Cold shock response of Bacillus subtilis: isoleucine-dependent switch in the fatty acid branching pattern for membrane adaptation to low temperatures

Transposon disruption of the complex I NADH oxidoreductase gene (snoD) in Staphylococcus aureus is associated with reduced susceptibility to the microbicidal activity of thrombin-induced platelet microbicidal protein 1

The cationic molecule thrombin-induced platelet microbicidal protein 1 (tPMP-1) exerts potent activity against Staphylococcus aureus. We previously reported that a Tn551 S. aureus transposon mutant, ISP479R, and two bacteriophage back-transductants, TxA and TxB, exhibit reduced in vitro susceptibility to tPMP-1 (tPMP-1(r)) compared to the parental strain, ISP479C (V. Dhawan, M. R. Yeaman, A. L. Cheung, E. Kim, P. M. Sullam, and A. S. Bayer, Infect. Immun. 65:3293-3299, 1997). In the current study, the genetic basis for tPMP-1(r) in these mutants was identified. GenBank homology searches using sequence corresponding to chromosomal DNA flanking Tn551 mutant strains showed that the fourth gene in the staphylococcal mnh operon (mnhABCDEFG) was insertionally inactivated. This operon was previously reported to encode a Na(+)/H(+) antiporter involved in pH tolerance and halotolerance. However, the capacity of ISP479R to grow at pH extremes and in high NaCl concentrations (1 to 3 M), coupled with its loss of transmembrane potential (DeltaPsi) during postexponential growth, suggested that the mnh gene products are not functioning as a secondary (i.e., passive) Na(+)/H(+) antiporter. Moreover, we identified protein homologies between mnhD and the nuo genes of Escherichia coli that encode components of a complex I NADH:ubiquinone oxidoreductase. Consistent with these data, exposures of tPMP-1-susceptible (tPMP-1(s)) parental strains (both clinical and laboratory derived) with either CCCP (a proton ionophore which collapses the proton motive force) or pieracidin A (a specific complex I enzyme inhibitor) significantly reduced tPMP-induced killing to levels seen in the tPMP-1(r) mutants. To reflect the energization of the gene products encoded by the mnh operon, we have renamed the locus sno (S. aureus nuo orthologue). These novel findings indicate that disruption of a complex I enzyme locus can confer reduced in vitro susceptibility to tPMP-1 in S. aureus.

Chill activation of compatible solute transporters in Corynebacterium glutamicum at the level of transport activity

The gram-positive soil bacterium Corynebacterium glutamicum harbors four osmoregulated secondary uptake systems for compatible solutes, BetP, EctP, LcoP, and ProP. When reconstituted in proteoliposomes, BetP was shown to sense hyperosmotic conditions via the increase in luminal K(+) and to respond by instant activation. To study further putative ways of stimulus perception and signal transduction, we have investigated the responses of EctP, LcoP, and BetP, all belonging to the betaine-carnitine-choline transporter family, to chill stress at the level of activity. When fully activated by hyperosmotic stress, they showed the expected increase of activity at increasing temperature. In the absence of osmotic stress, EctP was not activated by chill and LcoP to only a very low extent, whereas BetP was significantly stimulated at low temperature. BetP was maximally activated at 10 degrees C, reaching the same transport rate as that observed under hyperosmotic conditions at this temperature. A role of cytoplasmic K(+) in chill-dependent activation of BetP was ruled out, since (i) the cytoplasmic K(+) concentration did not change significantly at lower temperatures and (ii) a mutant BetP lacking the C-terminal 25 amino acids, which was previously shown to have lost the ability to be activated by luminal K(+), was fully competent in chill sensing. When heterologously expressed in Escherichia coli, BetP did not respond to chill stress. This may indicate that the membrane in which BetP is inserted plays an important role in chill activation and thus in signal transduction by BetP, different from the previously established K(+)-mediated process.

