[The effect of Streptococcus mutans luxS gene on mixed-species biofilm communities]

PURPOSE

To evaluate the effect of S.mutans luxS gene on mixed-species biofilms communities.

METHODS

Biofilms were formed by S. mutans (wild type strain, its luxS overexpression strain and luxS knockout strain) and Lactobacillus acidophilus (ATCC4356) with a ratio of 1:1 at 37℃ for 4 h, 14 h and 24 h. MTT assay was used to detect the quantification of the biofilms formed. The structures of biofilms were observed under confocal laser scanning microscopy after 24 h, and expression of biofilm-related genes (ftf, smu630, brpA, gbpB, gtfB, vicR, comDE and relA) was investigated by real-time PCR. Statistical analysis was performed with SPSS17.0 software package.

RESULTS

The results showed that biofilm formed by S. mutans(wild type strain, its luxS overexpression strain and luxS knockout strain) and L.acidophilus after 14 h were 0.481±0.024, 0.591±0.023 and 0.279±0.019, respectively. The same findings were present after 24 h, the biofilm formed by S.mutans overexpression strain with L.acidophilus was higher than wild type strain, and the biofilm formed by knockout strain significantly decreased; but there was no significant difference at 4 h time points. CLSM images revealed that both S.mutans overexpression strain and its wild type strain tended to aggregate into distinct clusters and dense structures, whereas the luxS knockout strain appeared relatively sparse. Compared with wild type strain, all of the genes examined were upregulated in the biofilms formed by the overexpression strain, and were downregulated in the biofilms formed by the luxS mutant strain in mixed-species biofilm.

CONCLUSIONS

S.mutans luxS gene can affect mixed-species biofilm formation with L.acidophilus, which provides evidences for further study.

Quorum sensing LuxS/autoinducer-2 inhibits Enterococcus faecalis biofilm formation ability

OBJECTIVE

To investigate the relation between biofilm formation ability and quorum sensing gene LuxS/AI-2.

MATERIALS AND METHODS

Enterococcus faecalis (E. faecalis) standard strain ATCC 29212 was used in the study. Long flanking homology polymerase chain reaction method was used to build the LuxS gene knockout strain. Sequential culture turbidity measurement and CFU counting were used to assess the proliferation ability of E. faecalis after the depletion of LuxS. 96-well plate assay was used to quantify the biofilm formation ability; CLSM was used to observe the attached bacteria areas, while scanning electron microscopy (SEM) was performed to observe biofilm microstructure conditions.

RESULTS

LuxS gene knockout strains were successfully constructed and identified. The results showed that proliferation ability of E. faecalis was not affected by the depletion of the luxS gene, and the biofilm formation ability of ΔLuxS 29212 significantly decreased (P<0.05).

CONCLUSIONS

Collectively, our studies provide the LuxS gene's key role in controlling biofilm formation of E. faecalis, which presented a negative regulation, and furthermore, providing us a possible way to conquer the persistent apical periodontitis.

Biogenesis and functions of mammalian iron-sulfur proteins in the regulation of iron homeostasis and pivotal metabolic pathways

Fe-S cofactors are composed of iron and inorganic sulfur in various stoichiometries. A complex assembly pathway conducts their initial synthesis and subsequent binding to recipient proteins. In this minireview, we discuss how discovery of the role of the mammalian cytosolic aconitase, known as iron regulatory protein 1 (IRP1), led to the characterization of the function of its Fe-S cluster in sensing and regulating cellular iron homeostasis. Moreover, we present an overview of recent studies that have provided insights into the mechanism of Fe-S cluster transfer to recipient Fe-S proteins.

The catabolic enzyme methionine gamma-lyase limits methionine accumulation in potato tubers

Increasing methionine in potato tubers is desirable, both to increase the availability of this limiting essential amino acid and to enhance the aroma of baked and fried potatoes. Previous attempts to elevate potato methionine content using transgenic approaches have focused on increasing methionine biosynthesis. Higher isoleucine accumulation in these transgenic tubers suggested that the potatoes compensate for increased methionine biosynthesis with enhanced catabolism via methionine gamma-lyase (MGL), thereby producing 2-ketybutyrate for isoleucine biosynthesis. In the current study, we show that potato StMGL1 encodes a functional MGL in potato tubers. In planta silencing of StMGL1 results in an increased methionine to isoleucine ratio in the free amino acid profile of potato tubers and, in some transgenic lines, elevated accumulation of free methionine. In both wild-type and transgenic tubers, the ratio of methionine to isoleucine is negatively correlated with the level of StMGL1 transcript. A three-dimensional distribution of free amino acids in potato tubers is also described.

LsrR quorum sensing "switch" is revealed by a bottom-up approach

Quorum sensing (QS) enables bacterial multicellularity and selective advantage for communicating populations. While genetic “switching” phenomena are a common feature, their mechanistic underpinnings have remained elusive. The interplay between circuit components and their regulation are intertwined and embedded. Observable phenotypes are complex and context dependent. We employed a combination of experimental work and mathematical models to decipher network connectivity and signal transduction in the autoinducer-2 (AI-2) quorum sensing system of E. coli. Negative and positive feedback mechanisms were examined by separating the network architecture into sub-networks. A new unreported negative feedback interaction was hypothesized and tested via a simple mathematical model. Also, the importance of the LsrR regulator and its determinant role in the E. coli QS “switch”, normally masked by interfering regulatory loops, were revealed. Our simple model allowed mechanistic understanding of the interplay among regulatory sub-structures and their contributions to the overall native functioning network. This “bottom up” approach in understanding gene regulation will serve to unravel complex QS network architectures and lead to the directed coordination of emergent behaviors.

