Cardiolipin synthase from Escherichia coli

Optimizing Archaeal Lipid Biosynthesis in Escherichia coli

Membrane lipid chemistry is remarkably different in archaea compared with bacteria and eukaryotes. In the evolutionary context, this is also termed the lipid divide and is reflected by distinct biosynthetic pathways. Contemporary organisms have almost without exception only one type of membrane lipid. During early membrane evolution, mixed membrane stages likely occurred, and it was hypothesized that the instability of such mixtures was the driving force for the lipid divide. To examine the compatibility between archaeal and bacterial lipids, the bacterium Escherichia coli has been engineered to contain both types of lipids with varying success. Only limited production of archaeal lipid archaetidylethanolamine was achieved. Here, we substantially increased its production in E. coli by overexpression of an archaeal phosphatidylserine synthase needed for ethanolamine headgroup attachment. Furthermore, we introduced a synthetic isoprenoid utilization pathway to increase the supply of isopentenyl-diphosphate and dimethylallyl diphosphate. This improved archaeal lipid production substantially. The archaeal phospholipids also served as a substrate for the E. coli cardiolipin synthase, resulting in archaeal and novel hybrid archaeal/bacterial cardiolipin species not seen in living organisms before. Growth of the E. coli strain with the mixed membrane shows an enhanced sensitivity to the inhibitor of fatty acid biosynthesis, cerulenin, indicating a critical dependence of the engineered E. coli strain on its native phospholipids.

Description of Luteibacter aegosomatis sp. nov., Luteibacter aegosomaticola sp. nov., and Luteibacter aegosomatissinici sp. nov. isolated from the Intestines of Aegosoma sinicum Larvae

Three novel bacterial strains, 321^T, 335^T, and 353^T, were isolated from the intestines of Aegosoma sinicum larvae collected from Paju-Si, South Korea. The strains were Gram-negative, obligate aerobe and had rod-shaped cells with a single flagellum. The three strains belonged to the genus Luteibacter in the family Rhodanobacteraceae and shared < 99.2% similarity in their 16S rRNA gene sequence and < 83.56% similarity in thier whole genome sequence. Strains 321^T, 335^T, and 353^T formed a monophyletic clade with Luteibacter yeojuensis KACC 11405^T, L. anthropi KACC 17855^T, and L. rhizovicinus KACC 12830^T, with sequence similarities of 98.77-98.91%, 98.44-98.58%, and 97.88-98.02%, respectively. Further genomic analyses, including the construction of the Up-to-date Bacterial Core Gene (UBCG) tree and assessment of other genome-related indices, indicated that these strains were novel species belonging to the genus Luteibacter. All three strains contained ubiquinone Q8 as their major isoprenoid quinone and iso-C_15:0 and summed feature 9 (C_16:0 10-methyl and/or iso-C_17:1 ω9c) as their major cellular fatty acids. Phosphatidylethanolamine and diphosphatidylglycerol were the major polar lipids in all the strains. The genomic DNA G + C contents of strains 321^T, 335^T, and 353^T were 66.0, 64.5, and 64.5 mol%, respectively. Based on multiphasic classification, strains 321^T, 335^T, and 353^T were classified into the genus Luteibacter as the type strains of novel species, for which the names Luteibacter aegosomatis sp. nov., Luteibacter aegosomaticola sp. nov., and Luteibacter aegosomatissinici sp. nov. are proposed, respectively.

The AAA+ ATPase RavA and its binding partner ViaA modulate E. coli aminoglycoside sensitivity through interaction with the inner membrane

Enteric bacteria have to adapt to environmental stresses in the human gastrointestinal tract such as acid and nutrient stress, oxygen limitation and exposure to antibiotics. Membrane lipid composition has recently emerged as a key factor for stress adaptation. The E. coli ravA-viaA operon is essential for aminoglycoside bactericidal activity under anaerobiosis but its mechanism of action is unclear. Here we characterise the VWA domain-protein ViaA and its interaction with the AAA+ ATPase RavA, and find that both proteins localise at the inner cell membrane. We demonstrate that RavA and ViaA target specific phospholipids and subsequently identify their lipid-binding sites. We further show that mutations abolishing interaction with lipids restore induced changes in cell membrane morphology and lipid composition. Finally we reveal that these mutations render E. coli gentamicin-resistant under fumarate respiration conditions. Our work thus uncovers a ravA-viaA-based pathway which is mobilised in response to aminoglycosides under anaerobiosis and engaged in cell membrane regulation.

Localized cardiolipin synthesis is required for the assembly of MreB during the polarized cell division of Chlamydia trachomatis

Pathogenic Chlamydia species are coccoid bacteria that use the rod-shape determining protein MreB to direct septal peptidoglycan synthesis during their polarized cell division process. How the site of polarized budding is determined in this bacterium, where contextual features like membrane curvature are seemingly identical, is unclear. We hypothesized that the accumulation of the phospholipid, cardiolipin (CL), in specific regions of the cell membrane induces localized membrane changes that trigger the recruitment of MreB to the site where the bud will arise. To test this, we ectopically expressed cardiolipin synthase (Cls) and observed a polar distribution for this enzyme in Chlamydia trachomatis. In early division intermediates, Cls was restricted to the bud site where MreB is localized and peptidoglycan synthesis is initiated. The localization profile of 6xHis tagged Cls (Cls_6xH) throughout division mimicked the distribution of lipids that stain with NAO, a dye that labels CL. Treatment of Chlamydia with 3’,6-dinonylneamine (diNN), an antibiotic targeting CL-containing membrane domains, resulted in redistribution of Cls_6xH and NAO-staining phospholipids. In addition, 6xHis tagged MreB localization was altered by diNN treatment, suggesting an upstream regulatory role for CL-containing membranes in directing the assembly of MreB. This hypothesis is consistent with the observation that the clustered localization of Cls_6xH is not dependent upon MreB function or peptidoglycan synthesis. Furthermore, expression of a CL-binding protein at the inner membrane of C. trachomatis dramatically inhibited bacterial growth supporting the importance of CL in the division process. Our findings implicate a critical role for localized CL synthesis in driving MreB assembly at the bud site during the polarized cell division of Chlamydia.

Immunoinformatic analysis of the whole proteome for vaccine design: An application to Clostridium perfringens

Clostridium perfringens is a dangerous bacterium and known biological warfare weapon associated with several diseases, whose lethal toxins can produce necrosis in humans. However, there is no safe and fully effective vaccine against C. perfringens for humans yet. To address this problem, we computationally screened its whole proteome, identifying highly immunogenic proteins, domains, and epitopes. First, we identified that the proteins with the highest epitope density are Collagenase A, Exo-alpha-sialidase, alpha n-acetylglucosaminidase and hyaluronoglucosaminidase, representing potential recombinant vaccine candidates. Second, we further explored the toxins, finding that the non-toxic domain of Perfringolysin O is enriched in CTL and HTL epitopes. This domain could be used as a potential sub-unit vaccine to combat gas gangrene. And third, we designed a multi-epitope protein containing 24 HTL-epitopes and 34 CTL-epitopes from extracellular regions of transmembrane proteins. Also, we analyzed the structural properties of this novel protein using molecular dynamics. Altogether, we are presenting a thorough immunoinformatic exploration of the whole proteome of C. perfringens, as well as promising whole-protein, domain-based and multi-epitope vaccine candidates. These can be evaluated in preclinical trials to assess their immunogenicity and protection against C. perfringens infection.

