Specialized Listeria monocytogenes produce tailocins to provide a population-level competitive growth advantage.

Tailocins are phage tail-like bacteriocins produced by various bacterial species to kill kin competitors. Given that tailocin release is dependent upon cell lysis, regulation of tailocin production at the single-cell and population level remains unclear. Here we used flow cytometry, competition assays and structural characterization of tailocin production in a human bacterial pathogen, Listeria monocytogenes. We revealed that a specialized subpopulation, constituting less than 1% of the total bacterial population, differentiates to produce, assemble and store thousands of tailocin particles. Tailocins are packed in a highly ordered manner, clustered in a liquid crystalline phase that occupies a substantial volume of the cell. Tailocin production confers a competitive growth advantage for the rest of the population. This study provides molecular insights into tailocin production as a form of altruism, showing how cell specialization within bacterial populations can confer competitive advantages at the population level.

A dual-function phage regulator controls the response of cohabiting phage elements via regulation of the bacterial SOS response.

Listeria monocytogenes strain 10403S harbors two phage elements in its chromosome; one produces infective virions and the other tailocins. It was previously demonstrated that induction of the two elements is coordinated, as they are regulated by the same anti-repressor. In this study, we identified AriS as another phage regulator that controls the two elements, bearing the capacity to inhibit their lytic induction under SOS conditions. AriS is a two-domain protein that possesses two distinct activities, one regulating the genes of its encoding phage and the other downregulating the bacterial SOS response. While the first activity associates with the AriS N-terminal AntA/AntB domain, the second associates with its C-terminal ANT/KilAC domain. The ANT/KilAC domain is conserved in many AriS-like proteins of listerial and non-listerial prophages, suggesting that temperate phages acquired such dual-function regulators to align their response with the other phage elements that cohabit the genome.

Listeria monocytogenes TcyKLMN Cystine/Cysteine Transporter Facilitates Glutathione Synthesis and Virulence Gene Expression.

Listeria monocytogenes is a saprophyte and a human intracellular pathogen. Upon invasion into mammalian cells, it senses multiple metabolic and environmental signals that collectively trigger its transition to the pathogenic state. One of these signals is the tripeptide glutathione, which acts as an allosteric activator of L. monocytogenes's master virulence regulator, PrfA. While glutathione synthesis by L. monocytogenes was shown to be critical for PrfA activation and virulence gene expression, it remains unclear how this tripeptide is synthesized in changing environments, especially in light of the observation that L. monocytogenes is auxotrophic to one of its precursors, cysteine. Here, we show that the ABC transporter TcyKLMN is a cystine/cysteine importer that supplies cysteine for glutathione synthesis, hence mediating the induction of the virulence genes. Further, we demonstrate that this transporter is negatively regulated by three metabolic regulators, CodY, CymR, and CysK, which sense and respond to changing concentrations of branched-chain amino acids (BCAA) and cysteine. The data indicate that under low concentrations of BCAA, TcyKLMN is upregulated, driving the production of glutathione by supplying cysteine, thereby facilitating PrfA activation. These findings provide molecular insight into the coupling of L. monocytogenes metabolism and virulence, connecting BCAA sensing to cysteine uptake and glutathione biosynthesis as a mechanism that controls virulence gene expression. This study exemplifies how bacterial pathogens sense their intracellular environment and exploit essential metabolites as effectors of virulence. IMPORTANCE Bacterial pathogens sense the repertoire of metabolites in the mammalian niche and use this information to shift into the pathogenic state to accomplish a successful infection. Glutathione is a virulence-activating signal that is synthesized by L. monocytogenes during infection of mammalian cells. In this study, we show that cysteine uptake via TcyKLMN drives glutathione synthesis and virulence gene expression. The data emphasize the intimate cross-regulation between metabolism and virulence in bacterial pathogens.

The aadE*-sat4-aphA-3 Gene Cluster of Mycoplasma bovirhinis HAZ141_2 Undergoes Genomic Rearrangements Influencing the Primary Promoter Sequence.