Quorum sensing is a mechanism by which bacterial cells communicate within a population. One particular form of communication in E. coli is through a universal signaling molecule known as autoinducer 2. Although the importance of this form of cell-cell interaction has been recognized in the formation of biofilms and virulent infections, the mechanisms by which this form of communication is regulated is still not well understood. In this paper, we presented a method of unraveling these mechanisms by using a combination of experimental work and mathematical models. We took apart the network architecture and isolated the different components. The examination of these isolated sub-networks provided us with a better understanding of the underlying mechanisms that control and regulate bacterial quorum sensing. We were also able to predict new network interactions with the help of our mathematical models. This bottom up approach, combined with our modeling efforts, proved effective in unraveling the mechanisms of quorum sensing in E. coli.

Regulation of Vibrio alginolyticus virulence by the LuxS quorum-sensing system

Quorum sensing (QS) is a bacterial intercommunication system that controls the expression of multiple genes in response to population density. The LuxS QS system regulates the expression of several virulence factors in a wide variety of pathogenic bacteria. LuxS has been characterized to be responsible for producing a type of autoinducer, AI-2, which stimulates the expression of the luciferase operon in Vibrio harveyi. Vibrio alginolyticus is established as an opportunistic pathogen of several marine animals, and its LuxS QS system remains undefined. To investigate the pathogenic role of luxS in V. alginolyticus, the luxS mutants of both the standard strain ATCC 33787 and a fish-clinical isolate MVP01, named MYJS and MYJM, respectively, were constructed. The mutation resulted in reduced lethality to Pagrus major. Intraperitoneal LD(50) of MYJS and MYJM increased by 15- and 93-fold, respectively. The two luxS mutants exhibited a lower growth rate and defective flagellar biosynthesis. They also showed a significant decrease in protease production and an increase in both extracellular polysaccharide production and biofilm development. The results suggest that the LuxS QS system plays an important role in regulating the expression of virulence factors in V. alginolyticus.

Production of shiga toxin by a luxS mutant of Escherichia coli O157:H7 in vivo and in vitro

Quorum sensing is a type of bacterial communication mediated by chemical signaling molecules called autoinducers (AIs). The production of AI-2 and AI-3 is dependent on the luxS gene in Escherichia coli O157:H7. A luxS mutation caused a minimal decrease (about 2-fold) in Shiga toxin (Stx) production in in vitro cultures using Luria-Bertani broth. The effect of a luxS mutation on the virulence of E. coli O157:H7 was examined by using germfree mice. There were no differences between the luxS mutant and the wild-type in the bacterial counts in feces shedding, Stx production, or the survival of the mice. The treatment of ciprofloxacin decreased the bacteria in feces but increased the Stx production. However, even treatment with ciprofloxacin did not make any difference between the luxS mutant and the wild-type in animal experiments.

The role of luxS in the fire blight pathogen Erwinia amylovora is limited to metabolism and does not involve quorum sensing

Erwinia amylovora is a gram-negative phytopathogen that causes fire blight of pome fruit and related members of the family Rosaceae. We sequenced the putative autoinducer-2 (AI-2) synthase gene luxS from E. amylovora. Diversity analysis indicated that this gene is extremely conserved among E. amylovora strains. Quorum sensing mediated by LuxS has been implicated in coordinated gene expression, growth, and virulence in other enterobacteria; however, our evidence suggests this is not the function in E. amylovora. Mutational analysis pointed to a role in colonization of apple blossoms, the primary infection court for fire blight, although little if any role in virulence on apple shoots and pear fruit was observed. Expression of key virulence genes hrpL and dspA/E was reduced in mutants of two E. amylovora strains. Stronger effects on gene expression were observed for metabolic genes involved in the activated methyl cycle with mutants having greater levels of expression. No quorum-sensing effect was observed in coculture experiments with wild-type and mutant strains either in vitro or in apple blossoms. Known receptors essential for AI-2 quorum sensing, the LuxPQ sensor kinase or the Lsr ABC-transporter, are absent in E. amylovora, further suggesting a primarily metabolic role for luxS in this bacterium.

Implication of quorum sensing in Salmonella enterica serovar typhimurium virulence: the luxS gene is necessary for expression of genes in pathogenicity island 1

Despite the fact that the regulatory system sensing density of cell population and its signaling molecule have been identified in Salmonella enterica, the biological significance of this phenomenon termed as quorum sensing remains unknown. In this report, we provide evidence that the luxS gene is necessary for Salmonella virulence phenotypes. Transcription assays showed that the cell-density-dependent induction of the invF gene was abolished in a Salmonella strain with the luxS gene deleted. The effect of the luxS deletion was also investigated in other InvF-regulated genes expressed from Salmonella pathogenicity island 1 (SPI-1). The decreased expression of SPI-1 genes in the strain with luxS deleted could be restored by either the addition of a synthetic signal molecule or the introduction of a plasmid copy of the luxS gene. Thus, the reduced expression of invF and its regulated genes in Salmonella cells lacking quorum sensing resulted in the attenuation of virulence phenotypes both in vitro and in vivo.