Effects of Growth Stage on the Characterization of Enterotoxin A-Producing Staphylococcus aureus‐Derived Membrane vesicles

Virulence factors, such as staphylococcal enterotoxin A (SEA), are contained within membrane vesicles (MVs) in the cell membrane of Staphylococcus aureus. In this study, the effects of the growth stage on quantitative and qualitative changes in the components contained in the MVs of S. aureus SEA-producing strains were examined. Changes in the expression levels of S. aureus genes were examined at each growth stage; phenol-soluble modulin (PSM) gene reached a maximum after 8 h, and the expression of cell membrane-related genes was decreased after 6 h. Based on these gene expression patterns, MVs were prepared at 6, 17, and 24 h. The particle size of MVs did not change depending on the growth stage. MVs prepared after culture for 17 h maintained their particle size when stored at 23 °C. The amount of SEA in the culture supernatant and MVs were not correlated. Bifunctional autolysin, a protein involved in cell wall biosynthesis/degradation, was increased in MVs at 17 h. The expression pattern of inflammation-related genes in human adult low calcium high temperature (HaCaT) cells induced by MVs was different for each growth stage. The inclusion components of S. aureus-derived MVs are selective, depend on the stage of growth, and may play an important role in toxicity.

Identification of Two Variants of Acinetobacter baumannii Strain ATCC 17978 with Distinct Genotypes and Phenotypes

Acinetobacter baumannii is a nosocomial pathogen that exhibits substantial genomic plasticity. Here, the identification of two variants of A. baumannii ATCC 17978 that differ based on the presence of a 44-kb accessory locus, named AbaAL44 (A. baumannii accessory locus 44 kb), is described. Analyses of existing deposited data suggest that both variants are found in published studies of A. baumannii ATCC 17978 and that American Type Culture Collection (ATCC)-derived laboratory stocks comprise a mix of these two variants. Yet, each variant exhibits distinct interactions with the host in vitro and in vivo. Infection with the variant that harbors AbaAL44 (A. baumannii 17978 UN) results in decreased bacterial burdens and increased neutrophilic lung inflammation in a mouse model of pneumonia, and affects the production of interleukin 1 beta (IL-1β) and IL-10 by infected macrophages. AbaAL44 harbors putative pathogenesis genes, including those predicted to encode a type I pilus cluster, a catalase, and a cardiolipin synthase. The accessory catalase increases A. baumannii resistance to oxidative stress and neutrophil-mediated killing in vitro. The accessory cardiolipin synthase plays a dichotomous role by promoting bacterial uptake and increasing IL-1β production by macrophages, but also by enhancing bacterial resistance to cell envelope stress. Collectively, these findings highlight the phenotypic consequences of the genomic dynamism of A. baumannii through the evolution of two variants of a common type strain with distinct infection-related attributes.

A promiscuous archaeal cardiolipin synthase enables construction of diverse natural and unnatural phospholipids

Cardiolipins (CL) are a class of lipids involved in the structural organization of membranes, enzyme functioning, and osmoregulation. Biosynthesis of CLs has been studied in eukaryotes and bacteria, but has been barely explored in archaea. Unlike the common fatty acyl chain–based ester phospholipids, archaeal membranes are made up of the structurally different isoprenoid-based ether phospholipids, possibly involving a different cardiolipin biosynthesis mechanism. Here, we identified a phospholipase D motif–containing cardiolipin synthase (MhCls) from the methanogen Methanospirillum hungatei. The enzyme was overexpressed in Escherichia coli, purified, and its activity was characterized by LC-MS analysis of substrates/products. MhCls utilizes two archaetidylglycerol (AG) molecules in a transesterification reaction to synthesize glycerol-di-archaetidyl-cardiolipin (Gro-DACL) and glycerol. The enzyme is nonselective to the stereochemistry of the glycerol backbone and the nature of the lipid tail, as it also accepts phosphatidylglycerol (PG) to generate glycerol-di-phosphatidyl-cardiolipin (Gro-DPCL). Remarkably, in the presence of AG and PG, MhCls formed glycerol-archaetidyl-phosphatidyl-cardiolipin (Gro-APCL), an archaeal-bacterial hybrid cardiolipin species that so far has not been observed in nature. Due to the reversibility of the transesterification, in the presence of glycerol, Gro-DPCL can be converted back into two PG molecules. In the presence of other compounds that contain primary hydroxyl groups (e.g., alcohols, water, sugars), various natural and unique unnatural phospholipid species could be synthesized, including multiple di-phosphatidyl-cardiolipin species. Moreover, MhCls can utilize a glycolipid in the presence of phosphatidylglycerol to form a glycosyl-mono-phosphatidyl-cardiolipin species, emphasizing the promiscuity of this cardiolipin synthase that could be of interest for bio-catalytic purposes.

Bacillus anthracis Responds to Targocil-Induced Envelope Damage through EdsRS Activation of Cardiolipin Synthesis

Bacillus anthracis is a spore-forming bacterium that causes devastating infections and has been used as a bioterror agent. This pathogen can survive hostile environments through the signaling activity of two-component systems, which couple environmental sensing with transcriptional activation to initiate a coordinated response to stress. In this work, we describe the identification of a two-component system, EdsRS, which mediates the B. anthracis response to the antimicrobial compound targocil. Targocil is a cell envelope-targeting compound that is toxic to B. anthracis at high concentrations. Exposure to targocil causes damage to the cellular barrier and activates EdsRS to induce expression of a previously uncharacterized cardiolipin synthase, which we have named ClsT. Both EdsRS and ClsT are required for protection against targocil-dependent damage. Induction of clsT by EdsRS during targocil treatment results in an increase in cardiolipin levels, which protects B. anthracis from envelope damage. Together, these results reveal that a two-component system signaling response to an envelope-targeting antimicrobial induces production of a phospholipid associated with stabilization of the membrane. Cardiolipin is then used to repair envelope damage and promote B. anthracis viability.IMPORTANCE Compromising the integrity of the bacterial cell barrier is a common action of antimicrobials. Targocil is an antimicrobial that is active against the bacterial envelope. We hypothesized that Bacillus anthracis, a potential weapon of bioterror, senses and responds to targocil to alleviate targocil-dependent cell damage. Here, we show that targocil treatment increases the permeability of the cellular envelope and is particularly toxic to B. anthracis spores during outgrowth. In vegetative cells, two-component system signaling through EdsRS is activated by targocil. This results in an increase in the production of cardiolipin via a cardiolipin synthase, ClsT, which restores the loss of barrier function, thereby reducing the effectiveness of targocil. By elucidating the B. anthracis response to targocil, we have uncovered an intrinsic mechanism that this pathogen employs to resist toxicity and have revealed therapeutic targets that are important for bacterial defense against structural damage.

Synthetic Minimal Cell: Self-Reproduction of the Boundary Layer

A critical aspect in the bottom-up construction of a synthetic minimal cell is to develop an entity that is capable of self-reproduction. A key role in this process is the expansion and division of the boundary layer that surrounds the compartment, a process in which content loss has to be avoided and the barrier function maintained. Here, we describe the latest developments regarding self-reproduction of a boundary layer with a focus on the growth and division of phospholipid-based membranes in the context of a synthetic minimal cell.

