Role of guanosine tetraphosphate in gene expression and the survival of glucose or seryl-tRNA starved cells of Escherichia coli K12

The concentration of guanosine 3,5-bispyrophosphate (ppGpp) increases in bacteria in response to amino acid or carbon/energy source starvation. An Escherichia coli K12 relAspoT mutant lacking the ability to synthesize ppGpp lost viability at an increased rate during both glucose and seryl-tRNA starvation. Also, the deleterious effect of chloramphenicol on starved wild-type cells could be overcome by inducing expression of RelA from a plasmid carrying the relA gene transcribed from a tac promoter, prior to starvation and chloramphenicol treatment. As demonstrated by two dimensional gel electrophoresis, this induction of the RelA protein resulted in global alterations in gene expression including increased synthesis of some rpoS-dependent proteins. The relAspoT mutant maintained high expression of several ribosomal proteins during starvation and appeared to exhibit significantly decreased translational fidelity, as demonstrated by an unusual heterogeneity in the isoelectric point of several proteins and the failure to express higher molecular weight proteins during starvation. Moreover, both rpoS-dependent and independent genes failed to exhibit increased expression in the mutant. It is suggested that the deleterious effects on the cells of the relA, spoT deletions are not due solely to the inability of these cells to induce the sigma factor ^s, but also to deficiencies in translational fidelity and failure to exert classical stringent regulation.