The 54 kb GC-rich prophage region of Mycoplasma bovirhinis HAZ141_2 contains three structural 'compartments', one of which is a highly transmittable cluster of three genes, aadE-like (aadE*), sat4, and aphA-3. In this study, we characterized recombination events and their consequences occurred within the aadE*-sat4-aphA-3 containing region. Analysis of this region revealed direct repeats (DRs) of 155 and invert repeats (IRs) of 197 base pairs (bps) each, flanking and overlapping with the primary promoter P* located upstream of the aadE*. Two recombination events, including inversions via both 197 and 155-bp IRs (the latter become inverted after the initial 197-bp IRs associated inversion) and the excision of the aadE*-sat4-aphA-3 cluster, were confirmed. Inversion via 155-IRs results in changes within the P* promoter region. Using Escherichia coli JM109 carrying plasmids containing derivatives of the aadE*-sat4-aphA-3 cluster, we validated the expression of those genes from different promoters. Our results showed no difference in the minimal inhibitory concentrations (MICs) to kanamycin and neomycin and only 2-fold decrease in MIC (from 512 to 256 µg/mL) to nourseothricin between the wild type and a P* derivative promoter. However, the MICs to kanamycin and neomycin were at least 4-fold lower in the construct where aphA-3 expressed under its P2 promoter (128 µg/mL) in comparison to the construct where aphA-3 expressed under P1″ promoter located within the sat4 gene (512-1024 µg/mL). PCR confirmed the excision of the aadE*-sat4-aphA-3 cluster via 197- and 155-bp DRs, but no selection of antibiotic-sensitive M. bovirhinis were obtained after 100 passages in kanamycin-free medium.

A Metzincin and TIMP-Like Protein Pair of a Phage Origin Sensitize Listeria monocytogenes to Phage Lysins and Other Cell Wall Targeting Agents.

Infection of mammalian cells by Listeria monocytogenes (Lm) was shown to be facilitated by its phage elements. In a search for additional phage remnants that play a role in Lm's lifecycle, we identified a conserved locus containing two XRE regulators and a pair of genes encoding a secreted metzincin protease and a lipoprotein structurally similar to a TIMP-family metzincin inhibitor. We found that the XRE regulators act as a classic CI/Cro regulatory switch that regulates the expression of the metzincin and TIMP-like genes under intracellular growth conditions. We established that when these genes are expressed, their products alter Lm morphology and increase its sensitivity to phage mediated lysis, thereby enhancing virion release. Expression of these proteins also sensitized the bacteria to cell wall targeting compounds, implying that they modulate the cell wall structure. Our data indicate that these effects are mediated by the cleavage of the TIMP-like protein by the metzincin, and its subsequent release to the extracellular milieu. While the importance of this locus to Lm pathogenicity remains unclear, the observation that this phage-associated protein pair act upon the bacterial cell wall may hold promise in the field of antibiotic potentiation to combat antibiotic resistant bacterial pathogens.

Renewable fatty acid ester production in Clostridium.

Bioproduction of renewable chemicals is considered as an urgent solution for fossil energy crisis. However, despite tremendous efforts, it is still challenging to generate microbial strains that can produce target biochemical to high levels. Here, we report an example of biosynthesis of high-value and easy-recoverable derivatives built upon natural microbial pathways, leading to improvement in bioproduction efficiency. By leveraging pathways in solventogenic clostridia for co-producing acyl-CoAs, acids and alcohols as precursors, through rational screening for host strains and enzymes, systematic metabolic engineering-including elimination of putative prophages, we develop strains that can produce 20.3 g/L butyl acetate and 1.6 g/L butyl butyrate. Techno-economic analysis results suggest the economic competitiveness of our developed bioprocess. Our principles of selecting the most appropriate host for specific bioproduction and engineering microbial chassis to produce high-value and easy-separable end products may be applicable to other bioprocesses.

A GC-Rich Prophage-Like Genomic Region of Mycoplasma bovirhinis HAZ141_2 Carries a Gene Cluster Encoding Resistance to Kanamycin and Neomycin.

