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Chapter 18 : Response to Osmotic Stress in a Haloarchaeal Genome: a Role for General Stress Proteins and Global Regulatory Mechanisms
Category: Applied and Industrial Microbiology; Environmental Microbiology
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Response to Osmotic Stress in a Haloarchaeal Genome: a Role for General Stress Proteins and Global Regulatory Mechanisms, Page 1 of 2< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781555815813/9781555814229_Chap18-1.gif /docserver/preview/fulltext/10.1128/9781555815813/9781555814229_Chap18-2.gif
Haloarchaea are highly specialized for life under extreme conditions. They can grow in saturated sodium chloride concentrations, and most of them require a minimum of 1.5 to 3M NaCl and 0.005 to 0.04M magnesium salts for growth. The description of different haloarchaeal genomes has created new means of understanding the biology of this group of organisms. The transcriptional response to different osmotic conditions appears to be quite widespread over the Haloferax volcanii genome. The authors have distinguished specific high-salt and low-salt responses, as well as more general stress behaviors such as responses to both low and high salt and to both osmotic stress and heat shock, which may help to understand the osmoadaptation processes and the connection between different networks of adaptation to environmental conditions. Organization of genes in gene clusters, not necessarily cotranscribed nor organized in operons, may allow global regulatory mechanisms such as DNA topology to play an effective role in adaptation to the environment. A general stress behavior, with response to heat shock, has also been observed for certain sequences responding to low-salt conditions, while it has not been observed for specific high-salt responses. Furthermore, the overlap of responses to heat shock and osmotic stress, particularly hypoosmotic stress, seems to be a frequent feature within the haloarchaeal genome. In fact, in haloarchaea, both hypoosmotic stress and heat shock would promote haloarchaeal protein destabilization and aggregation.
Key Concept Ranking
- General Stress Response
Transcriptional map of the Haloferax volcanii genome. The figure shows an overview of differentially transcribed regions in the chromosome and the pHV4 megaplasmid. Symbols are not drawn to scale and represent a summary of the most representative responses. Genome transcription analysis was mainly based on the use of cDNA probes to hybridize against restriction fragments of the cosmid clones of a genomic library of the organism ( Charlebois et al., 1991 ). Transcriptionally induced regions, over the whole genome, in cells growing in low (12% salts) and high (30% salts) salinity conditions were described previously ( Ferrer et al., 1996 ; Juez, 2004 ). Two genomic stretches, indicated by boxes, have been the subject of a more extensive analysis through the detection of transcripts arising from genomic regions (by Northern blot hybridization) and including the long-term response in cultures growing at different salinities (8, 10, 12, 15, 20, 25, 30, and 35% salt medium), as well as the immediate response after a downshift (30 to 10% salt medium), an upshift (10 to 30% salt medium) and a heat shock (37 to 55°C in 20% salt medium, indicated by asterisks) ( Juez, 2004 ; Soria and Juez, unpublished). A mixture of salts in the proportions found in seawater (30% salts containing in w/v: 23.4% NaCl, 1.95% MgCl2, 2.9% MgSO4, 0.12% CaCl2, 0.6% KCl, 0.03% NaHCO3, and 0.075% NaBr) was used, as described previously ( Rodríguez-Valera et al., 1980 ; Mojica et al., 1997 ). The map also includes minor and major signals (indicated as empty and solid circles, respectively) of heat-shock responses, as well as FII AT-rich regions containing IS elements (indicated by solid black bars below the distance scale), previously reported by Trieselmann and Charlebois (1992 ). A kilobase-pair distance scale and cosmid clones representing the genome are shown.
Protein sequence alignment of the DnaK chaperone. Conserved domains among the different types of organisms (external dashed boxed) and distinctive amino acid substitutions for haloarchaea and other Archaea (internal boxes) are indicated. A consensus sequence is also shown. For simplicity, a limited central region of the protein and sequences from representatives of different archaeal and bacterial genera are shown. Conserved domains (domains 4 to 8) correspond to Hsp70 signature (TVPAYFND), connect 1 (NEPTAA), phosphate 2 (LGGGTFD), Hinge residue (E), and nuclear localization signal (NLS), respectively.
Protein sequence alignment of the DnaJ cochaperone. A central region of the protein, including the zinc-finger sequences (CxxCxGxG) (indicated by external dashed boxes), is shown. Distinctive amino acid substitutions for haloarchaea and other Archaea (internal boxes) and a consensus sequence are also indicated.