Stringent factor from Escherichia coli directs ribosomal binding and release of uncharged tRNA

Uncharged tRNA is preferentially bound to the peptidyl site of the ribosome in the absence of stringent factor ,but in its presence is directed to the acceptor site. The synthesis of pppGpp and ppGpp is initiated by tRNA bound in the acceptor but not in the peptidyl site. In this reaction, tRNA is not permanently attached to the acceptor site. Uncharged [32P] tRNA but not 3H-labeled stringent factor is released from the ribosome after each round of stringent factor-dependent hydrolysis of ATP. ATP-32PPi-exchange experiments reveal that exchange is independent of the presence of GTP but strongly enhanced by the addition of stringent factor and tRNA. The tRNA release is suppressed when ATP is replaced by beta, gamma-imido adenosine 5'-triphosphate, 5'-AMP, or GTP.