Guanosine tetra- and pentaphosphate synthesis by bacterial stringent factor and eukaryotic ribosomes

Abstract

AMINO acid starvation of rel⁺ bacteria¹ results in a rapid pleiotropic response involving the cessation of stable RNA synthesis, a decrease in the rate of nucleoside transport, an altered pattern of mRNA synthesis, inhibition of phospholipid synthesis and increased protein turnover. This stringent response² is rapidly reversed when the amino acid starvation is relieved and is apparently mediated by two unusual nucleotides, guanosine 5′-diphosphate, 3′-diphosphate (ppGpp) and guanosine 5′-triphosphate, 3′-diphosphate (pppGpp)³. These nucleotides are synthesised on the ribosome in the presence of uncharged tRNA, ATP, GTP and an enzyme known as stringent factor³. Mutants which fail to exhibit the stringent response to amino acid starvation with the synthesis of ppGpp pppGpp are known as relaxed mutants⁴ and in Escherichia coli are mapped at three loci; relA (ref. 5), relB (ref. 6), and rplK (relC) (refs 7 and 8). The synthesis of ppGpp and pppGpp has also been observed in various prokaryotes including blue-green algae⁹. Here we demonstrate the ability of bacterial stringent factor to be stimulated by eukaryotic ribosomal preparations to synthesise both ppGpp and pppGpp.