Osmoregulation in Bacteria and Transport of Onium Compounds

Bacteria are able to adapt to environmental changes and generally respond to increases in the osmotic pressure of their surroundings by elevating the intracellular concentrations of osmoprotective compounds. Besides potassium, the most prevalent cellular cation, the preferred solute is glycine betaine which is accumulated either by uptake from the environment or via synthesis from choline. In Escherichia coli osmoregulatory uptake of glycine betaine is mediated by two transport systems, the constitutive low-affinity ProP system (K m = 40 μM), and the osmotically inducible ProU high-affinity system (K w = 1 μM). ProP is a single polypeptide and the energy for substrate transport is provided by the proton motive force. ProU is a periplasmic-binding-protein-dependent system encoded by three structural genes, proV, proW and proX