Recently, a complete genome sequence of Mycoplasma bovirhinis HAZ141_2 was published showing the presence of a 54-kB prophage-like region. Bioinformatic analysis revealed that this region has a more than 40% GC content and a chimeric organization with three structural elements-a prophage continuous region, a restriction-modification cassette, and a highly transmittable aadE-sat4-aphA-3 gene cluster found in both Gram-positive and Gram-negative bacteria. It is known that aadE confers resistance to streptomycin, sat4 governs resistance to streptothricin/nourseothricin, and aphA-3 is responsible for resistance to kanamycin and structurally related antibiotics. An aadE-like (aadE*) gene of strain HAZ141_2 encodes a 228-amino acid (aa) polypeptide whose carboxy-terminal domain (positions 44 to 206) is almost identical to that of a functional 302-aa AadE (positions 140 to 302). Transcription analysis of the aadE*-sat4-aphA-3 genes showed their cotranscription in M. bovirhinis HAZ141_2. Moreover, a common promoter for aadE*-sat4-aphA-3 was mapped upstream of aadE* using 5' rapid amplification of cDNA ends analysis. Determination of MICs to aminoglycosides and nourseothricin revealed that M. bovirhinis HAZ141_2 is highly resistant to kanamycin and neomycin (≥512 µg/ml). However, MICs to streptomycin (64 µg/ml) and nourseothricin (16 to 32 µg/ml) were similar to those identified in the prophageless M. bovirhinis type strain PG43 and Israeli field isolate 316981. We cloned the aadE*-sat4-aphA-3 genes into a low-copy-number vector and transferred them into antibiotic-sensitive Escherichia coli cells. While the obtained E. coli transformants were highly resistant to kanamycin, neomycin, and nourseothricin (MICs, ≥256 µg/ml), there were no changes in MICs to streptomycin, suggesting a functional defect of the aadE*.

Active Lysogeny in Listeria Monocytogenes Is a Bacteria-Phage Adaptive Response in the Mammalian Environment.

Some Listeria monocytogenes (Lm) strains harbor a prophage within the comK gene, which renders it inactive. During Lm infection of macrophage cells, the prophage turns into a molecular switch, promoting comK gene expression and therefore Lm intracellular growth. During this process, the prophage does not produce infective phages or cause bacterial lysis, suggesting it has acquired an adaptive behavior suited to the pathogenic lifestyle of its host. In this study, we demonstrate that this non-classical phage behavior, named active lysogeny, relies on a transcriptional response that is specific to the intracellular niche. While the prophage undergoes lytic induction, the process is arrested midway, preventing the transcription of the late genes. Further, we demonstrate key phage factors, such as LlgA transcription regulator and a DNA replicase, that support the phage adaptive behavior. This study provides molecular insights into the adaptation of phages to their pathogenic hosts, uncovering unusual cooperative interactions.

RRNPP-type quorum-sensing systems regulate solvent formation, sporulation and cell motility in Clostridium saccharoperbutylacetonicum.

BACKGROUND: Clostridium saccharoperbutylacetonicum N1-4 (HMT) is a strictly anaerobic, spore-forming Gram-positive bacterium capable of hyper-butanol production through the well-known acetone-butanol-ethanol fermentation process. Recently, five putative RRNPP-type QSSs (here designated as QSS1 to QSS5) were predicted in this bacterial strain, each of which comprises a putative RRNPP-type regulator (QssR1 to QssR5) and a cognate signaling peptide precursor (QssP1 to QssP5). In addition, both proteins are encoded by the same operon. The functions of these multiple RRNPP-type QSSs are unknown. RESULTS: To elucidate the function of multiple RRNPP-type QSSs as related to cell metabolism and solvent production in N1-4 (HMT), we constructed qssR-deficient mutants ΔR1, ΔR2, ΔR3 and ΔR5 through gene deletion using CRISPR-Cas9 and N1-4-dcas9-R4 (with the QssR4 expression suppressed using CRISPR-dCas9). We also constructed complementation strains by overexpressing the corresponding regulator gene. Based on systematic characterization, results indicate that QSS1, QSS2, QSS3, and QSS5 positively regulate the sol operon expression and thus solvent production, but they likely negatively regulate cell motility. Consequently, QSS4 might not directly regulate solvent production, but positively affect cell migration. In addition, QSS3 and QSS5 appear to positively regulate sporulation efficiency. CONCLUSIONS: Our study provides the first insights into the roles of multiple RRNPP-type QSSs of C. saccharoperbutylacetonicum for the regulation of solvent production, cell motility, and sporulation. Results of this study expand our knowledge of how multiple paralogous QSSs are involved in the regulation of essential bacterial metabolism pathways.

Genomics-based epidemiology of bovine Mycoplasma bovis strains in Israel.

