EVIDENCE FOR THE EXISTENCE OF A NOVEL COMPONENT OF BIOLOGICAL WATER-STRESS (ANHYDROTIC STRESS) IN ESCHERICHIA-COLI

The pathways for biosynthesis of pyrimidines, L-arginine and the polya mines are intimately interrelated in many microorganisms. We discovere d in this study that growth of wild-type Escherichia 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, K2S O4), nonionic and impermeable (e.g. sucrose), nonionic and freely perm eable (e.g. glycerol), or any mixture of these types. A mutant resista nt to such growth inhibition was isolated and was shown to harbour a b radytrophic mutation in argA, the gene encoding the first step in the L-arginine biosynthetic pathway. Mutations in argR, whose product is t he aporepressor of the same pathway, or exogenous supplementation with L-arginine or L-citrulline, also conferred resistance to uracil inhib ition in low-water-activity media. A similar uracil-sensitivity phenot ype, which was reversible by argA, argR, or L-arginine addition, was e xhibited even in media with a more moderate reduction in water activit y in two different situations: for a speC mutant (which is defective i n the enzyme ornithine decarboxylase required for biosynthesis of the polyamines) and for the wild-type strain in media additionally supplem ented with L-ornithine. On the basis of these observations, we propose a model in which high cytoplasmic levels of the intermediary metaboli te L-ornithine are inhibitory to growth of E. coli in media of low wat er activity. Our results also provide the first evidence for the exist ence of a third component of physiological water stress, which is elic ited by both impermeable and permeable dissolved solutes (the other tw o known components are ionic stress, which is elicited only by ionic s olutes, and osmotic stress, which is elicited only by impermeable solu tes either ionic or nonionic). We propose the term anhydrotic stress t o refer to this novel component of water stress.