Effects of the spoT and relA Mutation on the Synthesis and Accumulation of ppGpp and RNA during Glucose Starvation

Abstract

The effects of glucose starvation on the accumulation and synthesis of guanosine 3′,5′-bis(diphosphate) (ppGpp) were compared in four Escherichia coli K-12 strains having four different combinations of the spoT and relA alleles: spoT+relA+, spoT relA+, spoT+relA, and spoT relA. Glucose starvation caused a rapid and complete inhibition of RNA and protein accumulation and severe inhibition of RNA synthesis in all four strains. However, ppGpp accumulated only gradually in the relaxed (relA) strains and rapidly in the stringent (relA+) strains. Thus, ppGpp accumulation is not obligatory to the inhibition of RNA synthesis and accumulation during glucose starvation. During growth, relA strains synthesized ppGpp at a rate comparable with that in their relA+ partners. Glucose starvation did not affect the basal rate of ppGpp synthesis in the relA strains, but caused a transient stimulation of ppGpp synthesis in the relA+ strains. This suggests that glucose starvation causes transient amino-acid starvation. Since ppGpp accumulated in the relA strain without a change in its synthetic rate, it is inferred that ppGpp degradation decreased during glucose starvation. During growth, the turnover of ppGpp was considerably slower in the spoT strains than in the spoT+ strains. This suggests that the slower degradation of ppGpp in the spoT strains is counterbalanced by the slower synthesis of ppGpp. This difference in the rate of ppGpp synthesis became apparent when the relA strains were starved for glucose: The spoT relA strain accumulated ppGpp more slowly than did the spoT+relA strain.