Cellodextrin Metabolism and Phosphotransferase System-Catalyzed Uptake in Enterococcus faecalis

Two PTS transporters involved in the uptake of cellobiose and short cellooligosaccharides were identified in Enterococcus faecalis. Genes coding for the different EII proteins are found in a locus composed of three operonic structures expressing two distinct EIIC (CelC1 and CelC2), two identical EIIB (CelB1 and CelB2) and a unique EIIA (CelA1). The EIIA plays a central role in β-glucoside uptake because it is required not only for β-homodiholosides but also for the diheteroside N-acetylglucosamine-L-asparagine. Depending on their size, cellooligosaccharides are preferably transported either by CelC1 (di-saccharides) or by CelC2 (4 glycosidic residues and more), with tri-saccharides being taken up by both EIIC transporters. Moreover, CelA1B2C2 require CelGHI to be functional, three small proteins, the function of which remains unknown. CelA1B1C1 is the main but not exclusive transporter of cellobiose and chitobiose. It is involved in the transport of other β-glucodisaccharides, such as laminaribiose and sophorose. This PTS can be complemented by other transporters highlighting the existence of a network for β-glucoside uptake. This locus is under the control of CelR, a LevR-like transcription activator.

Transcriptional regulation of cellobiose utilization by PRD-domain containing Sigma54-dependent transcriptional activator (CelR) and catabolite control protein A (CcpA) in Bacillus thuringiensis

Cellobiose, a β-1,4-linked glucose dimer, is a major cellodextrin resulting from the enzymatic hydrolysis of cellulose. It is a major source of carbon for soil bacteria. In bacteria, the phosphoenolpyruvate (PEP): carbohydrate phosphotransferase system (PTS), encoded by the cel operon, is responsible for the transport and utilization of cellobiose. In this study, we analyzed the transcription and regulation of the cel operon in Bacillus thuringiensis (Bt). The cel operon is composed of five genes forming one transcription unit. β-Galactosidase assays revealed that cel operon transcription is induced by cellobiose, controlled by Sigma54, and positively regulated by CelR. The HTH-AAA+ domain of CelR recognized and specifically bound to three possible binding sites in the celA promoter region. CelR contains two PTS regulation domains (PRD1 and PRD2), which are separated by two PTS-like domains-the mannose transporter enzyme IIA component domain (EIIAMan) and the galactitol transporter enzyme IIB component domain (EIIBGat). Mutations of His-546 on the EIIAMan domain and Cys-682 on the EIIBGat domain resulted in decreased transcription of the cel operon, and mutations of His-839 on PRD2 increased transcription of the cel operon. Glucose repressed the transcription of the cel operon and catabolite control protein A (CcpA) positively regulated this process by binding the cel promoter. In the celABCDE and celR mutants, PTS activities were decreased, and cellobiose utilization was abolished, suggesting that the cel operon is essential for cellobiose utilization. Bt has been widely used as a biological pesticide. The metabolic properties of Bt are critical for fermentation. Nutrient utilization is also essential for the environmental adaptation of Bt. Glucose is the preferred energy source for many bacteria, and the presence of the phosphotransferase system allows bacteria to utilize other sugars in addition to glucose. Cellobiose utilization pathways have been of particular interest owing to their potential for developing alternative energy sources for bacteria. The data presented in this study improve our understanding of the transcription patterns of cel gene clusters. This will further help us to better understand how cellobiose is utilized for bacterial growth.

Elucidation of Sequence-Function Relationships for an Improved Biobutanol In Vivo Biosensor in E. coli

Transcription factor (TF)–promoter pairs have been repurposed from native hosts to provide tools to measure intracellular biochemical production titer and dynamically control gene expression. Most often, native TF–promoter systems require rigorous screening to obtain desirable characteristics optimized for biotechnological applications. High-throughput techniques may provide a rational and less labor-intensive strategy to engineer user-defined TF–promoter pairs using fluorescence-activated cell sorting and deep sequencing methods (sort-seq). Based on the designed promoter library’s distribution characteristics, we elucidate sequence–function interactions between the TF and DNA. In this work, we use the sort-seq method to study the sequence–function relationship of a σ⁵⁴-dependent, butanol-responsive TF–promoter pair, BmoR-P_BMO derived from Thauera butanivorans, at the nucleotide level to improve biosensor characteristics, specifically an improved dynamic range. Activities of promoters from a mutagenized P_BMO library were sorted based on gfp expression and subsequently deep sequenced to correlate site-specific sequences with changes in dynamic range. We identified site-specific mutations that increase the sensor output. Double mutant and a single mutant, CA(129,130)TC and G(205)A, in P_BMO promoter increased dynamic ranges of 4-fold and 1.65-fold compared with the native system, respectively. In addition, sort-seq identified essential sites required for the proper function of the σ⁵⁴-dependent promoter biosensor in the context of the host. This work can enable high-throughput screening methods for strain development.

