPCP and Phe-tRNA bound as much stringent factor as 70S ribosomes alone. This evidence suggests that the stringent and elongation factors recognize nonidentical site (s) on the ribosomes. When a 70S - poly U - tRNA complex was formed in the absence of stringent factor and was subsequently incubated with Nac-Phe-tRNA or Phe-tRNA only the binding of Nac-Phe-tRNA to the complex was inhibited. However, when the 70S-poly U-tRNA complex was formed in the presence of stringent factor and then was incubated with Nac-Phe-tRNA or Phe-tRNA only the binding of Phe-tRNA was inhibited. These results indicate that the stringent factor directs uncharged codon-specific tRNA into the acceptor site of the ribosome.