Comparative study of tributyltin toxicity on two bacteria of the genus Bacillus

Tributyltin is a potent biocide mainly used in marine anti-fouling paints. Owing to its widespread distribution in coast areas and its high toxicity to aquatic organisms, the use of this compound is generally restricted and under government regulation. Despite of that, it persists in the aquatic environment. Organotins used in industry have also been detected in terrestrial environments. The persistence and high lipophilicity explain bioaccumulation. The role of bacteria in recycling organic matter prompted us to study the interaction of tributyltin with two ubiquitous bacilli, B. stearothermophilus and B. subtilis, proposed as biological indicators of pollutants with ecological impact. These bacteria have been used as suitable models for the study of toxicity mechanisms of unselective lipophilic compounds (e.g., DDT and endosulfan). Drug effects on growth parameters, oxygen consumption and membrane organization were assessed. Bacteria growth in a liquid complex medium was disturbed by concentrations of TBT as low as 25 nM (8 microgL(-1)), close to the concentration in polluted environments. The respiratory activity is affected by TBT in both microorganisms. Membrane organization, assessed by fluorescence polarization of two fluidity probes, 1,6-diphenyl-1,3,5-hexatriene (DPH) and a propionic acid derivative (DPH-PA), was also perturbed by the xenobiotic. Alterations on growth, oxygen consumption and physical properties of membrane lipids are stronger in B. stearothermophilus as compared to B. subtilis. A putative relationship between growth inhibition and respiratory activity impairment induced by TBT and its effects on the physical behaviour of bacterial membrane lipids is suggested.

Comprehensive characterization of the contribution of individual SigB-dependent general stress genes to stress resistance of Bacillus subtilis

The sigma(B)-dependent general stress regulon of Bacillus subtilis comprises more than 150 members. Induction of this regulon by imposition of environmental or metabolic stress confers multiple, nonspecific, and preemptive stress resistance to nongrowing, nonsporulated cells of B. subtilis. In this study we performed a regulon-wide phenotypic screening analysis to determine the stress sensitivity profiles of 94 mutants defective in candidate members of the general stress regulon that were previously identified in our transcriptional profiling study of the general stress response of B. subtilis. The phenotypic screening analysis included analysis of adaptation to a growth-inhibiting concentration of ethanol (10%, vol/vol) or NaCl (10%, wt/vol), severe heat shock (54 degrees C), and low temperature (survival at 4 degrees C and growth at 12.5 degrees C). Surprisingly, 85% of the mutants tested displayed increased sensitivity at an alpha confidence level of < or =0.01 to at least one of the four stresses tested, and 62% still exhibited increased sensitivity at an alpha of < or =0.001. In essence, we were able to assign 63 genes (28 genes with an alpha of < or =0.001) to survival after ethanol shock, 37 genes (28 genes with an alpha of < or =0.001) to protection from NaCl shock, 34 genes (24 genes with an alpha of < or =0.001) to survival at 4 degrees C, and 10 genes (3 genes with an alpha of < or =0.001) to management of severe heat shock. Interestingly, there was a substantial overlap between the genes necessary for survival during ethanol shock and the genes necessary for survival at 4 degrees C, and there was also an overlap between genes required for survival during ethanol shock and genes required for survival during NaCl shock. Our data provide evidence for the importance of the sigma(B) regulon at low temperatures, not only for growth but also for survival. Moreover, the data imply that a secondary oxidative stress seems to be a common component of the severe stresses tested.

The Bacillus subtilis desaturase: a model to understand phospholipid modification and temperature sensing

Most fatty acid desaturases are members of a large superfamily of integral membrane, O2-dependent, iron-containing enzymes that insert double bonds into previously synthesized fatty acyl chains. The cold shock-induced, membrane-bound desaturase from Bacillus subtilis (Delta5-Des) uses existing phospholipids as substrates to introduce a cis-double bond at the fifth position of the fatty acyl chain. While essentially no three-dimensional structural information is available for these difficult-to-purify enzymes, experimental analysis of the topology of Delta5-Des has provided a model that might be extended to most acyl-lipid desaturases. In addition, studies of the cold-induced expression of Delta5-Des led to the identification of a two-component system composed of a membrane-associated kinase, DesK, and a transcriptional regulator, DesR, which stringently controls the transcription of the des gene, coding for the desaturase. A model for sensing and transduction of low-temperature signals has emerged from our results, which we discuss in the context of transcriptional regulation of membrane lipid fluidity homeostasis.

