Evidence for the existence of a novel component of biological water stress (anhydrotic stress) inEscherichia coli

The pathways for biosynthesis of pyrimidines, L-arginine and the polyamines are intimately interrelated in many microorganisms. We discovered in this study that growth of wild-typeEscherichia coli in low-water-activity minimal media is inhibited by the addition of uracil. Uracil sensitivity was observed irrespective of whether the dissolved solute(s) contributing to decreased water activity was ionic (e.g. NaCl, K2SO4), nonionic and impermeable (e.g. sucrose), nonionic and freely permeable (e.g. glycerol), or any mixture of these types. A mutant resistant to such growth inhibition was isolated and was shown to harbour a bradytrophic mutation inargA, the gene encoding the first step in the L-arginine biosynthetic pathway. Mutations inargR, whose product is the aporepressor of the same pathway, or exogenous supplementation with L-arginine or L-citrulline, also conferred resistance to uracil inhibition in low-water-activity media. A similar uracil-sensitivity phenotype, which was reversible byargA, argR, or L-arginine addition, was exhibited even in media with a more moderate reduction in water activity in two different situations: for aspeC mutant (which is defective in the enzyme ornithine decarboxylase required for biosynthesis of the polyamines) and for the wild-type strain in media additionally supplemented with L-ornithine. On the basis of these observations, we propose a model in which high cytoplasmic levels of the intermediary metabolite L-ornithine are inhibitory to growth ofE. coli in media of low water activity. Our results also provide the first evidence for the existence of a third component of physiological water stress, which is elicited by both impermeable and permeable dissolved solutes (the other two known components are ionic stress, which is elicited only by ionic solutes, and osmotic stress, which is elicited only by impermeable solutes either ionic or nonionic). We propose the term anhydrotic stress to refer to this novel component of water stress.