Autoinducer 2 is required for biofilm growth of Aggregatibacter (Actinobacillus) actinomycetemcomitans

Autoinducer 2 (AI-2) is required for the growth of Aggregatibacter (Actinobacillus) actinomycetemcomitans in culture under conditions of iron limitation. However, in vivo this organism thrives in a complex multispecies biofilm that forms in the human oral cavity. In this report, we show that adherent growth of A. actinomycetemcomitans on a saliva-coated surface, but not planktonic growth under iron-replete conditions, is defective in a LuxS-deficient background. Biofilm growth of the luxS mutant exhibited lower total biomass and lower biofilm depth than those for the wild-type strain. Normal biofilm growth of the luxS mutant was restored genetically by introduction of a functional copy of luxS and biochemically by addition of partially purified AI-2. Furthermore, introduction of S-adenosylhomocysteine hydrolase, which restores the metabolism of S-adenosylmethionine in the absence of LuxS, into A. actinomycetemcomitans did not complement the luxS mutation unless AI-2 was added in trans. This suggests that AI-2 itself is required for biofilm growth by A. actinomycetemcomitans. A biofilm growth deficiency similar to that of the LuxS-deficient strain was also observed when a gene encoding the AI-2-interacting protein RbsB or LsrB was inactivated. Biofilm formation by A. actinomycetemcomitans was virtually eliminated upon inactivation of both rbsB and lsrB. In addition, biofilm growth by wild-type A. actinomycetemcomitans was reduced in the presence of ribose, which competes with AI-2 for binding to RbsB. These results suggest that RbsB and LsrB function as AI-2 receptors in A. actinomycetemcomitans and that the development of A. actinomycetemcomitans biofilms requires AI-2.

The quorum-sensing molecule autoinducer 2 regulates motility and flagellar morphogenesis in Helicobacter pylori

The genome of the gastric pathogen Helicobacter pylori contains a homologue of the gene luxS, which has been shown to be responsible for production of the quorum-sensing signal autoinducer 2 (AI-2). We report here that deletion of the luxS gene in strain G27 resulted in decreased motility on soft agar plates, a defect that was complemented by a wild-type copy of the luxS gene and by the addition of cell-free supernatant containing AI-2. The flagella of the luxS mutant appeared normal; however, in genetic backgrounds lacking any of three flagellar regulators--the two-component sensor kinase flgS, the sigma factor sigma28 (also called fliA), and the anti-sigma factor flgM--loss of luxS altered flagellar morphology. In all cases, the double mutant phenotypes were restored to the luxS+ phenotype by the addition of synthetic 4,5-dihydroxy-2,3-pentanedione (DPD), which cyclizes to form AI-2. Furthermore, in all mutant backgrounds loss of luxS caused a decrease in transcript levels of the flagellar regulator flhA. Addition of DPD to luxS cells induced flhA transcription in a dose-dependent manner. Deletion of flhA in a wild-type or luxS mutant background resulted in identical loss of motility, flagella, and flagellar gene expression. These data demonstrate that AI-2 functions as a secreted signaling molecule upstream of FlhA and plays a critical role in global regulation of flagellar gene transcription in H. pylori.

The possible influence of LuxS in the in vivo virulence of rabbit enteropathogenic Escherichia coli

Attaching and effacing (A/E) organisms, such as rabbit enteropathogenic Escherichia coli (EPEC), human EPEC or enterohemorrhagic E. coli (EHEC) share attaching and effacing phenotype and LEE pathogenicity island responsible for A/E. The present study was undertaken to investigate the impact of the LuxS quorum sensing (QS) signaling system in vitro and in vivo pathogenicity of A/E organisms using rabbit EPEC (rEPEC) strain E22 (O103:H2). Analysis of the bioluminescence indicated abolished production of the QS signal AI-2 by luxS mutant (E22DeltaluxS). Strain E22Deltalux also exhibited impaired expression of several normally secreted proteins and reduced adherence to cultured HeLa cells. Complementation of the intact luxS gene to E22DeltaluxS restored secreted protein expression comparable to the WT type but not adherence to HeLa cells. In experimentally infected rabbits, the isogenic luxS mutant induced clinical illness and intimate adherence to the intestinal mucosa, albeit to a less extent, comparable to that seen with the parent virulent strain. It is worth noting that reduced fecal bacterial shedding, mucosal adherence and improved cumulative weight gain were seen for the mutant strain complemented with luxS when compared to the WT. It appears that the luxS gene is not essential for in vivo pathogenicity by rEPEC where exogenous QS signals are present in the gut. The impact of AI-2 provided by multicopy plasmid on bacterial virulence is discussed.