Omega-3 fatty acids in cellular membranes: a unified concept

The Omega-3 fatty acid DHA (docosahexaenoic acid, 22:6) and its sister molecule EPA (eicosapentaenoic acid, 20:5) are highlighted here. These highly unsaturated fatty acids are widespread in nature, especially in the marine environment, and are essential in membranes ranging from deep sea bacteria to human neurons. Studies of DHA/EPA in bacteria have led to a working model on the structural roles of these molecules and are described in this review. The main points are: (a) genomic analysis shows that genes encoding the DHA/EPA pathways are similar, supporting the idea that structural roles in bacteria might be similar, (b) biochemical analysis shows that DHA and EPA are produced in bacteria by a polyketide process distinct from the pathway of plants and animals; this allows DHA and EPA to be produced in anaerobic or oxygen-limited environments, (c) regulatory systems triggered by temperature and pressure have been identified and studied, and add to the understanding of the roles of these molecules, (d) DHA/EPA bacteria are located almost exclusively in the marine environment, raising the prospect of an important linkage between membrane processes and marine conditions, (e) physiological studies of an EPA recombinant of E. coli show that EPA phospholipids contribute essential fluidity to the bilayer and that an EPA-enriched membrane supports a respiratory lifestyle dependent on proton bioenergetics; the EPA recombinant displays other physiological properties likely attributed to high levels of EPA in the bilayer, and (f) chemical studies such as chemical dynamic modeling support the idea that DHA and presumably EPA contribute hyperfluidizing properties to the membrane. We hypothesize that DHA/EPA phospholipids contribute fluidity and other properties to the bilayer which distinguish these highly unsaturated chains from monounsaturates and polyunsaturates such as 18:2 and 18:3. We further hypothesize that the structural properties of DHA/EPA functioning in bacteria are also harnessed by higher organisms for enhancing crucial membrane processes including photosynthesis and energy transduction.

Treatment with oxidizing agents damages the inner membrane of spores of Bacillus subtilis and sensitizes spores to subsequent stress

AIMS

To determine if treatment of Bacillus subtilis spores with a variety of oxidizing agents causes damage to the spore's inner membrane.

METHODS AND RESULTS

Spores of B. subtilis were killed 80-99% with wet heat or a variety of oxidizing agents, including betadine, chlorine dioxide, cumene hydroperoxide, hydrogen peroxide, Oxone, ozone, sodium hypochlorite and t-butylhydroperoxide, and the agents neutralized and/or removed. Survivors of spores pretreated with oxidizing agents exhibited increased sensitivity to killing by a normally minimal lethal heat treatment, while spores pretreated with wet heat did not. In addition, spores treated with wet heat or the oxidizing agents, except sodium hypochlorite, were more sensitive to high NaCl in plating media than were untreated spores. The core region of spores treated with at least two oxidizing agents was also penetrated much more readily by methylamine than was the core of untreated spores, and spores treated with oxidizing agents but not wet heat germinated faster with dodecylamine than did untreated spores. Spores of strains with very different levels of unsaturated fatty acids in their inner membrane exhibited essentially identical resistance to oxidizing agents.

CONCLUSIONS

Treatment of spores with oxidizing agents has been suggested to cause damage to the spore's inner membrane, a membrane whose integrity is essential for spore viability. The sensitization of spores to killing by heat and to high salt after pretreatment with oxidizing agents is consistent with and supports this suggestion. Presumably mild pretreatment with oxidizing agents causes some damage to the spore's inner membrane. While this damage may not be lethal under normal conditions, the damaged inner membrane may be less able to maintain its integrity, when dormant spores are exposed to high temperature or when germinated spores are faced with osmotic stress. Triggering of spore germination by dodecylamine likely involves action by this agent on the spore's inner membrane allowing release of the spore core's depot of dipicolinic acid. Presumably dodecylamine more readily alters the permeability of a damaged inner membrane and thus more readily triggers germination of spores pretreated with oxidizing agents. Damage to the inner spore membrane by oxidizing agents is also consistent with the more rapid penetration of methylamine into the core of treated spores, as the inner membrane is likely the crucial permeability barrier to methylamine entry into the spore core. As spores of strains with very different levels of unsaturated fatty acids in their inner membrane exhibited essentially identical resistance to oxidizing agents, it is not through oxidation of unsaturated fatty acids that oxidizing agents kill and/or damage spores. Perhaps these agents work by causing oxidative damage to key proteins in the spore's inner membrane.

SIGNIFICANCE AND IMPACT OF THE STUDY

The more rapid heat killing and germination with dodecylamine, the greater permeability of the spore core and the osmotic stress sensitivity in outgrowth of spores pretreated with oxidizing agents is consistent with such agents causing damage to the spore's inner membrane, even if this damage is not lethal under normal conditions. It may be possible to take advantage of this phenomenon to devise improved, less costly regimens for spore inactivation.