BACKGROUND: Mycoplasma bovis is an important etiologic agent of bovine mycoplasmosis affecting cattle production and animal welfare. In the past in Israel, M. bovis has been most frequently associated with bovine respiratory disease (BRD) and was rarely isolated from mastitis. This situation changed in 2008 when M. bovis-associated mastitis emerged in Israel. The aim of this study was to utilize whole genome sequencing to evaluate the molecular epidemiology and genomic diversity of M. bovis mastitis-associated strains and their genetic relatedness to M. bovis strains isolated from BRD in local feedlot calves and those imported to Israel from different European countries and Australia. RESULTS: Phylogeny based on total single nucleotide polymorphism (SNP) analysis of 225 M. bovis genomes clearly showed clustering of isolates on the basis of geographical origin: strains isolated from European countries clustered together and separately from Australian and Chinese isolates, while Israeli isolates were found in the both groups. The dominant genotype was identified among local mastitis-associated M. bovis isolates. This genotype showed a close genomic relatedness to M. bovis strains isolated from calves imported to Israel from Australia, to original Australian M. bovis strains, as well as to strains isolated in China. CONCLUSIONS: This study represents the first comprehensive high-resolution genome-based epidemiological analysis of M. bovis in Israel and illustrates the possible dissemination of the pathogen across the globe by cattle trade.

Coordination of cohabiting phage elements supports bacteria-phage cooperation.

Bacterial pathogens often carry multiple prophages and other phage-derived elements within their genome, some of which can produce viral particles in response to stress. Listeria monocytogenes 10403S harbors two phage elements in its chromosome, both of which can trigger bacterial lysis under stress: an active prophage (ϕ10403S) that promotes the virulence of its host and can produce infective virions, and a locus encoding phage tail-like bacteriocins. Here, we show that the two phage elements are co-regulated, with the bacteriocin locus controlling the induction of the prophage and thus its activity as a virulence-associated molecular switch. More specifically, a metalloprotease encoded in the bacteriocin locus is upregulated in response to stress and acts as an anti-repressor for CI-like repressors encoded in each phage element. Our results provide molecular insight into the phenomenon of polylysogeny and its intricate adaptation to complex environments.

Distinctive ligand-binding specificities of tandem PA14 biomass-sensory elements from Clostridium thermocellum and Clostridium clariflavum.

Cellulolytic clostridia use a highly efficient cellulosome system to degrade polysaccharides. To regulate genes encoding enzymes of the multi-enzyme cellulosome complex, certain clostridia contain alternative sigma I (σI ) factors that have cognate membrane-associated anti-σI factors (RsgIs) which act as polysaccharide sensors. In this work, we analyzed the structure-function relationship of the extracellular sensory elements of Clostridium (Ruminiclostridium) thermocellum and Clostridium clariflavum (RsgI3 and RsgI4, respectively). These elements were selected for comparison, as each comprised two tandem PA14-superfamily motifs. The X-ray structures of the PA14 modular dyads from the two bacterial species were determined, both of which showed a high degree of structural and sequence similarity, although their binding preferences differed. Bioinformatic approaches indicated that the DNA sequence of promoter of sigI/rsgI operons represents a strong signature, which helps to differentiate binding specificity of the structurally similar modules. The σI4 -dependent C. clariflavum promoter sequence correlates with binding of RsgI4_PA14 to xylan and was identified in genes encoding xylanases, whereas the σI3 -dependent C. thermocellum promoter sequence correlates with RsgI3_PA14 binding to pectin and regulates pectin degradation-related genes. Structural similarity between clostridial PA14 dyads to PA14-containing proteins in yeast helped identify another crucial signature element: the calcium-binding loop 2 (CBL2), which governs binding specificity. Variations in the five amino acids that constitute this loop distinguish the pectin vs xylan specificities. We propose that the first module (PA14A ) is dominant in directing the binding to the ligand in both bacteria. The two X-ray structures of the different PA14 dyads represent the first reported structures of tandem PA14 modules.

Faecal Proteases from Pouchitis Patients Activate Protease Activating Receptor-2 to Disrupt the Epithelial Barrier.

