Guanosine tetraphosphate inhibits protein synthesis in vivo. A possible protective mechanism for starvation stress in Escherichia coli.

Guanosine 3',5'-bispyrophosphate (ppGpp) accumulates in bacteria in response to either amino acid or energy source starvation. Here we demonstrate that levels of ppGpp similar to those induced by amino acid starvation inhibit the rate of protein synthesis by 84-91%. The intracellular concentration of ppGpp is manipulated in our studies by increasing the expression of a truncated relA gene encoding a smaller but catalytically active peptide with ppGpp synthetase activity. We find that the intracellular activity of the truncated RelA peptide is insensitive to chloramphenicol, unlike the product of the wild-type relA gene, ppGpp synthetase I. Previously, this same ppGpp expression system was used (Schreiber, G., Metzger, S., Aizenman, E., Roza, S., Cashel, M., and Glaser, G. (1991) J. Biol. Chem. 226, 3760-3767) to demonstrate that increasing the ppGpp concentration inhibits growth and ribosomal RNA transcription, and they found suggestive evidence for ppGpp inhibition of protein synthesis. We further investigated the effect of ppGpp on protein synthesis and find that ppGpp is a potent inhibitor of protein synthesis as well as glycerol accumulation but has no effect on transport of methionine, the amino acid used in measuring protein synthesis rates, or on uptake of alpha-methylglucoside, a non-metabolizable analogue of glucose.