Global transcription profiles and intracellular pH regulation measured in Bacillus licheniformis upon external pH upshifts

For optimization of propagation conditions for an industrially used Bacillus licheniformis, this study examines the effect of transferring cells at the early-stationary growth phase (pH 5.3) to fresh growth medium at pH 5.0-8.0. Intracellular pH (pH(i)) was measured on a single-cell level, using fluorescence ratio imaging microscopy after staining with 5(6)-carboxyfluorescein diacetate succinimidyl ester. Transcription profiles were determined using a genome DNA microarray. The optimum extracellular pH (pH(ex)) value for growth of B. licheniformis was found to be pH 7.0, resulting in the shortest lag phase, highest maximum specific growth rate and maximum biomass formation. An average pH gradient (Delta pH = pH(i) - pH(ex)) of approx. 1.0 was found in B. licheniformis 15 min after transfer to pH(ex) 5.0-8.0. Up-regulation of genes involved in sucrose uptake at pH 7.0 could be related to the optimum growth observed. Transcription profiles indicated that the organism was experiencing phosphate starvation upon transfer to pH 7.0 and pH 8.0. Mechanisms involved in pH(i) regulation appeared to include changes in fatty acid synthesis to yield a more rigid cell membrane structure at low pH(ex) values and conversion of pyruvate to acetoin instead of acetate for neutralization of low pH(ex) values.

Expression of the pyrG gene determines the pool sizes of CTP and dCTP in Lactococcus lactis

The pyrG gene from Lactococcus lactis encodes CTP synthase (EC 6.4.3.2), an enzyme converting UTP to CTP. A series of strains were constructed with different levels of pyrG expression by insertion of synthetic constitutive promoters with different strengths in front of pyrG. These strains expressed pyrG levels in a range from 3 to 665% relative to the wild-type expression level. Decreasing the level of CTP synthase to 43% had no effect on the growth rate, showing that the capacity of CTP synthase in the cell is in excess in a wild-type strain. We then studied how pyrG expression affected the intracellular pool sizes of nucleotides and the correlation between pyrG expression and nucleotide pool sizes was quantified using metabolic control analysis in terms of inherent control coefficients. At the wild-type expression level, CTP synthase had full control of the CTP concentration with a concentration control coefficient close to one and a negative concentration control coefficient of -0.28 for the UTP concentration. Additionally, a concentration control coefficient of 0.49 was calculated for the dCTP concentration. Implications for the homeostasis of nucleotide pools are discussed.

Cold induction of the Bacillus subtilis bkd operon is mediated by increased mRNA stability

Recently it has been demonstrated that the ptb - bcd - buk - lpdV - bkdAA - bkdAB - bkdB operon ( bkdoperon) of Bacillus subtilis, which encodes the enzymes that catalyze the degradation of branched-chain amino acids, is inducible by a temperature downshift from 37 to 18 degrees C. Deamination and oxidative decarboxylation of isoleucine generates 2-methyl-butyryl-CoA, which serves as the precursor of anteiso-branched fatty acid species. Most probably, the induction of this operon upon cold shock ensures an increase in the content of anteiso-branched fatty acids in the membrane lipids at low temperature, thus permitting maintenance of membrane fluidity at lower temperatures. In the present study, we have analyzed the mechanism of cold induction of the bkd operon and of four further cold-inducible transcriptional units in B. subtilis. We demonstrate that cold induction of these genes is mediated by an increase in the stability of the corresponding mRNAs. None of the promoters that control the five transcriptional units analyzed is actually cold-inducible. Furthermore, the results of this study indicate that the 5' leader regions are not involved in the cold-induced stabilization of the mRNAs. The structural elements that enhance mRNA stability must therefore be restricted to the 3'-ends and/or the coding regions.

Characterization and localization of fluorescent Pseudomonas cold shock protein(s) by monospecific polyclonal antibodies

Cold shock protein (CSP) from Pseudomonas fluorescens MTCC 103 and cold resistant protein (CRP) from its mutant CRPF8 of 14 and 35 kd, respectively were purified to homogeneity by HPLC. Polyclonal antibodies were raised against these proteins and the expression level was checked at different temperatures, i.e., 4, 10, 20, 30 and 37 C. Furthermore, morphological changes in P. fluorescens MTCC 103 and its mutant (CRPF8) were analyzed by transmission electron microscopy (TEM). Localization of CSP and CRP documented with immunoelectron microscopy, using colloidal gold particles conjugated with secondary antibodies being the probe were used. Nevertheless, the results of cytosolic localization of CSP and CRP were evident. Furthermore, the expression of CSP and CRP increased with decrease in temperature and the cell wall thickness of the mutant exhibited 2-fold increase, thus facilitating low temperature survival.

