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Chapter 9 : Osmotic Stress
Category: Microbial Genetics and Molecular Biology
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Osmotic stress tolerance mechanisms determine whether bacteria survive or grow because osmotic stress profoundly affects the structure, physics, and chemistry of bacterial cells. In vitro studies have shown that K+ glutamate differentially modulates transcription mediated by the σ70 and σS RNA polymerases of Escherichia coli, the latter being central to many stress response. Progress toward understanding the structural changes associated with the opening of representative channels is discussed in this chapter. The study of osmoregulatory proteins is motivated partly by a desire to understand how cells sense osmotic pressure (osmosensing). During the last decade, representative osmoregulatory transporters and mechanosensitive (MS) channels have been shown to both sense osmotic pressure changes (osmosensing) and respond by modulating transmembrane solute distribution (osmoregulation) after purification and reconstitution in proteoliposomes. The osmoregulation of protein activity is discussed by focusing on representative proteins that have been studied. In many bacteria, the proportion of anionic phospholipids increases and the fatty acid composition changes with cultivation at high salinity. For E. coli, growth at high osmolality increases the proportion of CL at the expense of PE without changing the proportion of PG or the fatty acid composition. Interest in the osmoregulation of transcription was stimulated by a desire to understand how osmolality can direct gene expression. Studies focused on promoter identification and the identification of transcriptional regulatory proteins were complicated by multiple factors, including transcription that depends on multiple promoters and σ factors.
Key Concept Ranking
- Nuclear Magnetic Resonance Spectroscopy
Organic solutes that accumulate in osmotically stressed bacteria A. Glutamate accumulates as a K+ counterion. Compatible solutes such as trehalose, glycine betaine, proline, and ectoine are synthesized from endogenous substrates or transported into the cytoplasm. Extended lists of solutes that accumulate in response to osmotic and other stresses are provided elsewhere (Roberts, 2006 ).
Bacterial osmoregulatory mechanisms. Osmotic shifts cause water to flow across the phospholipid bilayer. Aqua-porins may accelerate osmotic swelling or shrinkage by contributing to that passive flux. When the external osmolality increases, K+ is immediately pumped into the cytoplasm and organic anions of metabolic origin may accumulate as counterions. Compatible solutes are synthesized from endogenous substrates. If osmoprotectants are available in the external medium, they are transported into the cell by osmoprotectant transporters where they act as, or are converted to, compatible solutes. This may attenuate the K+ response. When the external osmolality decreases abruptly, MS channels open to release solutes and cell lysis is averted.
MS channel MscS. Crystal structures provide valuable insights into osmosensory mechanisms even though they do not directly represent the structures of molecules in strained biological membranes. This figure represents the crystal structures of native MS channel MscS from E. coli (left, Protein Data Bank Identification Number [PDB ID] 2OAU [Bass et al., 2002 ]) and its variant MscS-A106V (right, PDB ID 2VV5 [Wang et al., 2008 ]). Each is shown as it would appear from the membrane plane (top) and as it would appear from the periplasm (bottom). Residues 94–112 of each subunit, colored black, constitute the pore-lining α-helices. The crystal structure of the native protein (left) is believed to represent the closed channel whereas the crystal structure of MscS-A106V (right) is believed to represent an open conformation (see further discussion in the text). Crystal structures that are believed to represent closed and expanded intermediate states of channel MscL have also been published (PDB ID 2OAR [Chang et al., 1998 ] and PDB ID 3HZQ [Liu et al., 2009 ], respectively).