Development of Freeze-Thaw Tolerant Lactobacillus rhamnosus GG by Adaptive Laboratory Evolution

The industrial application of microorganisms as starters or probiotics requires their preservation to assure viability and metabolic activity. Freezing is routinely used for this purpose, but the cold damage caused by ice crystal formation may result in severe decrease in microbial activity. In this study, adaptive laboratory evolution (ALE) technique was applied to a lactic acid bacterium to select tolerant strains against freezing and thawing stresses. Lactobacillus rhamnosus GG was subjected to freeze-thaw-growth (FTG) for 150 cycles with four replicates. After 150 cycles, FTG-evolved mutants showed improved fitness (survival rates), faster growth rate, and shortened lag phase than those of the ancestor. Genome sequencing analysis of two evolved mutants showed genetic variants at distant loci in six genes and one intergenic space. Loss-of-function mutations were thought to alter the structure of the microbial cell membrane (one insertion in cls), peptidoglycan (two missense mutations in dacA and murQ), and capsular polysaccharides (one missense mutation in wze), resulting in an increase in cellular fluidity. Consequently, L. rhamnosus GG was successfully evolved into stress-tolerant mutants using FTG-ALE in a concerted mode at distal loci of DNA. This study reports for the first time the functioning of dacA and murQ in freeze-thaw sensitivity of cells and demonstrates that simple treatment of ALE designed appropriately can lead to an intelligent genetic changes at multiple target genes in the host microbial cell.

Perspective: challenges and opportunities for the study of cardiolipin, a key player in bacterial cell structure and function

Cardiolipin (CL) is a key player in bacterial cell biology. CL accumulates at the poles of rod-shaped cells; the polar localization and function of diverse bacterial proteins are CL-dependent. Cardiolipin (CL) is an unusual phospholipid comprised of a glycerol headgroup coupled with two phosphatidate moieties. CL-rich membrane domains are often visualized with the fluorescent indicator 10-N-nonyl-acridine orange (NAO). Recent data show that NAO can also indicate phosphatidylglycerol localization under different experimental conditions, in the absence of CL. The formation of CL-rich membrane domains at bacterial cell poles was predicted to occur spontaneously, by lipid microphase separation arising from the conical CL shape. New data reveal that membrane-anchored cardiolipin synthase A is targeted to the cytoplasmic membrane surface at bacterial cell poles. Thus, localized CL synthesis, interaction of CL with ClsA, and membrane curvature could all contribute to retention of CL at cell poles. These observations provide new insight regarding the mechanism for assembly of CL-rich membrane domains in prokaryotes and eukaryotes.

Cardiolipin synthase A colocalizes with cardiolipin and osmosensing transporter ProP at the poles of Escherichia coli cells

Osmosensing by transporter ProP is modulated by its cardiolipin (CL)-dependent concentration at the poles of Escherichia coli cells. Other contributors to this phenomenon were sought with the BACterial Two-Hybrid System (BACTH). The BACTH-tagged variants T18-ProP and T25-ProP retained ProP function and localization. Their interaction confirmed the ProP homo-dimerization previously established by protein crosslinking. YdhP, YjbJ and ClsA were prominent among the putative ProP interactors identified by the BACTH system. The functions of YdhP and YjbJ are unknown, although YjbJ is an abundant, osmotically induced, soluble protein. ClsA (CL Synthase A) had been shown to determine ProP localization by mediating CL synthesis. Unlike a deletion of clsA, deletion of ydhP or yjbJ had no effect on ProP localization or function. All three proteins were concentrated at the cell poles, but only ClsA localization was CL-dependent. ClsA was shown to be N-terminally processed and membrane-anchored, with dual, cytoplasmic, catalytic domains. Active site amino acid replacements (H224A plus H404A) inactivated ClsA and compromised ProP localization. YdhP and YjbJ may be ClsA effectors, and interactions of YdhP, YjbJ and ClsA with ProP may reflect their colocalization at the cell poles. Targeted CL synthesis may contribute to the polar localization of CL, ClsA and ProP.

Cross-species complementation of bacterial- and eukaryotic-type cardiolipin synthases

The glycerophospholipid cardiolipin is a unique constituent of bacterial and mitochondrial membranes. It is involved in forming and stabilizing high molecular mass membrane protein complexes and in maintaining membrane architecture. Absence of cardiolipin leads to reduced efficiency of the electron transport chain, decreased membrane potential, and, ultimately, impaired respiratory metabolism. For the protozoan parasite Trypanosoma brucei cardiolipin synthesis is essential for survival, indicating that the enzymes involved in cardiolipin production represent potential drug targets. T. brucei cardiolipin synthase (TbCLS) is unique as it belongs to the family of phospholipases D (PLD), harboring a prokaryotic-type cardiolipin synthase (CLS) active site domain. In contrast, most other eukaryotic CLS, including the yeast ortholog ScCrd1, are members of the CDP-alcohol phosphatidyltransferase family. To study if these mechanistically distinct CLS enzymes are able to catalyze cardiolipin production in a cell that normally expresses a different type of CLS, we expressed TbCLS and ScCrd1 in CLS-deficient yeast and trypanosome strains, respectively. Our results show that TbCLS complemented cardiolipin production in CRD1 knockout yeast and partly restored wild-type colony forming capability under stress conditions. Remarkably, CL remodeling appeared to be impaired in the transgenic construct, suggesting that CL production and remodeling are tightly coupled processes that may require a clustering of the involved proteins into specific CL-synthesizing domains. In contrast, no complementation was observed by heterologous expression of ScCrd1 in conditional TbCLS knockout trypanosomes, despite proper mitochondrial targeting of the protein.

Unraveling the expression of genes involved in the biosynthesis pathway of cardiolipin and phosphatidylethanolamine in Salmonella Hadar grown under static magnetic field 200 mT

We aimed in this work to evaluate the effect of static magnetic field 200 mT (SMF) on the expression of genes involved in the biosynthetic pathway of cardiolipin: g3pd, plsB, cdsA, pgsA, pgpA, cls and phosphatidylethanolamine: pssA and psd in Salmonella enterica subsp enterica serovar Hadar. Bacteria were exposed to a SMF during 3, 6 and 9 h. RNA extraction was followed by Reverse Transcriptase Polymerase Chain Reaction RT-PCR. The relative quantification of mRNA expression levels using 16S rRNA doesn't change during the time exposure. RT-PCR was done for two exposure experiments. The gene expression using RT-PCR present no significant difference in case of plsB, cdsA, pgpA, pgsA and psd genes during the different exposure times. However, a significant increase was observed in the expression of g3pd and pssA genes after 6 h and for cls gene after 3 h of exposure, but any variation was notified after 9 h of exposure. So we can conclude from this study that cls, g3pd and pssA genes are required in the adaptation of Salmonella Hadar to SMF.

Membrane insertion of F0 c subunit of F0F1 ATPase depends on glycolipozyme MPIase and is stimulated by YidC

The F₀ c subunit of F₀F₁ ATPase (F₀-c) possesses two membrane-spanning stretches with N- and C-termini exposed to the periplasmic (extracellular) side of the cytoplasmic membrane of E. coli. Although F₀-c insertion has been extensively analyzed in vitro by means of protease protection assaying, it is unclear whether such assays allow elucidation of the insertion process faithfully, since the membrane-protected fragment, an index of membrane insertion, is a full-length polypeptide of F₀-c, which is the same as the protease-resistant conformation without membrane insertion. We found that the protease-resistant conformation could be discriminated from membrane-insertion by including octyl glucoside on protease digestion. By means of this system, we found that F₀-c insertion depends on MPIase, a glycolipozyme involved in membrane insertion, and is stimulated by YidC. In addition, we found that acidic phospholipids PG and CL transform F₀-c into a protease-resistant form, while MPIase prevents the acquisition of such a protease-resistant conformation.