Genetic evidence for the temperature-sensing ability of the membrane domain of the Bacillus subtilis histidine kinase DesK

A decrease in environmental temperature leads to the synthesis of Delta5-unsaturated fatty acids in Bacillus subtilis by the fatty acid desaturase Des. Des is regulated by the two-component system DesKR. To understand the mechanism of cold signal perception and transduction by the membrane domain and the cytosolic domain of DesK, we expressed the cytosolic domain of DesK in trans under the control of a xylose-inducible promoter without the membrane domain. We performed growth experiments and a Northern blot analysis. Our results show that the kinase function of the cytosolic domain of DesK is temperature-independent, leading to a constitutive expression of the des gene. These findings support the conclusion that the membrane domain of DesK is the temperature-sensing element of the two-component system.

Chill induction of the SigB-dependent general stress response in Bacillus subtilis and its contribution to low-temperature adaptation

A variety of environmental and metabolic cues trigger the transient activation of the alternative transcription factor SigB of Bacillus subtilis, which subsequently leads to the induction of more than 150 general stress genes. This general stress regulon provides nongrowing and nonsporulated cells with a multiple, nonspecific, and preemptive stress resistance. By a proteome approach we have detected the expression of the SigB regulon during continuous growth at low temperature (15 degrees C). Using a combination of Western blot analysis and SigB-dependent reporter gene fusions, we provide evidence for high-level and persistent induction of the sigB operon and the SigB regulon, respectively, in cells continuously exposed to low temperatures. In contrast to all SigB-activating stimuli described thus far, induction by low temperatures does not depend on the positive regulatory protein RsbV or its regulatory phosphatases RsbU and RsbP, indicating the presence of an entirely new pathway for the activation of SigB by chill stress in B. subtilis. The physiological importance of the induction of the general stress response for the adaptation of B. subtilis to low temperatures is emphasized by the observation that growth of a sigB mutant is drastically impaired at 15 degrees C. Inclusion of the compatible solute glycine betaine in the growth medium not only improved the growth of the wild-type strain but rescued the growth defect of the sigB mutant, indicating that the induction of the general stress regulon and the accumulation of glycine betaine are independent means by which B. subtilis cells cope with chill stress.

The Bacillus subtilis acyl lipid desaturase is a delta5 desaturase

Bacillus subtilis was recently reported to synthesize unsaturated fatty acids (UFAs) with a double bond at positions delta5, delta7, and delta9 (M. H. Weber, W. Klein, L. Muller, U. M. Niess, and M. A. Marahiel, Mol. Microbiol. 39:1321-1329, 2001). Since this finding would have considerable importance in the double-bond positional specificity displayed by the B. subtilis acyl lipid desaturase, we have attempted to confirm this observation. We report that the double bond of UFAs synthesized by B. subtilis is located exclusively at the delta5 position, regardless of the growth temperature and the length chain of the fatty acids.

Idiomarina baltica sp. nov., a marine bacterium with a high optimum growth temperature isolated from surface water of the central Baltic Sea

Two bacterial strains isolated from the Baltic Sea, OS145T and OS146, were characterized on the basis of their physiological and biochemical features, their fatty acid profiles and their phylogenetic position based on 16S rDNA sequence analyses. The strains were isolated from the upperoxic water column of the central Baltic Sea. Phylogenetic analyses of the 16S rDNA gene sequences revealed a clear affiliation of the novel strains with members of the genus Idiomarina, of the Gammaproteobacteria. Closest sequence similarity was seen with Idiomarina abyssalis and Idiomarina zobellii (95-96%). The mean G + C content of the DNA of strains OS145T and OS146 was 49.7 mol%. Both strains were non-pigmented, Gram-negative, polarly flagellated organisms that were strictly aerobic. Growth of the strains was observed at salinities ranging from 0.8 to 10% NaCl. Temperature range for growth was rather broad and high for marine bacteria: both strains grew between 8 and 46 degrees C, showed good growth between 20 and 44 degrees C, and had an optimum between 30 and 40 degrees C. The fatty acids of the two strains were dominated by iso-branched fatty acids (54-80%), with a high abundance of C15:0 iso (36%), C16:1 omega7c, C17:o iso and C17:1 iso omega9c. Growth temperature (8-40 degrees C) influenced the fatty acid composition of the strains in a way that the content of iso-branching fatty acids increased with increasing temperatures, while the mono-unsaturated fattyacids increased with decreasing temperatures. Salinity (1.7-10% NaCl) had only a minor effect on the fatty acid composition. According to their morphology, physiology, fatty acid composition and 16S rDNA sequences, strains OS145T and OS146 fitted well into the genus Idiomarina, but could be easily distinguished from the recognized species of the genus. Because of their unique nature, it is proposed that the strains isolated from the Baltic Sea represent a novel species, for which the name Idiomarina baltica (type strain OS145T =DSM 15154T =LMG 21691T) is proposed.