Chloroplast iron-sulfur cluster protein maturation requires the essential cysteine desulfurase CpNifS

NifS-like proteins provide the sulfur (S) for the formation of iron-sulfur (Fe-S) clusters, an ancient and essential type of cofactor found in all three domains of life. Plants are known to contain two distinct NifS-like proteins, localized in the mitochondria (MtNifS) and the chloroplast (CpNifS). In the chloroplast, five different Fe-S cluster types are required in various proteins. These plastid Fe-S proteins are involved in a variety of biochemical pathways including photosynthetic electron transport and nitrogen and sulfur assimilation. In vitro, the chloroplastic cysteine desulfurase CpNifS can release elemental sulfur from cysteine for Fe-S cluster biogenesis in ferredoxin. However, because of the lack of a suitable mutant allele, the role of CpNifS has not been studied thus far in planta. To study the role of CpNifS in Fe-S cluster biogenesis in vivo, the gene was silenced by using an inducible RNAi (interference) approach. Plants with reduced CpNifS expression exhibited chlorosis, a disorganized chloroplast structure, and stunted growth and eventually became necrotic and died before seed set. Photosynthetic electron transport and carbon dioxide assimilation were severely impaired in the silenced plant lines. The silencing of CpNifS decreased the abundance of all chloroplastic Fe-S proteins tested, representing all five Fe-S cluster types. Mitochondrial Fe-S proteins and respiration were not affected, suggesting that mitochondrial and chloroplastic Fe-S assembly operate independently. These findings indicate that CpNifS is necessary for the maturation of all plastidic Fe-S proteins and, thus, essential for plant growth.

Autoinducer-2 of the fire blight pathogen Erwinia amylovora and other plant-associated bacteria

Autoinducers are important for cellular communication of bacteria. The luxS gene has a central role in the synthesis of autoinducer-2 (AI-2). The gene was identified in a shotgun library of Erwinia amylovora and primers designed for PCR amplification from bacterial DNA. Supernatants of several Erwinia amylovora strains were assayed for AI-2 activity with a Vibrio harveyi mutant and were positive. Many other plant-associated bacteria also showed AI-2 activity such as Erwinia pyrifoliae and Erwinia tasmaniensis. The luxS genes of several bacteria were cloned, sequenced, and complemented Escherichia coli strain DH5alpha and a Salmonella typhimurium mutant, both defective in luxS, for synthesis of AI-2. Assays to detect AI-2 activity in culture supernatants of several Pseudomonas syringae pathovars failed, which may indicate the absence of AI-2 or synthesis of another type. Several reporter strains did not detect synthesis of an acyl homoserine lactone (AHL, AI-1) by Erwinia amylovora, but confirmed AHL-synthesis for Erwinia carotovora ssp. atroseptica and Pantoea stewartii.

Autoinducer 3 and epinephrine signaling in the kinetics of locus of enterocyte effacement gene expression in enterohemorrhagic Escherichia coli

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 is responsible for causing outbreaks of bloody diarrhea and hemolytic-uremic syndrome throughout the world. The locus of enterocyte effacement (LEE) consists of five major operons and is required for the formation of attaching and effacing lesions that disrupt intestinal epithelial microvilli. We have previously reported that expression of EHEC LEE genes is regulated by the luxS quorum-sensing system. The luxS gene in EHEC affects the production of autoinducer 3 (AI-3), which activates the LEE. Epinephrine and norepinephrine also activate the LEE in a manner similar to that of AI-3. Previous studies of quorum-sensing regulation of LEE transcription have thus far been restricted to using reporter systems in an E. coli K-12 background. Here, we examined the kinetics of LEE gene transcription, protein expression, and function of the LEE type III secretion apparatus in wild-type (WT) EHEC and an isogenic luxS mutant. The results revealed that the luxS mutant had diminished transcription from the LEE promoters during the mid-exponential growth phase; decreased protein levels of EscJ, Tir, and EspA; and reduced secretion of EspA and EspB. The luxS mutation also caused a delay in the formation of attaching and effacing lesions on cultured epithelial cells compared to the wild type. Epinephrine enhanced LEE expression in both the WT and the luxS mutant, but the WT still exhibited greater LEE activation. The results suggest a possible synergistic relationship between AI-3 and epinephrine. The combined effects of these two signaling molecules may lead to greater LEE expression and a more efficient infection.

Functional analysis of luxS in the probiotic strain Lactobacillus rhamnosus GG reveals a central metabolic role important for growth and biofilm formation