Cardiolipin plays an essential role in the formation of intracellular membranes in Escherichia coli

Mitochondria, chloroplasts and photosynthetic bacteria are characterized by the presence of complex and intricate membrane systems. In contrast, non-photosynthetic bacteria lack membrane structures within their cytoplasm. However, large scale over-production of some membrane proteins, such as the fumarate reductase, the mannitol permease MtlA, the glycerol acyl transferase PlsB, the chemotaxis receptor Tsr or the ATP synthase subunit b, can induce the proliferation of intra cellular membranes (ICMs) in the cytoplasm of Escherichia coli. These ICMs are particularly rich in cardiolipin (CL). Here, we have studied the effect of CL in the generation of these membranous structures. We have deleted the three genes (clsA, clsB and clsC) responsible of CL biosynthesis in E. coli and analysed the effect of these mutations by fluorescent and electron microscopy and by lipid mass spectrometry. We have found that CL is essential in the formation of non-lamellar structures in the cytoplasm of E. coli cells. These results could help to understand the structuration of membranes in E. coli and other membrane organelles, such as mitochondria and ER.

Thermotolerant genes essential for survival at a critical high temperature in thermotolerant ethanologenic Zymomonas mobilis TISTR 548

BACKGROUND

High-temperature fermentation (HTF) technology is expected to reduce the cost of bioconversion of biomass to fuels or chemicals. For stable HTF, the development of a thermotolerant microbe is indispensable. Elucidation of the molecular mechanism of thermotolerance would enable the thermal stability of microbes to be improved.

RESULTS

Thermotolerant genes that are essential for survival at a critical high temperature (CHT) were identified via transposon mutagenesis in ethanologenic, thermotolerant Zymomonas mobilis TISTR 548. Surprisingly, no genes for general heat shock proteins except for degP were included. Cells with transposon insertion in these genes showed a defect in growth at around 39 °C but grew normally at 30 °C. Of those, more than 60% were found to be sensitive to ethanol at 30 °C, indicating that the mechanism of thermotolerance partially overlaps with that of ethanol tolerance in the organism. Products of these genes were classified into nine categories of metabolism, membrane stabilization, transporter, DNA repair, tRNA modification, protein quality control, translation control, cell division, and transcriptional regulation.

CONCLUSIONS

The thermotolerant genes of Escherichia coli and Acetobacter tropicalis that had been identified can be functionally classified into 9 categories according to the classification of those of Z. mobilis, and the ratio of thermotolerant genes to total genomic genes in Z. mobilis is nearly the same as that in E. coli, though the ratio in A. tropicalis is relatively low. There are 7 conserved thermotolerant genes that are shared by these three or two microbes. These findings suggest that Z. mobilis possesses molecular mechanisms for its survival at a CHT that are similar to those in E. coli and A. tropicalis. The mechanisms may mainly contribute to membrane stabilization, protection and repair of damage of macromolecules and maintenance of cellular metabolism at a CHT. Notably, the contribution of heat shock proteins to such survival seems to be very low.

Bacterial aminoacyl phospholipids - Biosynthesis and role in basic cellular processes and pathogenicity

The bacterial cell membrane accomplishes the controlled exchange of molecules with the extracellular space and mediates specific interactions with the environment. However, the cytoplasmic membrane also includes vulnerable targets for antimicrobial agents. A common feature of cationic antimicrobial peptides (CAMPs) produced by other bacteria or by the host immune system is to utilize the negative charge of bacterial phospholipids such as phosphatidylglycerol (PG) or cardiolipin (CL) for initial adherence and subsequent penetration into the membrane bilayer. To resist cationic antimicrobials many bacteria integrate positive charges into the membrane surface. This is accomplished by aminoacylation of negatively charged (PG) or (CL) with alanine, arginine, or lysine residues. The Multiple Peptide Resistance Factor (MprF) of Staphylococcus aureus is the prototype of a highly conserved protein family of aminoacyl phosphatidylglycerol synthases (aaPGSs) which modify PG or CL with amino acids. MprF is an oligomerizing membrane protein responsible for both, synthesis of lysyl phosphatidylglycerol (LysPG) in the inner leaflet of the cytoplasmic membrane and translocation of LysPG to the outer leaflet. This review focuses on occurrence, synthesis and function of bacterial aminoacyl phospholipids (aaPLs) and on the role of such lipids in basic cellular processes and pathogenicity. This article is part of a Special Issue entitled: Bacterial Lipids edited by Russell E. Bishop.

Cardiolipin deficiency causes a dissociation of the b 6 c:caa 3 megacomplex in B. subtilis membranes

The associations among respiratory complexes in energy-transducing membranes have been established. In fact, it is known that the Gram-negative bacteria Paracoccus denitrificans and Escherichia coli have respiratory supercomplexes in their membranes. These supercomplexes are important for channeling substrates between enzymes in a metabolic pathway, and the assembly of these supercomplexes depends on the protein subunits and membrane lipids, mainly cardiolipin, which is present in both the mitochondrial inner membrane and bacterial membranes. The Gram-positive bacterium Bacillus subtilis has a branched respiratory chain, in which some complexes generate proton motive force whereas others constitute an escape valve of excess reducing power. Some peculiarities of this respiratory chain are the following: a type II NADH dehydrogenase, a unique b 6 c complex that has a b 6 type cytochrome with a covalently bound heme, and a c-type heme attached to the third subunit, which is similar to subunit IV of the photosynthetic b 6 f complex. Cytochrome c oxygen reductase (caa 3 ) contains a c-type cytochrome on subunit I. We previously showed that the b 6 c and the caa 3 complexes form a supercomplex. Both the b 6 c and the caa 3 together with the quinol oxygen reductase aa 3 generate the proton motive force in B. subtilis. In order to seek proof that this supercomplex is important for bacterial growth in aerobic conditions we compared the b 6 c: caa 3 supercomplex from wild type membranes with membranes from two mutants lacking cardiolipin. Both mutant complexes were found to have similar activity and heme content as the wild type. Clear native electrophoresis showed that mutants lacking cardiolipin had b 6 c:caa 3 supercomplexes of lower mass or even individual complexes after membrane solubilization with digitonin. The use of dodecyl maltoside revealed a more evident difference between wild-type and mutant supercomplexes. Here we provide evidence showing that cardiolipin plays a role in the stability of the b 6 c:caa 3 supercomplex in B. subtilis.

The magnesium transporter A is activated by cardiolipin and is highly sensitive to free magnesium in vitro

The magnesium transporter A (MgtA) is a specialized P-type ATPase, believed to import Mg²⁺ into the cytoplasm. In Salmonella typhimurium and Escherichia coli, the virulence determining two-component system PhoQ/PhoP regulates the transcription of mgtA gene by sensing Mg²⁺ concentrations in the periplasm. However, the factors that affect MgtA function are not known. This study demonstrates, for the first time, that MgtA is highly dependent on anionic phospholipids and in particular, cardiolipin. Colocalization studies confirm that MgtA is found in the cardiolipin lipid domains in the membrane. The head group of cardiolipin plays major role in activation of MgtA suggesting that cardiolipin may act as a Mg²⁺ chaperone for MgtA. We further show that MgtA is highly sensitive to free Mg²⁺ (Mg²⁺_free) levels in the solution. MgtA is activated when the Mg²⁺_free concentration is reduced below 10 μM and is strongly inhibited above 1 mM, indicating that Mg²⁺_free acts as product inhibitor. Combined, our findings conclude that MgtA may act as a sensor as well as a transporter of Mg²⁺.