Bacterial cold shock responses

As a measure for molecular motion, temperature is one of the most important environmental factors for life as it directly influences structural and hence functional properties of cellular components. After a sudden increase in ambient temperature, which is termed heat shock, bacteria respond by expressing a specific set of genes whose protein products are designed to mainly cope with heat-induced alterations of protein conformation. This heat shock response comprises the expression of protein chaperones and proteases, and is under central control of an alternative sigma factor (sigma 32) which acts as a master regulator that specifically directs RNA polymerase to transcribe from the heat shock promotors. In a similar manner, bacteria express a well-defined set of proteins after a rapid decrease in temperature, which is termed cold shock. This protein set, however, is different from that expressed under heat shock conditions and predominantly comprises proteins such as helicases, nucleases, and ribosome-associated components that directly or indirectly interact with the biological information molecules DNA and RNA. Interestingly, in contrast to the heat shock response, to date no cold-specific sigma factor has been identified. Rather, it appears that the cold shock response is organized as a complex stimulon in which post-transcriptional events play an important role. In this review, we present a summary of research results that have been acquired in recent years by examinations of bacterial cold shock responses. Important processes such as cold signal perception, membrane adaptation, and the modification of the translation apparatus are discussed together with many other cold-relevant aspects of bacterial physiology and first attempts are made to dissect the cold shock stimulon into less complex regulatory subunits. Special emphasis is placed on findings concerning the nucleic acid-binding cold shock proteins which play a fundamental role not only during cold shock adaptation but also under optimal growth conditions.

Genome-wide transcriptional profiling of the Bacillus subtilis cold-shock response

The transcriptome of Bacillus subtilis was analysed at different time points (30, 60 and 90 min) after a temperature downshift from 37 to 18 degrees C using DNA macroarrays. This approach allowed the identification of around 50 genes exhibiting an increased mRNA level and around 50 genes exhibiting a decreased mRNA level under cold-shock conditions. Many of the repressed genes encode enzymes involved in the biosynthesis of amino acids, nucleotides and coenzymes, indicating metabolic adaptation of the cells to the decreased growth rate at the lower temperature. The strongest cold-inducible gene encodes fatty acid desaturase, which forms unsaturated fatty acids from saturated phospholipid precursors, thereby increasing membrane fluidity. The cold-shock-induced increase of mRNA levels of the classical cold-shock genes cspB, cspC and cspD could be verified. Furthermore, besides many genes encoding proteins of unknown function, some genes encoding ribosomal proteins were transcriptionally up-regulated, which points to an adaptive reprogramming of the ribosomes under cold-shock conditions. Interestingly, the amount of mRNA specified by the operon ptb-bcd-buk-lpd-bkdA1-bkdA2-bkdB, which encodes enzymes involved in degradation of branched-chain amino acids, also increases after a temperature downshift. As cells utilize the isoleucine and valine degradation intermediates alpha-methylbutyryl-CoA and isobutyryl-CoA for synthesis of branched-chain fatty acids, this finding reflects the adaptation of membrane lipid composition, ensuring the maintenance of appropriate membrane fluidity at low temperatures. The results of the DNA array analyses were verified for several selected genes by RNA slot-blot analysis and compared with two-dimensional PAGE analyses.

Genomewide transcriptional analysis of the cold shock response in Bacillus subtilis

Previous studies with two-dimensional gel electrophoresis techniques revealed that the cold shock response in Bacillus subtilis is characterized by rapid induction and accumulation of two classes of specific proteins, which have been termed cold-induced proteins (CIPs) and cold acclimatization proteins (CAPs), respectively. Only recently, the B. subtilis two-component system encoded by the desKR operon has been demonstrated to be essential for the cold-induced expression of the lipid-modifying desaturase Des, which is required for efficient cold adaptation of the membrane in the absence of isoleucine. At present, one of the most intriguing questions in this research field is whether DesKR plays a global role in cold signal perception and transduction in B. subtilis. In this report, we present the first genomewide transcriptional analysis of a cold-exposed bacterium and demonstrate that the B. subtilis two-component system DesKR exclusively controls the desaturase gene des and is not the cold-triggered regulatory system of global relevance. In addition to this, we identified a set of genes that might participate as novel players in the cold shock adaptation of B. subtilis. Two cold-induced genes, the elongation factor homolog ylaG and the sigma(L)-dependent transcriptional activator homolog yplP, have been examined by construction and analysis of deletion mutants.

