Escherichia coli stringent factor binds to ribosomes at a site different from that of elongation factor Tu or G.

The binding of Escherichia coli stringent factor to ribosomes has been studied; the reaction depends on 50S and 30S ribosomal subunits and poly(U) as messenger RNA. The ribosome-stringent factor complex is formed at 5-10 mM magnesium acetate; NH4 ions are inhibitory. Binding of the stringent factor to the 70S-mRNA complex is not stimulated by uncharged tRNA. The ribosomal binding site(s) for the stringent factor does not overlap with the one known for the elongation factor Tu (EF-Tu) or G (EF-G). Ribosomes carrying either EF-Tu or EF-G are active in binding the stringent factor; however, they are inactive in synthesizing guanosine 5'-triphosphate 3'-diphosphate (pppGpp) and guanosine 5'-diphosphate 3'-diphosphate (ppGpp). The latter result is due to the blockage of the ribosomal acceptor site by the aminoacyl-tRNA and/or elongation factors. That stringent and elongation factors do not compete for identical ribosomal region(s) is supported by: (1) the reverse experiments where ribosomes charged with the stringent factor are fully active in EF-Tu or EF-G dependent functions; (2) ribosomes that lack the two ribosomal proteins L7 and L12 known to be essential for EF-Tu and EF-G functions bind the stringent factor and are active in synthesizing pppGpp and ppGpp.