DOI:http://dx.doi.org/10.7554/eLife.11407.001

Magnesium is an essential element for living cells, meaning that organisms from bacteria to humans need magnesium to survive. All cells are surrounded by a membrane made of fatty molecules called lipids, which is also embedded with proteins. Magnesium, like other metal ions, is transported inside cells across the cell’s membrane by specific membrane proteins.

A species of gut bacteria called E. coli has two separate magnesium transport systems: one that works at high concentrations of magnesium and one at lower concentrations. The latter system involves a membrane protein called magnesium transporter A (or MgtA for short), which works like a molecular pump. However, it was not known exactly how this transporter was affected by magnesium nor how sensitive it was to this divalent metal ion. It was also unclear whether MgtA worked alone in the bacterial membrane or if it worked in conjunction with other molecules.

Now Subramani et al. have managed to show that MgtA can sense magnesium ions down to micromolar concentrations, which is the equivalent to a pinch (1 gram) of magnesium salt in 10,000 liters of water. The experiments also showed that this detection system depended on a specific lipid molecule in the membrane called cardiolipin. MgtA and cardiolipin were found together in the membrane of living E. coli suggesting that the two do indeed work together.

The discovery that a membrane transporter that pumps ions needs cardiolipin to work suggests that cells could indirectly control the movement of ions by changing the levels of specific lipids in their membranes. Subramani et al. now hope to use techniques, such as X-ray crystallography, to visualize how magnesium and cardiolipin bind to MtgA and explore how the three molecules work together as a complete system. Information about these interactions could in the future help researchers understand how these bacteria try to protect themself in the hostile environment in the human gut or cells of the immune systems. Further studies of this system could be used to develop biological sensors for magnesium or to design antibiotics that interfere with the magnesium transporter to treat bacterial infections.

DOI:http://dx.doi.org/10.7554/eLife.11407.002

Septal localization by membrane targeting sequences and a conserved sequence essential for activity at the COOH-terminus of Bacillus subtilis cardiolipin synthase

The acidic phospholipid cardiolipin (CL) is localized on polar and septal membranes and plays an important physiological role in Bacillus subtilis cells. ClsA, the enzyme responsible for CL synthesis, is also localized on septal membranes. We found that GFP fusion proteins of the enzyme with NH2-terminal and internal deletions retained septal localization. However, derivatives with deletions starting from the COOH-terminus (Leu482) ceased to localize to the septum once the deletion passed the Ile residue at 448, indicating that the sequence responsible for septal localization is confined within a short distance from the COOH-terminus. Two sequences, Ile436-Leu450 and Leu466-Leu478, are predicted to individually form an amphipathic α-helix. This configuration is known as a membrane targeting sequence (MTS) and we therefore refer to them as MTS2 and MTS1, respectively. Either one has the ability to affect septal localization, and each of these sequences by itself localizes to the septum. Membrane association of the constructs of this enzyme containing the MTSs was verified by subcellular fractionation of the cells. CL synthesis, in contrast, was abolished after deleting just the last residue, Leu482, in the COOH-terminal four amino acid residue sequence, Ser-Pro-Ile-Leu, which is highly conserved among bacterial CL synthases.

Biosynthesis of Membrane Lipids

The pathways in Escherichia coli and (largely by analogy) S. enterica remain the paradigm of bacterial lipid synthetic pathways, although recently considerable diversity among bacteria in the specific areas of lipid synthesis has been demonstrated. The structural biology of the fatty acid synthetic proteins is essentially complete. However, the membrane-bound enzymes of phospholipid synthesis remain recalcitrant to structural analyses. Recent advances in genetic technology have allowed the essentialgenes of lipid synthesis to be tested with rigor, and as expected most genes are essential under standard growth conditions. Conditionally lethal mutants are available in numerous genes, which facilitates physiological analyses. The array of genetic constructs facilitates analysis of the functions of genes from other organisms. Advances in mass spectroscopy have allowed very accurate and detailed analyses of lipid compositions as well as detection of the interactions of lipid biosynthetic proteins with one another and with proteins outside the lipid pathway. The combination of these advances has resulted in use of E. coli and S. enterica for discovery of new antimicrobials targeted to lipid synthesis and in deciphering the molecular actions of known antimicrobials. Finally,roles for bacterial fatty acids other than as membrane lipid structural components have been uncovered. For example, fatty acid synthesis plays major roles in the synthesis of the essential enzyme cofactors, biotin and lipoic acid. Although other roles for bacterial fatty acids, such as synthesis of acyl-homoserine quorum-sensing molecules, are not native to E. coli introduction of the relevant gene(s) synthesis of these foreign molecules readily proceeds and the sophisticated tools available can used to decipher the mechanisms of synthesis of these molecules.

Phosphatidylglycerol-derived phospholipids have a universal, domain-crossing role in stress responses

Phosphatidylglycerol and phospholipids derived from it are widely distributed throughout the three domains of life. Cardiolipin is the best characterized of these phospholipids, and plays a key role in the response to environmental variations. Phosphatidylglycerol-derived phospholipids confer cell membranes with a wide range of responses, including changes in surface charge, fluidity, flexibility, morphology, biosynthesis and remodeling, that adapt the cell to these situations. Furthermore, the synthesis and remodeling of these phospholipids is finely regulated, highlighting the importance of these lipids in cell homeostasis and responses during stressful situations. In this article, we review the most important roles of these anionic phospholipids across domains, focusing on the biophysical basis by which these phospholipids are used in stress responses.

Crosstalk between DnaA protein, the initiator of Escherichia coli chromosomal replication, and acidic phospholipids present in bacterial membranes

Anionic (i.e., acidic) phospholipids such as phosphotidylglycerol (PG) and cardiolipin (CL), participate in several cellular functions. Here we review intriguing in vitro and in vivo evidence that suggest emergent roles for acidic phospholipids in regulating DnaA protein-mediated initiation of Escherichia coli chromosomal replication. In vitro acidic phospholipids in a fluid bilayer promote the conversion of inactive ADP-DnaA to replicatively proficient ATP-DnaA, yet both PG and CL also can inhibit the DNA-binding activity of DnaA protein. We discuss how cellular acidic phospholipids may positively and negatively influence the initiation activity of DnaA protein to help assure chromosomal replication occurs once, but only once, per cell-cycle. Fluorescence microscopy has revealed that PG and CL exist in domains located at the cell poles and mid-cell, and several studies link membrane curvature with sub-cellular localization of various integral and peripheral membrane proteins. E. coli DnaA itself is found at the cell membrane and forms helical structures along the longitudinal axis of the cell. We propose that there is cross-talk between acidic phospholipids in the bacterial membrane and DnaA protein as a means to help control the spatial and temporal regulation of chromosomal replication in bacteria.