Mechanism of membrane fluidity optimization: isothermal control of the Bacillus subtilis acyl-lipid desaturase

The Des pathway of Bacillus subtilis regulates the expression of the acyl-lipid desaturase, Des, thereby controlling the synthesis of unsaturated fatty acids (UFAs) from saturated phospholipid precursors. Previously, we showed that the master switch for the Des pathway is a two-component regulatory system composed of a membrane-associated kinase, DesK, and a soluble transcriptional regulator, DesR, which stringently controls transcription of the des gene. Activation of this pathway takes place when cells are shifted to low growth temperature. Here, we report on the mechanism by which isoleucine regulates the Des pathway. We found that exogenous isoleucine sources, as well as its alpha-keto acid derivative, which is a branched-chain fatty acid precursor, negatively regulate the expression of the des gene at 37 degrees C. The DesK-DesR two-component system mediates this response, as both partners are required to sense and transduce the isoleucine signal at 37 degrees C. Fatty acid profiles strongly indicate that isoleucine affects the signalling state of the DesK sensor protein by dramatically increasing the incorporation of the lower-melting-point anteiso-branched-chain fatty acids into membrane phospholipids. We propose that both a decrease in membrane fluidity at constant temperature and a temperature downshift induce des by the same mechanism. Thus, the Des pathway would provide a novel mechanism to optimize membrane lipid fluidity at a constant temperature.

Coping with the cold: the cold shock response in the Gram-positive soil bacterium Bacillus subtilis

All organisms examined to date, respond to a sudden change in environmental temperature with a specific cascade of adaptation reactions that, in some cases, have been identified and monitored at the molecular level. According to the type of temperature change, this response has been termed heat shock response (HSR) or cold shock response (CSR). During the HSR, a specialized sigma factor has been shown to play a central regulatory role in controlling expression of genes predominantly required to cope with heat-induced alteration of protein conformation. In contrast, after cold shock, nucleic acid structure and proteins interacting with the biological information molecules DNA and RNA appear to play a major cellular role. Currently, no cold-specific sigma factor has been identified. Therefore, unlike the HSR, the CSR appears to be organized as a complex stimulon rather than resembling a regulon. This review has been designed to draw a refined picture of our current understanding of the CSR in Bacillus subtilis. Important processes such as temperature sensing, membrane adaptation, modification of the translation apparatus, as well as nucleoid reorganization and some metabolic aspects, are discussed in brief. Special emphasis is placed on recent findings concerning the nucleic acid binding cold shock proteins, which play a fundamental role, not only during cold shock adaptation but also under optimal growth conditions.

Regulation of the desaturation of fatty acids and its role in tolerance to cold and salt stress

The expression of cold-inducible genes is regulated by a two-component system in Synechocystis and Bacillus subtilis. The cold sensors are membrane-bound histidine kinases and it seems likely that they sense and transduce changes in the fluidity of membranes. Desaturation of fatty acids in membrane lipids has been implicated in tolerance to cold and salt stress.

Conversion of L-leucine to isovaleric acid by Propionibacterium freudenreichii TL 34 and ITGP23

Several branched-chain volatile compounds are involved in the flavor of Swiss cheese. These compounds are probably produced by enzymatic conversion of branched-chain amino acids, but the flora and the pathways involved remain hypothetical. Our aim was to determine the ability of Propionibacterium freudenreichii, which is one of the main components of the secondary flora of Swiss cheese, to produce flavor compounds during leucine catabolism. Cell extracts and resting cells of two strains were incubated in the presence of L-leucine, alpha-ketoglutaric acid, and cofactors, and the metabolites produced were determined by high-performance liquid chromatography and gas chromatography. The first step of leucine catabolism was a transamination that produced alpha-ketoisocaproic acid, which was enzymatically converted to isovaleric acid. Both reactions were faster at pH 8.0 than at acidic pHs. Cell extracts catalyzed only the transamination step under our experimental conditions. Small amounts of 3-methylbutanol were also produced by resting cells, but neither 3-methylbutanal noralpha-hydroxyisocaproic acid was detected. L-Isoleucine and L-valine were also converted to the corresponding acids and alcohols. Isovaleric acid was produced by both strains during growth in a complex medium, even under conditions simulating Swiss cheese conditions (2.1% NaCl, pH 5.4, 24 degrees C). Our results show that P. frendenreichii could play a significant role in the formation of isovaleric acid during ripening.