BACKGROUND AND AIMS: The pathogenesis of pouch inflammation may involve epithelial barrier disruption. We investigated whether faecal proteolytic activity is increased during pouchitis and results in epithelial barrier dysfunction through protease activating receptor [PAR] activation, and assessed whether the intestinal microbiome may be the source of the proteases. METHODS: Faecal samples were measured for protease activity using a fluorescein isothiocyanate [FITC]-casein florescence assay. Caco-2 cell monolayers were exposed to faecal supernatants to assess permeability to FITC-dextran. Tight junction protein integrity and PAR activation were assessed by immunoblot and immunofluorescence. A truncated PAR2 protein in Caco-2 cells was achieved by stable transfection using CRISPR/Cas9 plasmid. PAR2 activation in pouch biopsies was examined using antibodies directed to the N-terminus of the protein. Microbial composition was analysed based on 16S rRNA gene sequence analysis. RESULTS: Ten pouchitis patients, six normal pouch [NP] patients and nine healthy controls [HC] were recruited. The pouchitis patients exhibited a 5.19- and 5.35-fold higher faecal protease [FP] activity [p ≤ 0.05] compared to the NP and HC participants, respectively. The genus Haemophilus was positively associated with FP activity [R = 0.718, false discovery rate < 0.1]. Faecal supernatants from pouchitis patients activated PAR2 on Caco-2 monolayers, disrupted tight junction proteins and increased epithelial permeability. PAR2 truncation in Caco-2 abrogated faecal protease-mediated permeability. Pouch biopsies obtained from pouchitis patients, but not from NP patients, displayed PAR2 activation. CONCLUSIONS: Protease-producing bacteria may increase faecal proteolytic activity that results in pouch inflammation through disruption of tight junction proteins and increased epithelial permeability in a PAR2-dependent manner. This mechanism may initiate or propagate pouch inflammation.

Controlled branched-chain amino acids auxotrophy in Listeria monocytogenes allows isoleucine to serve as a host signal and virulence effector.

Listeria monocytogenes (Lm) is a saprophyte and intracellular pathogen. Transition to the pathogenic state relies on sensing of host-derived metabolites, yet it remains unclear how these are recognized and how they mediate virulence gene regulation. We previously found that low availability of isoleucine signals Lm to activate the virulent state. This response is dependent on CodY, a global regulator and isoleucine sensor. Isoleucine-bound CodY represses metabolic pathways including branched-chain amino acids (BCAA) biosynthesis, however under BCAA depletion, as occurs during infection, BCAA biosynthesis is upregulated and isoleucine-unbound CodY activates virulence genes. While isoleucine was revealed as an important input signal, it was not identified how internal levels are controlled during infection. Here we show that Lm regulates BCAA biosynthesis via CodY and via a riboregulator located upstream to the BCAA biosynthesis genes, named Rli60. rli60 is transcribed when BCAA levels drop, forming a ribosome-mediated attenuator that cis-regulates the downstream genes according to BCAA supply. Notably, we found that Rli60 restricts BCAA production, essentially starving Lm, a mechanism that is directly linked to virulence, as it controls the internal isoleucine pool and thereby CodY activity. This controlled BCAA auxotrophy likely evolved to enable isoleucine to serve as a host signal and virulence effector.

Novel ATP-cone-driven allosteric regulation of ribonucleotide reductase via the radical-generating subunit.

Ribonucleotide reductases (RNRs) are key enzymes in DNA metabolism, with allosteric mechanisms controlling substrate specificity and overall activity. In RNRs, the activity master-switch, the ATP-cone, has been found exclusively in the catalytic subunit. In two class I RNR subclasses whose catalytic subunit lacks the ATP-cone, we discovered ATP-cones in the radical-generating subunit. The ATP-cone in the Leeuwenhoekiella blandensis radical-generating subunit regulates activity via quaternary structure induced by binding of nucleotides. ATP induces enzymatically competent dimers, whereas dATP induces non-productive tetramers, resulting in different holoenzymes. The tetramer forms by interactions between ATP-cones, shown by a 2.45 Å crystal structure. We also present evidence for an MnIIIMnIV metal center. In summary, lack of an ATP-cone domain in the catalytic subunit was compensated by transfer of the domain to the radical-generating subunit. To our knowledge, this represents the first observation of transfer of an allosteric domain between components of the same enzyme complex.

Pan-Cellulosomics of Mesophilic Clostridia: Variations on a Theme.