The SHOCT domain: a widespread domain under-represented in model organisms

We have identified a new protein domain, which we have named the SHOCT domain (Short C-terminal domain). This domain is widespread in bacteria with over a thousand examples. But we found it is missing from the most commonly studied model organisms, despite being present in closely related species. It's predominantly C-terminal location, co-occurrence with numerous other domains and short size is reminiscent of the Gram-positive anchor motif, however it is present in a much wider range of species. We suggest several hypotheses about the function of SHOCT, including oligomerisation and nucleic acid binding. Our initial experiments do not support its role as an oligomerisation domain.

Quantification of cardiolipin molecular species in Escherichia coli lipid extracts using liquid chromatography/electrospray ionization mass spectrometry

RATIONALE

Cardiolipin (CL), a glycerophospholipid containing four acyl chains, is found in most organisms including Gram-negative bacteria such as Escherichia coli. While CL composes only a fraction of the total glycerophospholipids, the four acyl chains lead to a large number of possible molecular species as defined by the total number of carbons and unsaturations in the acyl chains. Understanding the molecular composition of CL, and how it changes under different growth conditions, will aid in understanding the complex role of CL in E. coli.

METHODS

Normal-phase liquid chromatography/electrospray ionization mass spectrometry was used to quantify the CL molecular species (as defined by the total number of carbons:unsaturations in the acyl chains) in lipid extracts prepared from E. coli grown at 15 °C, 30 °C, 37 °C and 42 °C.

RESULTS

Fifty-six different CL species were identified as [M-2H](2-) ions in E. coli lipid extracts ranging from 60:0 to 72:4. CL species with an increased total number of unsaturations were more abundant in lipid extracts prepared from cells grown at 15 °C as compared to higher temperatures.

CONCLUSIONS

This work characterizes the CL composition of E. coli cells grown at various temperatures. By quantifying CL species at a molecular level we have illuminated the molecular complexity of the CL in this relatively simple model organism. This data will be useful for understanding CL function in E. coli and other organisms.

Cardiolipin biosynthesis in Streptococcus mutans is regulated in response to external pH

Streptococcus mutans, a causative agent of dental caries in humans, adapts to changing environmental conditions, such as pH, in order to survive and cause disease in the oral cavity. Previously, we have shown that S. mutans increases the proportion of monounsaturated membrane fatty acids as part of its acid-adaptive strategy. Membrane lipids function as carriers of membrane fatty acids and therefore it was hypothesized that lipid backbones themselves could participate in the acid adaptation process. Lipids have been shown to protect other bacterial species from rapid changes in their environment, such as shifts in osmolality and the need for long-term survival. In the present study, we have determined the contribution of cardiolipin (CL) to acid resistance in S. mutans. Two ORFs have been identified in the S. mutans genome that encode presumptive synthetic enzymes for the acidic phospholipids: phosphatidylglycerol (PG) synthase (pgsA, SMU.2151c) and CL synthase (cls, SMU.988), which is responsible for condensing two molecules of PG to create CL. A deletion mutant of the presumptive cls gene was created using PCR-mediated cloning; however, attempts to delete pgsA were unsuccessful, indicating that pgsA may be essential. Loss of the presumptive cls gene resulted in the inability of the mutant strain to produce CL, indicating that SMU.988 encodes CL synthase. The defect in cls rendered the mutant acid sensitive, indicating that CL is required for acid adaptation in S. mutans. Addition of exogenous CL to the mutant strain alleviated acid sensitivity. MS indicated that S. mutans could assimilate exogenous CL into the membrane, halting endogenous CL incorporation. This phenomenon was not due to repression, as a cls gene transcriptional reporter fusion exhibited elevated activity when cells were supplemented with exogenous CL. Lipid analysis, via MS, indicated that CL is a reservoir for monounsaturated fatty acids in S. mutans. We demonstrated that the cls mutant exhibits elevated F-ATPase activity but it is nevertheless unable to maintain the normal membrane proton gradient, indicating cytoplasmic acidification. We conclude that the control of lipid backbone synthesis is part of the acid-adaptive repertoire of S. mutans.

Cardiolipin binding in bacterial respiratory complexes: structural and functional implications

The structural and functional integrity of biological membranes is vital to life. The interplay of lipids and membrane proteins is crucial for numerous fundamental processes ranging from respiration, photosynthesis, signal transduction, solute transport to motility. Evidence is accumulating that specific lipids play important roles in membrane proteins, but how specific lipids interact with and enable membrane proteins to achieve their full functionality remains unclear. X-ray structures of membrane proteins have revealed tight and specific binding of lipids. For instance, cardiolipin, an anionic phospholipid, has been found to be associated to a number of eukaryotic and prokaryotic respiratory complexes. Moreover, polar and septal accumulation of cardiolipin in a number of prokaryotes may ensure proper spatial segregation and/or activity of proteins. In this review, we describe current knowledge of the functions associated with cardiolipin binding to respiratory complexes in prokaryotes as a frame to discuss how specific lipid binding may tune their reactivity towards quinone and participate to supercomplex formation of both aerobic and anaerobic respiratory chains. This article is part of a Special Issue entitled: 17th European Bioenergetics Conference (EBEC 2012).

The evolution of cardiolipin biosynthesis and maturation pathways and its implications for the evolution of eukaryotes

Background

Cardiolipin (CL) is an important component in mitochondrial inner and bacterial membranes. Its appearance in these two biomembranes has been considered as evidence of the endosymbiotic origin of mitochondria. But CL was reported to be synthesized through two distinct enzymes--CLS_cap and CLS_pld in eukaryotes and bacteria. Therefore, how the CL biosynthesis pathway evolved is an interesting question.

Results

Phylogenetic distribution investigation of CL synthase (CLS) showed: most bacteria have CLS_pld pathway, but in partial bacteria including proteobacteria and actinobacteria CLS_cap pathway has already appeared; in eukaryotes, Supergroup Opisthokonta and Archaeplastida, and Subgroup Stramenopiles, which all contain multicellular organisms, possess CLS_cap pathway, while Supergroup Amoebozoa and Excavata and Subgroup Alveolata, which all consist exclusively of unicellular eukaryotes, bear CLS_pld pathway; amitochondriate protists in any supergroups have neither. Phylogenetic analysis indicated the CLS_cap in eukaryotes have the closest relationship with those of alpha proteobacteria, while the CLS_pld in eukaryotes share a common ancestor but have no close correlation with those of any particular bacteria.

Conclusions

The first eukaryote common ancestor (FECA) inherited the CLS_pld from its bacterial ancestor (e. g. the bacterial partner according to any of the hypotheses about eukaryote evolution); later, when the FECA evolved into the last eukaryote common ancestor (LECA), the endosymbiotic mitochondria (alpha proteobacteria) brought in CLS_cap, and then in some LECA individuals the CLS_cap substituted the CLS_pld, and these LECAs would evolve into the protist lineages from which multicellular eukaryotes could arise, while in the other LECAs the CLS_pld was retained and the CLS_cap was lost, and these LECAs would evolve into the protist lineages possessing CLS_pld. Besides, our work indicated CL maturation pathway arose after the emergence of eukaryotes probably through mechanisms such as duplication of other genes, and gene duplication and loss occurred frequently at different lineage levels, increasing the pathway diversity probably to fit the complicated cellular process in various cells. Our work also implies the classification putting Stramenopiles and Alveolata together to form Chromalveolata may be unreasonable; the absence of CL synthesis and maturation pathways in amitochondriate protists is most probably due to secondary loss.