Quorum sensing is involved in the regulation of multicellular behavior through communication via small molecules. Given the high number and diversity of the gastrointestinal microbiota, it is postulated that members of this community communicate to coordinate a variety of adaptive processes. AI-2 is suggested to be a universal bacterial signaling molecule synthesized by the LuxS enzyme, which forms an integral part of the activated methyl cycle. We have previously reported that the well-documented probiotic strain Lactobacillus rhamnosus GG, a human isolate, produces AI-2-like molecules. In this study, we identified the luxS homologue of L. rhamnosus GG. luxS seems to be located in an operon with a yxjH gene encoding a putative cobalamin-independent methionine synthase. In silico analysis revealed a methionine-specific T box in the leader sequence of the putative yxjH-luxS operon. However, transcriptional analysis showed that luxS is expressed mainly as a monocistronic transcript. Construction of a luxS knockout mutant confirmed that the luxS gene is responsible for AI-2 production in L. rhamnosus GG. However, this mutation also resulted in pleiotropic effects on the growth of this fastidious strain. Cysteine, pantothenate, folic acid, and biotin could partially complement growth, suggesting a central metabolic role for luxS in L. rhamnosus GG. Interestingly, the luxS mutant also showed a defect in monospecies biofilm formation. Experiments with chemically synthesized (S)-4,5-dihydroxy-2,3-pentanedione, coculture with the wild type, and nutritional complementation suggested that the main cause of this defect has a metabolic nature. Moreover, our data indicate that suppressor mutations are likely to occur in luxS mutants of L. rhamnosus GG. Therefore, results of luxS-related studies should be carefully interpreted.

RNA Silencing of Mitochondrial m-Nfs1 Reduces Fe-S Enzyme Activity Both in Mitochondria and Cytosol of Mammalian Cells

In prokaryotes and yeast, the general mechanism of biogenesis of iron-sulfur (Fe-S) clusters involves activities of several proteins among which IscS and Nfs1p provide, through cysteine desulfuration, elemental sulfide for Fe-S core formation. Although these proteins have been well characterized, the role of their mammalian homolog in Fe-S cluster biogenesis has never been evaluated. We report here the first functional study that implicates the putative cysteine desulfurase m-Nfs1 in the biogenesis of both mitochondrial and cytosolic mammalian Fe-S proteins. Depletion of m-Nfs1 in cultured fibroblasts through small interfering RNA-based gene silencing significantly inhibited the activities of mitochondrial NADH-ubiquinone oxidoreductase (complex I) and succinate-ubiquinone oxidoreductase (complex II) of the respiratory chain, as well as aconitase of the Krebs cycle, with no alteration in their protein levels. Activity of cytosolic xanthine oxidase, which holds a [2Fe-2S] cluster, was also specifically reduced, and iron-regulatory protein-1 was converted from its [4Fe-4S] aconitase form to its apo- or RNA-binding form. Reduction of Fe-S enzyme activities occurred earlier and more markedly in the cytosol than in mitochondria, suggesting that there is a mechanism that primarily dedicates m-Nfs1 to the biogenesis of mitochondrial Fe-S clusters in order to maintain cell survival. Finally, depletion of m-Nfs1, which conferred on apo-IRP-1 a high affinity for ferritin mRNA, was associated with the down-regulation of the iron storage protein ferritin.

Quorum sensing in Erwinia species

The term quorum sensing (QS) refers to the ability of bacteria to regulate gene expression according to the accumulation of signalling molecules that are made by every cell in the population. The erwiniae group of bacteria are often phytopathogens and the expression of a number of their important virulence determinants and secondary metabolites is under QS control. The erwiniae utilise two types of QS signalling molecules: N-acyl homoserine lactones and AI-2-type signalling molecules. Here, we review the regulatory networks involving QS in the soft rot erwiniae.

Quantitative effect of luxS gene inactivation on the fitness of Helicobacter pylori

Furanone metabolites called AI-2 (autoinducer 2), used by some bacterial species for signaling and cell density-regulated changes in gene expression, are made while regenerating S-adenosyl methionine (SAM) after its use as a methyl donor. The luxS-encoded enzyme, in particular, participates in this activated methyl cycle by generating both a pentanedione, which is transformed chemically into these AI-2 compounds, and homocysteine, a precursor of methionine and SAM. Helicobacter pylori seems to contain the genes for this activated methyl cycle, including luxS, but not genes for AI-2 uptake and transcriptional regulation. Here we report that deletion of luxS in H. pylori reference strain SS1 diminished its competitive ability in mice and motility in soft agar, whereas no such effect was seen with an equivalent Delta luxS derivative of the unrelated strain X47. These different outcomes are consistent with H. pylori's considerable genetic diversity and are reminiscent of phenotypes seen after deletion of another nonessential metabolic gene, that encoding polyphosphate kinase 1. We suggest that synthesis of AI-2 by H. pylori may be an inadvertent consequence of metabolite flux in its activated methyl cycle and that impairment of this cycle and/or pathways affected by it, rather than loss of quorum sensing, is deleterious for some H. pylori strains. Also tenable is a model in which AI-2 affects other microbes in H. pylori's gastric ecosystem and thereby modulates the gastric environment in ways to which certain H. pylori strains are particularly sensitive.

LuxS-mediated signalling in Streptococcus anginosus and its role in biofilm formation

The autoinducer-2 signal (AI-2) produced by several Gram-positive and Gram-negative bacteria mediates interspecies communication. In this study we were able to identify an orthologue of luxS, required for the synthesis of AI-2 signals, in Streptococcus anginosus. Comparative analyses revealed conserved sequences in the predicted S. anginosus LuxS. Expression of luxS was highest during early exponential growth phase. Compared to other oral streptococci, conditioned media from growth of members of the anginosus group were the most efficient in inducing bioluminescence in Vibrio harveyi, indicative of AI-2 signalling. Disruption of luxS in S. anginosus resulted in a mutant deficient in biofilm formation, whereas no effect on planktonic growth rate was observed under various growth conditions. S. anginosus is part of the human flora found in biofilms of the oral cavity, as well as of the upper respiratory, gastrointestinal and urogenital tracts. Such habitats harbour large varieties of bacterial species, among which cell-cell communication may play an important role. S. anginosus has also been associated with purulent infections and cancer in the upper digestive tract. Knowledge about the molecular mechanisms involved in S. anginosus communication is important for understanding its commensalism and its pathogenic transition.