Characterization of an operon required for growth on cellobiose in Clostridioides difficile

Cellobiose metabolism is linked to the virulence properties in numerous bacterial pathogens. Here, we characterized a putative cellobiose PTS operon of Clostridiodes difficile to investigate the role of cellobiose metabolism in C. difficile pathogenesis. Our gene knockout experiments demonstrated that the putative cellobiose operon enables uptake of cellobiose into C. difficile and allows growth when cellobiose is provided as the sole carbon source in minimal medium. Additionally, using reporter gene fusion assays and DNA pulldown experiments, we show that its transcription is regulated by CelR, a novel transcriptional repressor protein, which directly binds to the upstream region of the cellobiose operon to control its expression. We have also identified cellobiose metabolism to play a significant role in C. difficile physiology as observed by the reduction of sporulation efficiency when cellobiose uptake was compromised in the mutant strain. In corroboration to in vitro study findings, our in vivo hamster challenge experiment showed a significant reduction of pathogenicity by the cellobiose mutant strain in both the primary and the recurrent infection model - substantiating the role of cellobiose metabolism in C. difficile pathogenesis.

Proteomic Adaptation of Clostridioides difficile to Treatment with the Antimicrobial Peptide Nisin

Clostridioides difficile is the leading cause of antibiotic-associated diarrhea but can also result in more serious, life-threatening conditions. The incidence of C. difficile infections in hospitals is increasing, both in frequency and severity, and antibiotic-resistant C. difficile strains are advancing. Against this background antimicrobial peptides (AMPs) are an interesting alternative to classic antibiotics. Information on the effects of AMPs on C. difficile will not only enhance the knowledge for possible biomedical application but may also provide insights into mechanisms of C. difficile to adapt or counteract AMPs. This study applies state-of-the-art mass spectrometry methods to quantitatively investigate the proteomic response of C. difficile 630∆erm to sublethal concentrations of the AMP nisin allowing to follow the cellular stress adaptation in a time-resolved manner. The results do not only point at a heavy reorganization of the cellular envelope but also resulted in pronounced changes in central cellular processes such as carbohydrate metabolism. Further, the number of flagella per cell was increased during the adaptation process. The potential involvement of flagella in nisin adaptation was supported by a more resistant phenotype exhibited by a non-motile but hyper-flagellated mutant.

Transcriptome profile of carbon catabolite repression in an efficient l-(+)-lactic acid-producing bacterium Enterococcus mundtii QU25 grown in media with combinations of cellobiose, xylose, and glucose

Enterococcus mundtii QU25, a non-dairy lactic acid bacterium of the phylum Firmicutes, is capable of simultaneously fermenting cellobiose and xylose, and is described as a promising strain for the industrial production of optically pure l-lactic acid (≥ 99.9%) via homo-fermentation of lignocellulosic hydrolysates. Generally, Firmicutes bacteria show preferential consumption of sugar (usually glucose), termed carbon catabolite repression (CCR), while hampering the catabolism of other sugars. In our previous study, QU25 exhibited apparent CCR in a glucose-xylose mixture phenotypically, and transcriptional repression of the xylose operon encoding initial xylose metabolism genes, likely occurred in a CcpA-dependent manner. QU25 did not exhibit CCR phenotypically in a cellobiose-xylose mixture. The aim of the current study is to elucidate the transcriptional change associated with the simultaneous utilization of cellobiose and xylose. To this end, we performed RNA-seq analysis in the exponential growth phase of E. mundtii QU25 cells grown in glucose, cellobiose, and/or xylose as either sole or co-carbon sources. Our transcriptomic data showed that the xylose operon was weakly repressed in cells grown in a cellobiose-xylose mixture compared with that in cells grown in a glucose-xylose mixture. Furthermore, the gene expression of talC, the sole gene encoding transaldolase, is expected to be repressed by CcpA-mediated CCR. QU25 metabolized xylose without using transaldolase, which is necessary for homolactic fermentation from pentoses using the pentose-phosphate pathway. Hence, the metabolism of xylose in the presence of cellobiose by QU25 may have been due to 1) sufficient amounts of proteins encoded by the xylose operon genes for xylose metabolism despite of the slight repression of the operon, and 2) bypassing of the pentose-phosphate pathway without the TalC activity. Accordingly, we have determined the targets of genetic modification in QU25 to metabolize cellobiose, xylose and glucose simultaneously for application of the lactic fermentation from lignocellulosic hydrolysates.