Characterization of Staphylococcus aureus cardiolipin synthases 1 and 2 and their contribution to accumulation of cardiolipin in stationary phase and within phagocytes

In many bacteria, including Staphylococcus aureus, progression from the logarithmic to the stationary phase is accompanied by conversion of most of bacterial membrane phosphatidylglycerol (PG) to cardiolipin (CL). Phagocytosis of S. aureus by human neutrophils also induces the conversion of most bacterial PG to CL. The genome of all sequenced strains of S. aureus contains two open reading frames (ORFs) predicting proteins encoded with ∼30% identity to the principal CL synthase (cls) of Escherichia coli. To test whether these ORFs (cls1 and cls2) encode cardiolipin synthases and contribute to CL accumulation in S. aureus, we expressed these proteins in a cls strain of E. coli and created isogenic single and double mutants in S. aureus. The expression of either Cls1 or Cls2 in CL-deficient E. coli resulted in CL accumulation in the stationary phase. S. aureus with deletion of both cls1 and cls2 showed no detectable CL accumulation in the stationary phase or after phagocytosis by neutrophils. CL accumulation in the stationary phase was due almost solely to Cls2, whereas both Cls1 and Cls2 contributed to CL accumulation following phagocytosis by neutrophils. Differences in the relative contributions of Cls1 and Cls2 to CL accumulation under different triggering conditions suggest differences in the role and regulation of these two enzymes.

Cardiolipin and the osmotic stress responses of bacteria

Cells control their own hydration by accumulating solutes when they are exposed to high osmolality media and releasing solutes in response to osmotic down-shocks. Osmosensory transporters mediate solute accumulation and mechanosensitive channels mediate solute release. Escherichia coli serves as a paradigm for studies of cellular osmoregulation. Growth in media of high salinity alters the phospholipid headgroup and fatty acid compositions of bacterial cytoplasmic membranes, in many cases increasing the ratio of anionic to zwitterionic lipid. In E. coli, the proportion of cardiolipin (CL) increases as the proportion of phosphatidylethanolamine (PE) decreases when osmotic stress is imposed with an electrolyte or a non-electrolyte. Osmotic induction of the gene encoding CL synthase (cls) contributes to these changes. The proportion of phosphatidylglycerol (PG) increases at the expense of PE in cls(-) bacteria and, in Bacillus subtilis, the genes encoding CL and PG synthases (clsA and pgsA) are both osmotically regulated. CL is concentrated at the poles of diverse bacterial cells. A FlAsH-tagged variant of osmosensory transporter ProP is also concentrated at E. coli cell poles. Polar concentration of ProP is CL-dependent whereas polar concentration of its paralogue LacY, a H(+)-lactose symporter, is not. The proportion of anionic lipids (CL and PG) modulates the function of ProP in vivo and in vitro. These effects suggest that the osmotic induction of CL synthesis and co-localization of ProP with CL at the cell poles adjust the osmolality range over which ProP activity is controlled by placing it in a CL-rich membrane environment. In contrast, a GFP-tagged variant of mechanosensitive channel MscL is not concentrated at the cell poles but anionic lipids bind to a specific site on each subunit of MscL and influence its function in vitro. The sub-cellular locations and lipid dependencies of other osmosensory systems are not known. Varying CL content is a key element of osmotic adaptation by bacteria but much remains to be learned about its roles in the localization and function of osmoregulatory proteins.

Molecular genetic approaches to defining lipid function

Lipids fulfill multiple and diverse functions in cells. Establishing the molecular basis for these functions has been challenging due to the lack of catalytic activity of lipids and the pleiotropic effects of mutations that affect lipid composition. By combining molecular genetic manipulation of membrane lipid composition with biochemical characterization of the resulting phenotypes, the molecular details of novel lipid functions have been established. This review summarizes the results of such a combined approach to defining lipid function in bacteria.

Cardiolipin synthesis for the assembly of bacterial and mitochondrial membranes

In this article, the formation of prokaryotic and eukaryotic cardiolipin is reviewed in light of its biological function. I begin with a detailed account of the structure of cardiolipin, its stereochemistry, and the resulting physical properties, and I present structural analogs of cardiolipin that occur in some organisms. Then I continue to discuss i) the de novo formation of cardiolipin, ii) its acyl remodeling, iii) the assembly of cardiolipin into biological membranes, and iv) the degradation of cardiolipin, which may be involved in apoptosis and mitochondrial fusion. Thus, this article covers the entire metabolic cycle of this unique phospholipid. It is shown that mitochondria produce cardiolipin species with a high degree of structural uniformity and molecular symmetry, among which there is often a dominant form with four identical acyl chains. The subsequent assembly of cardiolipin into functional membranes is largely unknown, but the analysis of crystal structures of membrane proteins has revealed a first glimpse into the underlying principles of cardiolipin-protein interactions. Disturbances of cardiolipin metabolism are crucial in the pathophysiology of human Barth syndrome and perhaps also play a role in diabetes and ischemic heart disease.

Cardiolipin promotes polar localization of osmosensory transporter ProP in Escherichia coli

The osmolality required to activate osmosensory transporter ProP and the proportion of cardiolipin (CL) among the phospholipids of Escherichia coli rise with growth medium osmolality. Most CL synthesis has been attributed to the cls gene product. Transcription of cls increased with osmolality. The proportion of CL was low and osmolality-independent in cls(-) bacteria. It increased more dramatically on the transition to stationary phase in cls(-) than cls(+) bacteria. Thus, Cls is responsible for osmoregulated CL synthesis and other enzymes may contribute to CL accumulation during stationary phase. The proportion of phosphatidylglycerol (PG) was elevated and it increased with medium osmolality in cls(-) bacteria. A cls defect impaired growth of E. coli on solid and in liquid media at low and, more strongly, at high osmolality. Bacteria cultured at high osmolality without osmoprotectant were shorter and rounder than those cultured at low osmolality or with glycine betaine. Fluorescence microscopy showed that CL and ProP colocalize at the poles and near the septa of dividing E. coli cells. The polar localization of ProP was independent of its expression level but correlated with the proportion and polar localization of CL. Association with CL (and not PG) may be required for polar ProP localization.

Vincent G, Silman NJ, Brehm JK, Elmore MJ, Hudson MJ, Forsman M, Isherwood KE, Guryčová D, Minton NP, Titball RW, Pallen MJ, Vipond R. Genome sequencing shows that European isolates of Francisella tularensis subspecies tularensis are almost identical to US laboratory strain Schu S4

Background

Francisella tularensis causes tularaemia, a life-threatening zoonosis, and has potential as a biowarfare agent. F. tularensis subsp. tularensis, which causes the most severe form of tularaemia, is usually confined to North America. However, a handful of isolates from this subspecies was obtained in the 1980s from ticks and mites from Slovakia and Austria. Our aim was to uncover the origins of these enigmatic European isolates.

Methodology/Principal Findings

We determined the complete genome sequence of FSC198, a European isolate of F. tularensis subsp. tularensis, by whole-genome shotgun sequencing and compared it to that of the North American laboratory strain Schu S4. Apparent differences between the two genomes were resolved by re-sequencing discrepant loci in both strains. We found that the genome of FSC198 is almost identical to that of Schu S4, with only eight SNPs and three VNTR differences between the two sequences. Sequencing of these loci in two other European isolates of F. tularensis subsp. tularensis confirmed that all three European isolates are also closely related to, but distinct from Schu S4.