Roles of the Mammalian Cytosolic Cysteine Desulfurase, ISCS, and Scaffold Protein, ISCU, in Iron-Sulfur Cluster Assembly

Iron-sulfur clusters are prosthetic groups composed of sulfur and iron that are found in respiratory chain complexes and numerous enzymes. Iron-sulfur clusters are synthesized in a multistep process that utilizes cysteine desulfurases, scaffold proteins, chaperones, and iron donors. Assembly of iron-sulfur clusters occurs in the mitochondrial matrix of mammalian cells, but cytosolic isoforms of three major mammalian iron-sulfur cluster (ISC) assembly components have been found, raising the possibility that de novo iron-sulfur cluster biogenesis also occurs in cytosol. The human cysteine desulfurase, ISCS, has two isoforms, one of which targets to the mitochondria, whereas the other less abundant form is cytosolic and nuclear. The open-reading frame of cytosolic mammalian ISCS begins at the second AUG of the transcript and lacks mitochondrial targeting information. Yeast complementation experiments have suggested that the human cytosolic ISCS isoform (c-ISCS) cannot be functional. To evaluate function of c-ISCS, we overexpressed the human cytosolic ISCS in yeast Pichia pastoris and showed that the cytosolic form of ISCS is an active cysteine desulfurase that covalently binds 35S acquired from desulfuration of radiolabeled cysteine. Human cytosolic ISCS dimerized as efficiently as bacterial ISCS and formed a complex in vitro with overexpressed cytosolic human ISCU. When incubated with iron regulatory protein 1, cysteine, and iron, the cytosolic forms of ISCS and ISCU facilitated efficient formation of a [4Fe-4S] cluster on IRP1. Thus, the cytosolic form of ISCS is a functional cysteine desulfurase that can collaborate with cytosolic ISCU to promote de novo iron-sulfur cluster formation.

CpNifS-dependent iron-sulfur cluster biogenesis in chloroplasts

Iron-sulfur (Fe-S) clusters are important prosthetic groups in all organisms. The biosynthesis of Fe-S clusters has been studied extensively in bacteria and yeast. By contrast, much remains to be discovered about Fe-S cluster biogenesis in higher plants. Plant plastids are known to make their own Fe-S clusters. Plastid Fe-S proteins are involved in essential metabolic pathways, such as photosynthesis, nitrogen and sulfur assimilation, protein import, and chlorophyll transformation. This review aims to summarize the roles of Fe-S proteins in essential metabolic pathways and to give an overview of the latest findings on plastidic Fe-S assembly. The plastidic Fe-S biosynthetic machinery contains many homologues of bacterial mobilization of sulfur (SUF) proteins, but there are additional components and properties that may be plant-specific. These additional features could make the plastidic machinery more suitable for assembling Fe-S clusters in the presence of oxygen, and may enable it to be regulated in response to oxidative stress, iron status and light.

Thiamine biosynthesis in Escherichia coli: in vitro reconstitution of the thiazole synthase activity

The biosynthesis of thiamine in Escherichia coli requires the formation of an intermediate thiazole from tyrosine, 1-deoxy-d-xylulose-5-phosphate (Dxp), and cysteine using at least six structural proteins, ThiFSGH, IscS, and ThiI. We describe for the first time the reconstitution of thiazole synthase activity using cell-free extracts and proteins derived from adenosine-treated E. coli 83-1 cells. The addition of adenosine or adenine to growing cultures of Aerobacter aerogenes, Salmonella typhimurium, and E. coli has been shown previously to relieve the repression by thiamine of its own biosynthesis and increase the expression levels of the thiamine biosynthetic enzymes. By exploiting this effect, we show that the in vitro thiazole synthase activity of cleared lysates or desalted proteins from E. coli 83-1 cells is dependent upon the addition of purified ThiGH-His complex, tyrosine (but not cysteine or 1-deoxy-d-xylulose-5-phosphate), and an as yet unidentified intermediate present in the protein fraction from these cells. The activity is strongly stimulated by the addition of S-adenosylmethionine and NADPH.