Complementation of cold shock proteins by translation initiation factor IF1 in vivo

The cold shock response in both Escherichia coli and Bacillus subtilis is induced by an abrupt downshift in growth temperature and leads to a dramatic increase in the production of a homologous class of small, often highly acidic cold shock proteins. This protein family is the prototype of the cold shock domain (CSD) that is conserved from bacteria to humans. For B. subtilis it has been shown that at least one of the three resident cold shock proteins (CspB to D) is essential under optimal growth conditions as well as during cold shock. Analysis of the B. subtilis cspB cspC double deletion mutant revealed that removal of these csp genes results in pleiotropic alteration of protein synthesis, cell lysis during the entry of stationary growth phase, and the inability to differentiate into endospores. We show here that heterologous expression of the translation initiation factor IF1 from E. coli in a B. subtilis cspB cspC double deletion strain is able to cure both the growth and the sporulation defects observed for this mutant, suggesting that IF1 and cold shock proteins have at least in part overlapping cellular function(s). Two of the possible explanation models are discussed.

Responses of Gram-negative bacteria to certain environmental stressors

Bacteria in nature are exposed to variations in temperature, and are affected by the availability of nutrients and water and the presence of toxic molecules. Their reactions to these changes require a series of rapid adaptive responses. Although transcriptional regulation is of primary importance in these responses, translational regulation and even activation of 'silenced' enzymes are critical for survival in changing environments. Bacteria have developed a series of mechanisms at the membrane structure level to cope with high concentrations of solvents. In addition, solvent-tolerant strains express highly effective efflux pumps to remove solvents from the cytoplasm. Desiccation tolerance is based on the synthesis and accumulation of osmoprotectants together with changes in fatty acid composition to preserve membrane structure. Both cold shock and heat shock responses are mainly regulated at a post-transcriptional level, translation efficiency in the case of cold shock and mRNA half-life and sigma32 stability in the case of heat shock.

Role of the Bacillus subtilis fatty acid desaturase in membrane adaptation during cold shock

In our attempt to understand the cold shock response of Bacillus subtilis, we report on the role of the B. subtilis fatty acid desaturase (FA-D) Des during membrane adaptation to low temperatures and demonstrate its importance during cold shock. A des null mutant was constructed and analysed in comparison with its parental strain. Growth studies and large-scale comparative fatty acid (FA) analysis revealed a severe cold-sensitive phenotype of the des deletion mutant during the absence of isoleucine and showed that four unsaturated fatty acid (UFA) species differing in length, branching pattern and position of the double bond are synthesized in B. subtilis JH642 but not in the des null mutant. Apart from the lack of UFA synthesis, the FA-D deletion strain showed a dramatically altered saturated fatty acid (SFA) profile at the onset of the stationary growth phase in the presence of exogenous isoleucine sources. Expression of des integrated in trans at the amyE locus of the des deletion strain not only cured the cold-sensitive phenotype observed for the des mutant but allowed much better growth than in strain JH642 after a shift from 37 degrees C to 15 degrees C. These results show that, during cold shock adaptation, des expression can completely replace the isoleucine-dependent, long-term, FA branching adaptation mechanism. We conclude that the crucial aspect in cold adaptation of the cytoplasmic membrane is not its specific molecular composition but rather its physical status in terms of its fluidity.

Unusual lipid composition of a Bacillus sp. isolated from Lake Pomorie in Bulgaria

The lipid composition of a Bacillus sp., isolated from Lake Pomorie in Bulgaria, was unusual and consisted of 26 different fatty acids between C12 and C26, with anteiso C15-C17 saturated fatty acids predominating. The furan fatty acid, 10,13-epoxy-11-methyloctadeca-10,12-dienoic acid, was also identified, a new finding for this genus. The hydrocarbons consisted of 30 different monounsaturated hydrocarbons, between C25 and C30, with the iso-iso, iso-anteiso, anteiso-anteiso, iso-normal, and anteiso-normal methyl branching for odd-numbered chains, and the iso-iso, iso-anteiso, iso-normal, and anteiso-normal methyl branching for even-numbered chains. The double bond positions in these hydrocarbons were determined by dimethyl disulfide derivatization followed by GC-MS, and the double-bond cis configuration was confirmed by infrared spectroscopy. Some previously unknown hydrocarbons in bacteria, such as (Z)-3,21-dimethyl-9-tricosene, (Z)-3,21-dimethyl-10-tricosene, (Z)-2,24-dimethyl-11-pentacosene, and (Z)-2,25-dimethyl-13-hexacosene were identified. Sterols were detected and were based on the sitosterol nucleus.