Induction of stringency by hyperoxia in Escherichia coli

The effects of exposure to 4.2 atm. of oxygen on induction of the stringent genetic response (production of guanosine tetraphosphate) and the consequences for growth and survival were determined using E. coli strains NF161 stringent and NF162 relaxed. The stringent response was induced by hyperoxia, and guanosine tetraphosphate began to accumulate within 1 min in the stringent but not in the relaxed strain. Within 10 min ppGpp reached [447 ± 96] x 10³ molecules/cell, approximately 6 times control values. This apparently results from the previously reported inhibition of amino acid biosynthesis by hyperoxia. During recovery with air as the gas phase, resumption of growth was correlated with the decay of ppGpp and concentrations of ppGpp and GTP were inversely correlated both during hyperoxia and recovery. Exposure to hyperoxia resulted in rapid growth cessation of both strains and to some death of the relaxed strain during 5 hr in hypoxia. After 5 hr in hyperoxia, growth of the stringent strain recovered in about 2 hr, but the relaxed strain required almost 16 hr. Thus, stringency is involved in the mechanism by which hyperoxia inhibits growth in minimal medium, and also provides some protection.