The mannose phosphotransferase system (Man-PTS) - Mannose transporter and receptor for bacteriocins and bacteriophages

Mannose transporters constitute a superfamily (Man-PTS) of the Phosphoenolpyruvate Carbohydrate Phosphotransferase System (PTS). The membrane complexes are homotrimers of protomers consisting of two subunits, IIC and IID. The two subunits without recognizable sequence similarity assume the same fold, and in the protomer are structurally related by a two fold pseudosymmetry axis parallel to membrane-plane (Liu et al. (2019) Cell Research 29 680). Two reentrant loops and two transmembrane helices of each subunit together form the N-terminal transport domain. Two three-helix bundles, one of each subunit, form the scaffold domain. The protomer is stabilized by a helix swap between these bundles. The two C-terminal helices of IIC mediate the interprotomer contacts. PTS occur in bacteria and archaea but not in eukaryotes. Man-PTS are abundant in Gram-positive bacteria living on carbohydrate rich mucosal surfaces. A subgroup of IICIID complexes serve as receptors for class IIa bacteriocins and as channel for the penetration of bacteriophage lambda DNA across the inner membrane. Some Man-PTS are associated with host-pathogen and -symbiont processes.

Clostridioides difficile para-Cresol Production Is Induced by the Precursor para-Hydroxyphenylacetate

Clostridioides difficile is an etiological agent for antibiotic-associated diarrheal disease. C. difficile produces a phenolic compound, para-cresol, which selectively targets gammaproteobacteria in the gut, facilitating dysbiosis. C. difficile decarboxylates para-hydroxyphenylacetate (p-HPA) to produce p-cresol by the action of the HpdBCA decarboxylase encoded by the hpdBCA operon. Here, we investigate regulation of the hpdBCA operon and directly compare three independent reporter systems; SNAP-tag, glucuronidase gusA, and alkaline phosphatase phoZ reporters to detect basal and inducible expression. We show that expression of hpdBCA is upregulated in response to elevated p-HPA. In silico analysis identified three putative promoters upstream of hpdBCA operon-P₁, P₂, and Pσ⁵⁴; only the P₁ promoter was responsible for both basal and p-HPA-inducible expression of hpdBCA We demonstrated that turnover of tyrosine, a precursor for p-HPA, is insufficient to induce expression of the hpdBCA operon above basal levels because it is inefficiently converted to p-HPA in minimal media. We show that induction of the hpdBCA operon in response to p-HPA occurs in a dose-dependent manner. We also identified an inverted palindromic repeat (AAAAAG-N₁₃-CTTTTT) upstream of the hpdBCA start codon (ATG) that is essential for inducing transcription of the hpdBCA operon in response to p-HPA, which drives the production of p-cresol. This provides insights into the regulatory control of p-cresol production, which affords a competitive advantage for C. difficile over other intestinal bacteria, promoting dysbiosis.IMPORTANCE Clostridioides difficile infection results from antibiotic-associated dysbiosis. para-Cresol, a phenolic compound produced by C. difficile, selectively targets gammaproteobacteria in the gut, facilitating dysbiosis. Here, we demonstrate that expression of the hpdBCA operon, encoding the HpdBCA decarboxylase which converts p-HPA to p-cresol, is upregulated in response to elevated exogenous p-HPA, with induction occurring between >0.1 and ≤0.25 mg/ml. We determined a single promoter and an inverted palindromic repeat responsible for basal and p-HPA-inducible hpdBCA expression. We identified turnover of tyrosine, a p-HPA precursor, does not induce hpdBCA expression above basal level, indicating that exogenous p-HPA was required for p-cresol production. Identifying regulatory controls of p-cresol production will provide novel therapeutic targets to prevent p-cresol production, reducing C. difficile's competitive advantage.