The bacterial cellulosome is an extracellular, multi-enzyme machinery, which efficiently depolymerizes plant biomass by degrading plant cell wall polysaccharides. Several cellulolytic bacteria have evolved various elaborate modular architectures of active cellulosomes. We present here a genome-wide analysis of a dozen mesophilic clostridia species, including both well-studied and yet-undescribed cellulosome-producing bacteria. We first report here, the presence of cellulosomal elements, thus expanding our knowledge regarding the prevalence of the cellulosomal paradigm in nature. We explored the genomic organization of key cellulosome components by comparing the cellulosomal gene clusters in each bacterial species, and the conserved sequence features of the specific cellulosomal modules (cohesins and dockerins), on the background of their phylogenetic relationship. Additionally, we performed comparative analyses of the species-specific repertoire of carbohydrate-degrading enzymes for each of the clostridial species, and classified each cellulosomal enzyme into a specific CAZy family, thus indicating their putative enzymatic activity (e.g., cellulases, hemicellulases, and pectinases). Our work provides, for this large group of bacteria, a broad overview of the blueprints of their multi-component cellulosomal complexes. The high similarity of their scaffoldin clusters and dockerin-based recognition residues suggests a common ancestor, and/or extensive horizontal gene transfer, and potential cross-species recognition. In addition, the sporadic spatial organization of the numerous dockerin-containing genes in several of the genomes, suggests the importance of the cellulosome paradigm in the given bacterial species. The information gained in this work may be utilized directly or developed further by genetically engineering and optimizing designer cellulosome systems for enhanced biotechnological biomass deconstruction and biofuel production.

How does cellulosome composition influence deconstruction of lignocellulosic substrates in Clostridium (Ruminiclostridium) thermocellum DSM 1313?

BACKGROUND: Bioethanol production processes involve enzymatic hydrolysis of pretreated lignocellulosic biomass into fermentable sugars. Due to the relatively high cost of enzyme production, the development of potent and cost-effective cellulolytic cocktails is critical for increasing the cost-effectiveness of bioethanol production. In this context, the multi-protein cellulolytic complex of Clostridium (Ruminiclostridium) thermocellum, the cellulosome, was studied here. C. thermocellum is known to assemble cellulosomes of various subunit (enzyme) compositions, in response to the available carbon source. In the current study, different carbon sources were used, and their influence on both cellulosomal composition and the resultant activity was investigated. RESULTS: Glucose, cellobiose, microcrystalline cellulose, alkaline-pretreated switchgrass, alkaline-pretreated corn stover, and dilute acid-pretreated corn stover were used as sole carbon sources in the growth media of C. thermocellum strain DSM 1313. The purified cellulosomes were compared for their activity on selected cellulosic substrates. Interestingly, cellulosomes derived from cells grown on lignocellulosic biomass showed no advantage in hydrolyzing the original carbon source used for their production. Instead, microcrystalline cellulose- and glucose-derived cellulosomes were equal or superior in their capacity to deconstruct lignocellulosic biomass. Mass spectrometry analysis revealed differential composition of catalytic and structural subunits (scaffoldins) in the different cellulosome samples. The most abundant catalytic subunits in all cellulosome types include Cel48S, Cel9K, Cel9Q, Cel9R, and Cel5G. Microcrystalline cellulose- and glucose-derived cellulosome samples showed higher endoglucanase-to-exoglucanase ratios and higher catalytic subunit-per-scaffoldin ratios compared to lignocellulose-derived cellulosome types. CONCLUSION: The results reported here highlight the finding that cellulosomes derived from cells grown on glucose and microcrystalline cellulose are more efficient in their action on cellulosic substrates than other cellulosome preparations. These results should be considered in the future development of C. thermocellum-based cellulolytic cocktails, designer cellulosomes, or engineering of improved strains for deconstruction of lignocellulosic biomass.

Temperate bacteriophages as regulators of host behavior.

Bacteriophages are ubiquitous and affect most facets of life, from evolution of bacteria, through ecology and global biochemical cycling to human health. The interactions between phages and bacteria often lead to biological novelty and an important milestone in this process is the ability of phages to regulate their host's behavior. In this review article, we will focus on newly reported cases that demonstrate how temperate phages regulate bacterial gene expression and behavior in a variety of bacterial species, pathogenic and environmental. This regulation is mediated by diverse mechanisms such as transcription factors, sRNAs, DNA rearrangements, and even controlled bacterial lysis. The outcome is mutualistic relationships that enable adaptively enhanced communal phage-host fitness under specific conditions.

