Full text loading...
Chapter 7 : Cellular Response to Heat Shock and Cold Shock
Category: Microbial Genetics and Molecular Biology
Ebook: Choose a downloadable PDF or ePub file. Chapter is a downloadable PDF file. File must be downloaded within 48 hours of purchase
This chapter reviews the molecular response of bacteria to shifts in either high or low temperatures. It discusses the inputs to each response, the outputs needed to cope with the sudden stress, and the molecular circuitry that controls these stress responses, and reviews the strategies utilized to cope with sudden heat stress or cold shock by mesophilic bacteria, focusing on Escherichia coli, the most completely studied bacterial species. The chapter talks about the relationship between temperature and the steady-state growth rate for this organism, and considers how E. coli responds to shift to both high and low temperatures, with a goal of integrating our knowledge about each response. Although the focus is on E. coli, the strategies used by E. coli with those used by Bacillus subtilis, are compared. When cells are shifted within the normal growth range, there is little or no lag in adaptation to the new growth rate and neither a heat shock response (HSR) nor a cold shock response (CSR) is elicited. The demonstration that overproduction of a variety of unfolded proteins triggers the HSR without temperature shift solidified the idea that such molecules serve as a signal controlling the response. Molecular understanding of this signaling mechanism is described in the circuitry section. RhlB is present during cold shock and it is not clear whether degradosomes contain both helicases or whether different populations of degradosomes contain one or the other. More DnaK is associated with the degradosome during cold shock.
Key Concept Ranking
- CssRS Two-Component Regulatory System
Growth rate of E. coli B/r as a function of temperature. The specific growth rate (k, hour–1), log scale, is plotted against the inverse of absolute temperature (K). Individual datum points are marked with degrees Celsius: •, in a rich medium; ○, in a glucose-minimal medium. (Reprinted from Herendeen, S. L., R. A. VanBogelen, and F. C. Neidhardt. 1979. Levels of major proteins of Escherichia coli during growth at different temperatures. J. Bacteriol. 139:185–194, with permission of the publisher. Copyright 1979 by the American Society for Microbiology.)
Rates of HSP synthesis during temperature upshift and downshift. (A) HSP production during a temperature shift from 30° to 42°C reveals three distinct phases: induction, adaptation, and steady state. (B) Repression of HSP production during a temperature shift from 42° to 30°C.
Wiring diagram of σ32 regulation. There are three primary modes of regulation as follows: (i) excess free DnaK/J and GroEL/S chaperones directly bind to and inactivate σ32; (ii) the FtsH protease degrades σ32, with chaperones participating in this process; and (iii) temperature directly controls the rate of σ32 translation. Misfolded proteins titrate chaperones from these regulatory roles, allowing active σ32 to increase the synthesis of chaperones and proteases during conditions where they are needed. (Reprinted from Guisbert, E., T. Yura, V. A. Rhodius, and C. A. Gross. 2008. Convergence of molecular, modeling, and systems approaches for an understanding of the Escherichia coli heat shock response. Microbiol. Mol. Biol. Rev. 72:545–554, with permission of the publisher. Copyright 2008 by the American Society for Microbiology.)
Summary of the major transcription factors and their regulatory systems involved in the HSR of B. subtilis. (A) The immediate response is controlled by σB, which is negatively regulated by an anti-σ factor. During heat shock, signals release the anti-σ factor, thereby allowing σB to activate transcription of the heat shock regulon. (B) During heat and secretion stress, the CssRS two-component regulatory system recognizes unfolded proteins at the cell wall-membrane interface activating transcription of the proteases, htrA and htrB. (C) The inhibition of the repressor HrcA leads to transcription of the chaperone system GroEL/S. During heat shock, GroEL/S is occupied with unfolded proteins, unable to renature HrcA molecules, thereby decreasing active HrcA. (D) The degradation of the repressor CtsR leads to transcription of ClpP peptidase. During heat shock, the arginine kinase MscB is released from binding to the protease complex ClpCP upon an increase in unfolded proteins. This release allows MscB to phosphorylate CtsR, facilitating its recognition by ClpCP for degradation.
Summary of the major outputs during the CSR. (A) Regulation and modulation of the 30S and 70S ribosomal subunits by various proteins translated during cold shock; (B) formation and regulation of the degradosome; (C) production of trehalose, a major osmoprotectant during cold shock; (D) RNA chaperone activity and RNA protection by the RNA-binding cold shock proteins, CspA, CspB, CspE, CspG, and CspI; (E) regulation of RNase III activity by YmdB; and (F) transcriptional repression and DNA negative supercoiling by HNS and gyrase.
Expression regulation of infA and the genes in the metY operon region in the chromosome. (A) Several cold-shock induced genes are located downstream of the metY gene. Transcription occurs at several promoters in the region (Ishii et al., 1984 ; Regnier and Portier, 1986 ; Portier et al., 1987 ; Regnier and Grunberg-Manago, 1989 ; Granston et al., 1990 ; Regnier and Grunberg-Manago, 1990 ; Zaslaver et al., 2006 ), designated as “P”; however, transcription termination, designated as “T,” prevents robust transcription of downstream genes. During cold shock, the induction of the Csps prevents transcription termination, allowing for increased expression of downstreams genes, including pnp. Below the schematic of the genomic architecture are the documented mRNA transcripts that contain pnp. The RNase III cleavage site is designated as R III (Regnier and Grunberg-Manago, 1989 ). (B) An RNase III site exists between the two promoters driving infA expression. The regulatory expression components of infA are shown with the designations as described previously (Cummings et al., 1991 ; Giangrossi et al., 2007 ).
Localization and functional classification of σ32 regulon members