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As is evident from the above summary of the recent literature, plus many other papers not cited here, there is an extensive literature indicating the physiological significance of these amines. The most important studies can be summarized as follows. (a) Polyamines and their biosynthetic enzymes are ubiquitous. (b) Microbiological mutants have been described in which there is a definite requirement of polyamines for growth. (c) The concentration of polyamines and their biosynthesis enzymes increase when the growth rate increases. These increases usually precede or are simultaneous with increases in RNA, DNA, and protein levels. (d) Ornithine decarboxylase has a remarkably fast turnover rate in animal cells, and the level of this enzyme rapidly changes after a variety of growth stimuli. (e) Polyamines have a high affinity for nucleic acids and stabilize their secondary structure. They are found associated with DNA in bacteriophages and have a variety of stimulatory effects on DNA and RNA biosynthesis in vitro. (f) Polyamines stimulate protein synthesis in vivo and in vitro. (g) Polyamines protect spheroplasts and halophilic organisms for lysis, indicating their ability to stabilize membranes. Despite these observations, no specific mechanism has been firmly established for the action of the polyamines in vivo. It is clear that these compounds are physiologically important, however, and further work is necessary to establish the mechanism of their action.

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Wild-type Escherichia coli cells grow normally in 95% O2͞5% CO2. In contrast, cells that cannot make polyamines because of mutations in the biosynthetic pathway are rapidly killed by incubation in 95% O2͞5% CO2. Addition of polyamines prevents the toxic effect of oxygen, permitting cell survival and optimal growth. Oxygen toxicity can also be prevented if the growth medium contains an amino acid mixture or if the polyamine-deficient cells contain a manganese-superoxide dismutase (Mn-SOD) plasmid. Partial protection is afforded by the addition of 0.4 M sucrose or 0.4 M sorbitol to the growth medium. We also report that concentrations of H2O2 that are nontoxic to wild-type cells or to mutant cells pretreated with polyamines kill polyamine-deficient cells. These results show that polyamines are important in protecting cells from the toxic effects of oxygen. It is well known from the classic studies of Fridovich and his group (1-5) and others (6-8) that Escherichia coli cells, grown in air, have mechanisms for the protection of the cells from the toxic effects of superoxide and of oxygen radicals. Of particular importance is the protective effect exerted by the superoxide dismutase that is present in these cells. It has also been observed that E. coli cells can grow at a normal rate in an atmosphere of increased oxygen, presumably by operation of these protective mechanisms (7).Polyamines, the ubiquitous polycationic compounds, are associated with a variety of biological processes, such as nucleic acid biosynthesis, cell growth, and differentiation (9, 10). In this paper, we present data showing that another important factor in the protection of E. coli cells from the toxic effects of oxygen is the intracellular level of polyamines. We have found that polyamine-deficient cells are rapidly killed by incubation in oxygen, even though, as we previously reported, when incubated in air, these same cells are not killed and grow indefinitely (albeit at a somewhat reduced growth rate; ref. 11). Materials and MethodsThe strains used are listed in Table 1. All strains were maintained on LB plates. All incubations were at 37°C with vigorous shaking. The various mutants were grown on a purified polyamine-free Vogel-Bonner (VB) medium ref. 16) or M63 medium (Fig. 5; ref. 7) [supplemented with glucose and the auxotrophic requirements for EWH319 and HT306 (thiamin, pantothenate, proline, and threonine)] for at least 20-25 generations to deplete any polyamines remaining from the original LB medium. The cultures then were grown overnight in limiting glucose (0.025%) to an OD 600 of 0.1-0.2. Additional glucose (0.4%) was then added, plus polyamines where indicated, and the cultures were incubated for another 2 h. The cultures were then diluted to an OD 600 of 0.001 into the same medium with and without polyamines. † In some of the experiments an amino acid mixture (Fig. 3A), sucrose or sorbitol (Fig. 3B) was added at this point. For experiments presented in Fig. 5 (with pDT1.5, sodA plasmid), the cultures were grown in M63 medium...

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