Complexity of the Ruminococcus flavefaciens FD-1 cellulosome reflects an expansion of family-related protein-protein interactions.

Protein-protein interactions play a vital role in cellular processes as exemplified by assembly of the intricate multi-enzyme cellulosome complex. Cellulosomes are assembled by selective high-affinity binding of enzyme-borne dockerin modules to repeated cohesin modules of structural proteins termed scaffoldins. Recent sequencing of the fiber-degrading Ruminococcus flavefaciens FD-1 genome revealed a particularly elaborate cellulosome system. In total, 223 dockerin-bearing ORFs potentially involved in cellulosome assembly and a variety of multi-modular scaffoldins were identified, and the dockerins were classified into six major groups. Here, extensive screening employing three complementary medium- to high-throughput platforms was used to characterize the different cohesin-dockerin specificities. The platforms included (i) cellulose-coated microarray assay, (ii) enzyme-linked immunosorbent assay (ELISA) and (iii) in-vivo co-expression and screening in Escherichia coli. The data revealed a collection of unique cohesin-dockerin interactions and support the functional relevance of dockerin classification into groups. In contrast to observations reported previously, a dual-binding mode is involved in cellulosome cell-surface attachment, whereas single-binding interactions operate for cellulosome integration of enzymes. This sui generis cellulosome model enhances our understanding of the mechanisms governing the remarkable ability of R. flavefaciens to degrade carbohydrates in the bovine rumen and provides a basis for constructing efficient nano-machines applied to biological processes.

Revisiting the Regulation of the Primary Scaffoldin Gene in Clostridium thermocellum.

Cellulosomes are considered to be one of the most efficient systems for the degradation of plant cell wall polysaccharides. The central cellulosome component comprises a large, noncatalytic protein subunit called scaffoldin. Multiple saccharolytic enzymes are incorporated into the scaffoldins via specific high-affinity cohesin-dockerin interactions. Recently, the regulation of genes encoding certain cellulosomal components by multiple RNA polymerase alternative σI factors has been demonstrated in Clostridium (Ruminiclostridium) thermocellum In the present report, we provide experimental evidence demonstrating that the C. thermocellum cipA gene, which encodes the primary cellulosomal scaffoldin, is regulated by several alternative σI factors and by the vegetative σA factor. Furthermore, we show that previously suggested transcriptional start sites (TSSs) of C. thermocellum cipA are actually posttranscriptional processed sites. By using comparative bioinformatic analysis, we have also identified highly conserved σI- and σA-dependent promoters upstream of the primary scaffoldin-encoding genes of other clostridia, namely, Clostridium straminisolvens, Clostridium clariflavum, Acetivibrio cellulolyticus, and Clostridium sp. strain Bc-iso-3. Interestingly, a previously identified TSS of the primary scaffoldin CbpA gene of Clostridium cellulovorans matches the predicted σI-dependent promoter identified in the present work rather than the previously proposed σA promoter. With the exception of C. cellulovorans, both σI and σA promoters of primary scaffoldin genes are located more than 600 nucleotides upstream of the start codon, yielding long 5'-untranslated regions (5'-UTRs). Furthermore, these 5'-UTRs have highly conserved stem-loop structures located near the start codon. We propose that these large 5'-UTRs may be involved in the regulation of both the primary scaffoldin and other cellulosomal components.IMPORTANCE Cellulosome-producing bacteria are among the most effective cellulolytic microorganisms known. This group of bacteria has biotechnological potential for the production of second-generation biofuels and other biocommodities from cellulosic wastes. The efficiency of cellulose hydrolysis is due to their cellulosomes, which arrange enzymes in close proximity on the cellulosic substrate, thereby increasing synergism among the catalytic domains. The backbone of these multienzyme nanomachines is the scaffoldin subunit, which has been the subject of study for many years. However, its genetic regulation is poorly understood. Hence, from basic and applied points of view, it is imperative to unravel the regulatory mechanisms of the scaffoldin genes. The understanding of these regulatory mechanisms can help to improve the performance of the industrially relevant strains of C. thermocellum and related cellulosome-producing bacteria en route to the consolidated bioprocessing of biomass.