Requirement for IscS in biosynthesis of all thionucleosides in Escherichia coli

Escherichia coli tRNA contains four naturally occurring nucleosides modified with sulfur. Cysteine is the intracellular sulfur source for each of these modified bases. We previously found that the iscS gene, a member of the nifS cysteine desulfurase gene family, is required for 4-thiouridine biosynthesis in E. coli. Since IscS does not bind tRNA, its role is the mobilization and distribution of sulfur to enzymes that catalyze the sulfur insertion steps. In addition to iscS, E. coli contains two other nifS homologs, csdA and csdB, each of which has cysteine desulfurase activity and could potentially donate sulfur for thionucleoside biosynthesis. Double csdA csdB and iscS csdA mutants were prepared or obtained, and all mutants were analyzed for thionucleoside content. It was found that unfractionated tRNA isolated from the iscS mutant strain contained <5% of the level of sulfur found in the parent strain. High-pressure liquid chromatography analysis of tRNA nuclease digests from the mutant strain grown in the presence of [(35)S]cysteine showed that only a small fraction of 2-thiocytidine was present, while the other thionucleosides were absent when cells were isolated during log phase. As expected, digests from the iscS mutant strain contained 6-N-dimethylallyl adenosine (i(6)A) in place of 6-N-dimethylallyl-2-methylthioadenosine and 5-methylaminomethyl uridine (mnm(5)U) instead of 5-methylaminomethyl-2-thiouridine. Prolonged growth of the iscS and iscS csdA mutant strains revealed a gradual increase in levels of 2-thiocytidine and 6-N-dimethylallyl-2-methylthioadenosine with extended incubation (>24 h), while the thiouridines remained absent. This may be due to a residual level of Fe-S cluster biosynthesis in iscS deletion strains. An overall scheme for thionucleoside biosynthesis in E. coli is discussed.

The cysteine desulfurase IscS is required for synthesis of all five thiolated nucleosides present in tRNA from Salmonella enterica serovar typhimurium

Deficiency of a modified nucleoside in tRNA often mediates suppression of +1 frameshift mutations. In Salmonella enterica serovar Typhimurium strain TR970 (hisC3737), which requires histidine for growth, a potential +1 frameshifting site, CCC-CAA-UAA, exists within the frameshifting window created by insertion of a C in the hisC gene. This site may be suppressed by peptidyl-tRNAProcmo5UGG (cmo(5)U is uridine-5-oxyacetic acid), making a frameshift when decoding the near-cognate codon CCC, provided that a pause occurs by, e.g., a slow entry of the tRNAGlnmnm5s2UUG (mnm(5)s(2)U is 5-methylaminomethyl-2-thiouridine) to the CAA codon located in the A site. We selected mutants of strain TR970 that were able to grow without histidine, and one such mutant (iscS51) was shown to have an amino acid substitution in the L-cysteine desulfurase IscS. Moreover, the levels of all five thiolated nucleosides 2-thiocytidine, mnm(5)s(2)U, 5-carboxymethylaminomethyl-2-thiouridine, 4-thiouridine, and N-6-(4-hydroxyisopentenyl)-2-methylthioadenosine present in the tRNA of S. enterica were reduced in the iscS51 mutant. In logarithmically growing cells of Escherichia coli, a deletion of the iscS gene resulted in nondetectable levels of all thiolated nucleosides in tRNA except N-6-(4-hydroxyisopentenyl)-2-methylthioadenosine, which was present at only 1.6% of the wild-type level. After prolonged incubation of cells in stationary phase, a 20% level of 2-thiocytidine and a 2% level of N-6-(4-hydroxyisopentenyl)-2-methylthioadenosine was observed, whereas no 4-thiouridine, 5-carboxymethylaminomethyl-2-thiouridine, or mnm(5)s(2)U was found. We attribute the frameshifting ability mediated by the iscS51 mutation to a slow decoding of CAA by the tRNAGlnmnm5s2UUG due to mnm(5)s(2)U deficiency. Since the growth rate of the iscS deletion mutant in rich medium was similar to that of a mutant (mnmA) lacking only mnm(5)s(2)U, we suggest that the major cause for the reduced growth rate of the iscS deletion mutant is the lack of mnm(5)s(2)U and 5-carboxymethylaminomethyl-2-thiouridine and not the lack of any of the other three thiolated nucleosides that are also absent in the iscS deletion mutant.

The iscS gene is essential for the biosynthesis of 2-selenouridine in tRNA and the selenocysteine-containing formate dehydrogenase H

Three NifS-like proteins, IscS, CSD, and CsdB, from Escherichia coli catalyze the removal of sulfur and selenium from L-cysteine and L-selenocysteine, respectively, to form L-alanine. These enzymes are proposed to function as sulfur-delivery proteins for iron-sulfur cluster, thiamin, 4-thiouridine, biotin, and molybdopterin. Recently, it was reported that selenium mobilized from free selenocysteine is incorporated specifically into a selenoprotein and tRNA in vivo, supporting the involvement of the NifS-like proteins in selenium metabolism. We here report evidence that a strain lacking IscS is incapable of synthesizing 5-methylaminomethyl-2-selenouridine and its precursor 5-methylaminomethyl-2-thiouridine (mnm(5)s(2)U) in tRNA, suggesting that the sulfur atom released from L-cysteine by the action of IscS is incorporated into mnm(5)s(2)U. In contrast, neither CSD nor CsdB was essential for production of mnm(5)s(2)U and 5-methylaminomethyl-2-selenouridine. The lack of IscS also caused a significant loss of the selenium-containing polypeptide of formate dehydrogenase H. Together, these results suggest a dual function of IscS in sulfur and selenium metabolism.

