Structure and function of osmoregulated ABC transporters

In their natural habitats, microorganisms are often exposed to osmolality changes in the environment. For instance, soil bacteria such as Bacillus subtilis are alternately exposed to peri- ods of drought and rain, to which they have to adapt. Since the cytoplasmic membrane of bac- teria is highly permeable to water but forms an effective barrier for most solutes present in the medium and metabolites present in the cyto- plasm, water will flow out of the cell when the outside osmolality increases (‘osmotic upshift’). As a consequence of an osmotic upshift, the turgor pressure will decrease and ultimately the cell may plasmolyze. Upon osmotic down- shift, water will flow into the cell and thereby increase the turgor pressure. Maintenance of cell turgor is a prerequisite for almost any form of life, as it provides a mechanical force for the expansion of the cell envelope and regulates cell growth.