Adaptation of Escherichia coli to high osmolarity environments: Osmoregulation of the high-affinity glycine betaine transport system ProU

Abstract: A sudden increase in the osmolarity of the environment is highly detrimental to the growth and survival of Fscherichia coli and Salmonella typhimurium since it triggers a rapid efflux of water from the cell, resulting in a decreased turgor. Changes in the external osmolarity must therefore be sensed by the microorganisms and this information must be converted into an adaptation process that aims at the restoration of turgor. The physiological reaction of the cell to the changing environmental condition is a highly coordinated process. Loss of turgor triggers a rapid influx of K⁺ ions into the cell via specific transporters and the concomitant synthesis of counterions, such as glutamate. The increased intracellular concentration of K⁺-glutamate allows the adaptation of the cell to environments of moderately high osmolarities. At high osmolarity, K⁺-glutamate is insufficient to ensure cell growth, and the bacteria therefore replace the accumulated K⁺ ions with compounds that are less d eleterious for the cell's physiology. These compatible solutes include polyoles such as trehalose, amino acids such as proline, and methyl-amines such as glycine betaine. One of the most important compatible solutes for bacteria is glycine betaine. This potent osmoprotectant is widespread in nature, and its intracellular accumulation is achieved through uptake from the environment or synthesis from its precursor choline. In this overview, we discuss the properties of the high-affinity glycine betaine transport system ProU and the osmotic regulation of its structural genes.     

DNA bridging and antibridging: a role for bacterial nucleoid-associated proteins in regulating the expression of laterally acquired genes

Horizontal DNA transfer plays a major role in the evolution of bacteria. It allows them to acquire new traits rapidly and these may confer fitness advantages as the bacteria compete with others in the environment. Historically, the mechanisms of horizontal DNA transfer, chiefly conjugation, transformation and transduction, have received a great deal of attention. Less attention has been focused on the regulatory problems that may accompany the acquisition of new genes by lateral routes. How are these genes integrated into the existing regulatory circuits of the cell? Does a process of 'plug-and-play' operate, or are the new genes silenced pending the evolution of regulatory mechanisms that make their expression not only safe but also beneficial to both the gene and its new host? Recent research shows that bacterial nucleoid-associated proteins such as H-NS, HU and Fis are important contributors to the processes of regulatory integration that accompany horizontal gene transfer. A key emerging theme is the antagonism that exists between the DNA–protein–DNA bridging activity of the H-NS repressor and the DNA-bending and DNA-wrapping activities of regulatory proteins that oppose H-NS.     

Osmoregulation of the HtrA (DegP) protease of Escherichia coli: an Hha-H-NS complex represses HtrA expression at low osmolarity

The HtrA protein of Escherichia coli is a heat-shock inducible periplasmic protease, essential for bacterial survival at high temperatures. Expression of htrA gene depends on the alternative factor sigmaE and on the two-component regulatory system Cpx. These regulators systems respond, among others factors, to overproduction of misfolded proteins in the periplasm or to high level synthesis of various extracytoplasmic proteins. We describe in this report the osmoregulation of the expression of htrA gene. Low osmolarity conditions result in htrA repression. We report, as well, the role of the nucleoid associated proteins H-NS and Hha in the repression of htrA expression at low osmolarity.     

Characterization of the smtA gene encoding an S-adenosylmethionine-dependent methyltransferase of Escherichia coli

Abstract The mukB operon is located at 21 min on the Escherichia coli chromosome and seems to consist of four genes, orf30 ( smtA ), mukF , mukE , and mukB . Based on sequence similarity, the promoter-proximal gene, orf30 ( smtA ), could encode an S-adenosylmethionine-dependent methyltransferase. The smtA gene is not essential for cell growth and its expression is positively regulated by H-NS, an Escherichia coli histone-like protein.