Conclusions/Significance

The data presented here suggest that the Schu S4 laboratory strain is the most likely source of the European isolates of F. tularensis subsp. tularensis and indicate that anthropogenic activities, such as movement of strains or animal vectors, account for the presence of these isolates in Europe. Given the highly pathogenic nature of this subspecies, the possibility that it has become established wild in the heartland of Europe carries significant public health implications.

Anionic lipids enriched at the ExPortal of Streptococcus pyogenes

The ExPortal of Streptococcus pyogenes is a membrane microdomain dedicated to the secretion and folding of proteins. We investigated the lipid composition of the ExPortal by examining the distribution of anionic membrane phospholipids. Staining with 10-N-nonyl-acridine orange revealed a single microdomain enriched with an anionic phospholipid whose staining characteristics and behavior in a cardiolipin-deficient mutant were characteristic of phosphatidylglycerol. Furthermore, the location of the microdomain corresponded to the site of active protein secretion at the ExPortal. These results indicate that the ExPortal is an asymmetric lipid microdomain, whose enriched content of anionic phospholipids may play an important role in ExPortal organization and protein trafficking.

The osmotic activation of transporter ProP is tuned by both its C-terminal coiled-coil and osmotically induced changes in phospholipid composition

Transporter ProP of Escherichia coli (ProPEc) senses extracellular osmolality and mediates osmoprotectant uptake when it is rising or high. A replica of the ProPEc C terminus (Asp468-Arg497) forms an intermolecular alpha-helical coiled-coil. This structure is implicated in the osmoregulation of intact ProPEc, in vivo. Like that from Corynebacterium glutamicum (ProPCg), the ProP orthologue from Agrobacterium tumefaciens (ProPAt) sensed and responded to extracellular osmolality after expression in E. coli. The osmotic activation profiles of all three orthologues depended on the osmolality of the bacterial growth medium, the osmolality required for activation rising as the growth osmolality approached 0.7 mol/kg. Thus, each could undergo osmotic adaptation. The proportion of cardiolipin in a polar lipid extract from E. coli increased with extracellular osmolality so that the osmolality activating ProPEc was a direct function of membrane cardiolipin content. Group A ProP orthologues (ProPEc, ProPAt) share the C-terminal coiled-coil domain and were activated at low osmolalities. Like variant ProPEc-R488I, in which the C-terminal coiled-coil is disrupted, ProPEc derivatives that lack the coiled-coil and Group B orthologue ProPCg required a higher osmolality to activate. The amplitude of ProPEc activation was reduced 10-fold in its deletion derivatives. The coiled-coil structure is not essential for osmotic activation of ProP per se. However, it tunes Group A orthologues to osmoregulate over a low osmolality range. Coiled-coil lesions may impair both coiled-coil formation and interaction of ProPEc with amplifier protein ProQ. Cardiolipin may contribute to ProP adaptation by altering bulk membrane properties or by acting as a ProP ligand.

Cardiolipin synthase of Arabidopsis thaliana

Functional expression studies in microorganisms showed that the Arabidopsis thaliana gene At4g04870 represents the cardiolipin synthase (CLS) gene encoding a hydrophobic preprotein of 38 kDa with a cleavable signal peptide for the import into mitochondria. CLS of Arabidopsis over-expressed in Escherichia coli has an alkaline pH optimum, a strict requirement for divalent cations and a distinctly lower K(m) for cytidinediphosphate-diacylglycerol than for phosphatidylglycerol. It displayed a preference for both its substrates esterified with unsaturated acyl groups. Solubilization and purification experiments revealed that the protein requires a defined phospholipid environment, particularly the presence of cardiolipin, to acquire its catalytically active conformation.

Evolutionary genomics of ecological specialization

We used a combination of genomic techniques to monitor chromosomal evolution across hundreds of generations as Escherichia coli adapted to growth-limiting concentrations of either lactulose, methyl-galactoside, or a 72:28 mixture of the two. DNA microarrays identified 8 unique duplications and 16 unique deletions among 42 evolvants from 23 chemostat experiments. Each mutation was confirmed by sequencing PCR-amplified flanking genomic DNA and, except for one deletion, an insertion sequence was found at the break point. vPCR of insertion sequences identified these same mutations and 16 additional insertions (all confirmed by sequencing). The pattern of genomic evolution is highly reproducible. Statistical analyses show that duplications at lac and mutations in mgl are adaptations specific to lactulose and to methyl-galactoside, respectively. Adaptation to mixed sugars is characterized by similar mutations, but lac duplications and mgl mutations usually arise in different backgrounds, producing ecological specialists for each sugar. This suggests that an antagonistic pleiotropic tradeoff between duplications at lac and mutations in mgl retards the evolution of generalists. Other mutations that repeatedly appear in replicate experiments are adaptations to the chemostat environment and are not specific to one or the other sugar.

Bacterial Membrane Lipids: Where Do We Stand?

Phospholipids play multiple roles in bacterial cells. These are the establishment of the permeability barrier, provision of the environment for many enzyme and transporter proteins, and they influence membrane-related processes such as protein export and DNA replication. The lipid synthetic pathway also provides precursors for protein modification and for the synthesis of other molecules. This review concentrates on the phospholipid synthetic pathway and discusses recent data on the synthesis and function of phospholipids mainly in the bacterium Escherichia coli.

Phosphatidylglycerophosphate synthases from Arabidopsis thaliana

Two Arabidopsis thaliana genes were shown to encode phosphatidylglycerophosphate synthases (PGPS) of 25.4 and 32.2 kDa, respectively. Apart from their N-terminal regions, the two proteins exhibit high sequence similarity. Functional expression studies in yeast provided evidence that the 25.4 kDa protein is a microsomal PGPS while the 32.2 kDa protein represents a preprotein which can be imported into yeast mitochondria and processed to a mature PGPS. The two isozymes were solubilized and purified as fusion proteins carrying a His tag at their C-terminus. Enzyme assays with both membrane fractions and purified enzyme fractions revealed that the two A. thaliana isozymes have similar properties but differ in their CDP-diacylglycerol species specificity.

Phosphatidylinositol is an essential phospholipid of mycobacteria

Phosphatidylinositol (PI) and metabolically derived products such as the phosphatidylinositol mannosides and linear and mature branched lipomannan and lipoarabinomannan are prominent phospholipids/lipoglycans of Mycobacterium sp. believed to play important roles in the structure and physiology of the bacterium as well as during host infection. To determine if PI is an essential phospholipid of mycobacteria, we identified the pgsA gene of Mycobacterium tuberculosis encoding the phosphatidylinositol synthase enzyme and constructed a pgsA conditional mutant of Mycobacterium smegmatis. The ability of this mutant to synthesize phosphatidylinositol synthase and subsequently PI was dependent on the presence of a functional copy of the pgsA gene carried on a thermosensitive plasmid. The mutant grew like the control strain under permissive conditions (30 degrees C), but ceased growing when placed at 42 degrees C, a temperature at which the rescue plasmid is lost. Loss of cell viability at 42 degrees C was observed when PI and phosphatidylinositol dimannoside contents dropped to approximately 30 and 50% of the wild-type levels, respectively. This work provides the first evidence of the essentiality of PI to the survival of mycobacteria. PI synthase is thus an essential enzyme of Mycobacterium that shows promise as a drug target for anti-tuberculosis therapy.