Genetics and physiology of osmoregulation in Escherichia coli: mechanisms of osmotic regulation of trancription of the proU Operon

Abstract

The ability of organisms to adapt to fluctuations in growth conditions is of fundamental importance newlinefor their survival. One of the factors that directly affects growth of plants and bacteria is the osmolarity of newlineextracellular environment, and these organisms exhibit remarkable similarity in their response to hyperosmotic newlinestress conditions. Unicellular bacteria like Escherichia coli and Salmonella typhimurium are able newlineto survive and grow in a wide range of osmolarities by carrying out a set of active processes, and this newlinephenomenon of adaptation is known as osmoregulation. In these organisms, long-term adaptation to newlinehyper-osmotic stress involves the accumulation of certain solutes, called compatible solutes, such as newlinetrehalose, glutamate, proline and glycine betaine. Compatible solutes are so called because they exhibit newlineminimal inhibitory effect on cellular metabolism, even when accumulated to concentrations of several newlinehundreds of milimolar in the cytoplasm; amongst the compatible solutes, glycine betaine has been shown newlineto be the most potent in alleviating the inhibitory effects of osmotic stress. newlineThe proU locus in E coli and S. typhimurium encodes an osmotically inducible transport system newlinefor active uptake of glycine betaine and of proline, and thus plays an important role in osmoregulation in newlinethese organisms~ Molecular and genetic analysis has shown that proU is an operon consisting of three newlinegenes that encodes a binding-protein-dependent transport system. Studies on cis-regulation of the proU newlineoperon in E coli have shown the presence of two promoters P1 and P2, along with a negative regulatory newlineelement (NRE) present downstream of P2 and extending into the first structural gene proV. Promoter-lac newlinefusion studies indicated that expression from each of the two promoter P1 and P2 is separately osmotically newlineinducible, by about five- and eight-fold respectively, whereas deletion of the NRE results in derepression newlineof proU expression at low osmolarity.