REGULATION OF COMPATIBLE SOLUTE ACCUMULATION IN BACTERIA

In their natural habitats, microorganisms are often exposed to osmolal ity changes in the environment. The osmotic stress must be sensed and converted into an activity change of specific enzymes and transport pr oteins and/or it must trigger their synthesis such that the osmotic im balance can be rapidly restored. On the basis of the available literat ure, we conclude that representative Gram-negative and Grampositive ba cteria use different strategies to respond to osmotic stress. The main focus of this paper is on the initial response of bacteria to hyper- and hypoosmotic conditions, and in particular the osmosensing devices that allow the cell to rapidly activate and/or to synthesize the trans port systems necessary for uptake and excretion of compatible solutes. The experimental data allow us to discriminate the transport systems- by the physicochemical parameter that is sensed, which can be a change in external osmotic pressure, turgor pressure, membrane strain, inter nal osmolality and/or concentration of specific signal molecule. We al so evaluate the molecular basis for osmosensing by reviewing the uniqu e structural features of known osmoregulated transport systems.