Formation of methyl mercaptan from L-methionine by Porphyromonas gingivalis

Methyl mercaptan production by oral bacteria is thought to be one of the main causes of oral malodor. We examined the ability of periodontopathic Porphyromonas gingivalis to produce methyl mercaptan from L-methionine and found that the invasive strains W83 and W50 produced large amounts of methyl mercaptan. We cloned and sequenced the mgl gene encoding L-methionine-alpha-deamino-gamma-mercaptomethane-lyase (METase) from P. gingivalis W83. The structural mgl gene consisted of 1,200 bp and encoded a 43.3-kDa protein. To examine the role of methyl mercaptan in the pathogenesis of P. gingivalis, a METase-deficient mutant of P. gingivalis W83 was constructed. The methionine degradation activity and virulence of the mutant (M1217) and the parent strain (W83) in mice were compared. M1217 showed a marked decrease in the formation of methyl mercaptan from L-methionine and decreased virulence compared with the wild-type strain W83. These results suggest that methyl mercaptan not only is one of the sources of oral malodor, but may also play a role in the pathogenicity of P. gingivalis.

The iscS gene in Escherichia coli is required for the biosynthesis of 4-thiouridine, thiamin, and NAD

IscS, a cysteine desulfurase implicated in the repair of Fe-S clusters, was recently shown to act as a sulfurtransferase in the biosynthesis of 4-thiouridine (s(4)U) in tRNA (Kambampati, R., and Lauhon, C. T. (1999) Biochemistry 38, 16561-16568). In frame deletion of the iscS gene in Escherichia coli results in a mutant strain that lacks s(4)U in its tRNA. Assays of cell-free extracts isolated from the iscS(-) strain confirm the complete loss of tRNA sulfurtransferase activity. In addition to lacking s(4)U, the iscS(-) strain requires thiamin and nicotinic acid for growth in minimal media. The thiamin requirement can be relieved by the addition of the thiamin precursor 5-hydroxyethyl-4-methylthiazole, indicating that iscS is required specifically for thiazole biosynthesis. The growth rate of the iscS(-) strain is half that of the parent strain in rich medium. When the iscS(-) strain is switched from rich to minimal medium containing thiamin and nicotinate, growth is preceded by a considerable lag period relative to the parent strain. Addition of isoleucine results in a significant reduction in the duration of this lag phase. To examine the thiazole requirement, we have reconstituted the in vitro biosynthesis of ThiS thiocarboxylate, the ultimate sulfur donor in thiazole biosynthesis, and we show that IscS mobilizes sulfur for transfer to the C-terminal carboxylate of ThiS. ThiI, a known factor involved in both thiazole and s(4)U synthesis, stimulates this sulfur transfer step by 7-fold. Extracts from the iscS(-) strain show significantly reduced activity in the in vitro synthesis of ThiS thiocarboxylate. Transformation of the iscS(-) strain with an iscS expression plasmid complemented all of the observed phenotypic effects of the deletion mutant. Of the remaining two nifS-like genes in E. coli, neither can complement loss of iscS when each is overexpressed in the iscS(-) strain. Thus, IscS plays a significant and specific role at the top of a potentially broad sulfur transfer cascade that is required for the biosynthesis of thiamin, NAD, Fe-S clusters, and thionucleosides.

Role of a NifS-like protein from the cyanobacterium Synechocystis PCC 6803 in the maturation of FeS proteins

In Azotobacter vinelandii and Escherichia coli NifS or NifS-like proteins are involved in FeS protein assembly by mobilizing sulfur from free cysteine. This sulfur together with Fe(2+) is then incorporated into apo-FeS proteins to form an FeS center. A different activity termed C-DES [for cyst(e)ine desulfurylase] was recently isolated from the cyanobacterium Synechocystis PCC 6714 which also mobilized sulfur and which was able to incorporate the FeS center into apoferredoxin. In the genome of the cyanobacterium Synechocystis PCC 6803, there are three open reading frames (orfs) that are similar to NifS and one that is similar to C-DES, indicating that this bacterium might contain both activities, NifS and C-DES. One orf from Synechocystis PCC 6803 encoding a NifS-like protein, slr0387, was overexpressed in E. coli and purified. The molecular mass of the recombinant protein was determined to be about 82 kDa, indicating that it is a homodimer. The absorption spectrum was typical for PLP-containing proteins with an absorption maximum at 390 nm at pH 9.0 and at 425 nm at pH 6.5. The pH dependence of the absorption spectrum correlated with enzyme activity. Maximal activity measured as sulfide production was observed between pH 8.5 and 10. The activity decreased at lower pH values and was undetectable at pH 5.5. pH-dependent changes in the absorption spectrum and activity were attributed to protonation of the Schiff base formed by a lysine side chain and the PLP cofactor. Studies on substrate specificity demonstrated that cysteine derivatives other than cysteine methyl ester and cysteine-sulfinic acid could not serve as substrates for this enzyme. In particular, cystine was not a substrate for the Synechocystis NifS-like protein, whereas it is the best substrate for C-DES. In the presence of Fe(2+), cysteine, and a reductant, the NifS-like protein was able to produce holoferredoxin from apoferredoxin. The implications of two different activities for FeS center biosynthesis in Synechocystis are discussed.