Distribution and diversity of glycocin biosynthesis gene clusters beyond Firmicutes

Glycocins are the ribosomally synthesized glycosylated bacteriocins discovered and characterized in Firmicutes, only. These peptides have antimicrobial activity against several pathogenic bacteria, including Streptococcus pyogenes , methicillin-resistant Staphylococcus aureus and food-spoilage bacteria Listeria monocytogenes. Glycocins exhibit immunostimulatory properties and make a promising source of new antibiotics and food preservatives akin to Nisin. Biochemical studies of Sublancin, Glycocin F, Pallidocin and ASM1 prove that the nested disulfide-bonds are essential for their bioactivities. Using in silico approach of genome mining coupled with manual curation, here we identify 220 new putative glycocin biosynthesis gene clusters (PGBCs) spread across 153 bacterial species belonging to seven different bacterial phyla. Based on gene composition, we have grouped these PGBCs into five distinct conserved cluster Types I-V. All experimentally identified glycocins belong to Type I PGBCs. From protein sequence based phylograms, tanglegrams, global similarity heat-maps and cumulative mutual information analysis, it appears that glycocins may have originated from closely related bacteriocins, whereas recruitment of cognate glycosyltransferases (GTs) might be an independent event. Analysis further suggests that GTs may have coevolved with glycocins in cluster-specific manner to define distinctive donor specificities of GTs or to contribute to glycocin diversity across these clusters. We further identify 162 hitherto unreported PGBCs wherein the corresponding product glycocins have three or less than three cysteines. Secondary structure predictions suggest that these putative glycocins may not form di-nested disulfide-bonds. Therefore, production of such glycocins in heterologous host Escherichia coli is feasible and may provide novel antimicrobial spectrum and or mechanism of action for varied applications.

Ferrous-Iron-Activated Transcriptional Factor AdhR Regulates Redox Homeostasis in Clostridium beijerinckii

The AdhR regulatory protein is an activator of σ⁵⁴-dependent transcription of adhA1 and adhA2 genes, which are required for alcohol synthesis in Clostridium beijerinckii Here, we identified the signal perceived by AdhR and determined the regulatory mechanism of AdhR activity. By assaying the activity of AdhR in N-terminally truncated forms, a negative control mechanism of AdhR activity was identified in which the central AAA⁺ domain is subject to repression by the N-terminal GAF and PAS domains. Binding of Fe²⁺ to the GAF domain was found to relieve intramolecular repression and stimulate the ATPase activity of AdhR, allowing the AdhR to activate transcription. This control mechanism enables AdhR to regulate transcription of adhA1 and adhA2 in response to cellular redox status. The mutants deficient in AdhR or σ⁵⁴ showed large shifts in intracellular redox state indicated by the NADH/NAD⁺ ratio under conditions of increased electron availability or oxidative stress. We demonstrated that the Fe²⁺-activated transcriptional regulator AdhR and σ⁵⁴ control alcohol synthesis to maintain redox homeostasis in clostridial cells. Expression of N-terminally truncated forms of AdhR resulted in improved solvent production by C. beijerinckii IMPORTANCE Solventogenic clostridia are anaerobic bacteria that can produce butanol, ethanol, and acetone, which can be used as biofuels or building block chemicals. Here, we show that AdhR, a σ⁵⁴-dependent transcriptional activator, senses the intracellular redox status and controls alcohol synthesis in Clostridium beijerinckii AdhR provides a new example of a GAF domain coordinating a mononuclear non-heme iron to sense and transduce the redox signal. Our study reveals a previously unrecognized functional role of σ⁵⁴ in control of cellular redox balance and provides new insights into redox signaling and regulation in clostridia. Our results reveal AdhR as a novel engineering target for improving solvent production by C. beijerinckii and other solventogenic clostridia.

Control of solvent production by sigma-54 factor and the transcriptional activator AdhR in Clostridium beijerinckii

Clostridia are obligate anaerobic bacteria that can produce solvents such as acetone, butanol and ethanol. Alcohol dehydrogenases (ADHs) play a key role in solvent production; however, their regulatory mechanisms remain largely unknown. In this study, we characterized the regulatory mechanisms of two ADH-encoding genes in C. beijerinckii. SigL (sigma factor σ⁵⁴ ) was found to be required for transcription of adhA1 and adhA2 genes. Moreover, a novel transcriptional activator AdhR was identified, which binds to the σ⁵⁴ promoter and activates σ⁵⁴ -dependent transcription of adhA1 and adhA2. Clostridia beijerinckii mutants deficient in SigL or AdhR showed severely impaired butanol and ethanol production as well as altered acetone and butyrate synthesis. Overexpression of SigL resulted in significantly improved solvent production by C. beijerinckii when butyrate was added to cultures. Our results reveal SigL as a novel engineering target for improving solvent production by C. beijerinckii and other solvent-producing clostridia. Moreover, this study gains an insight into regulation of alcohol metabolism in diverse clostridia.

TargeTron Technology Applicable in Solventogenic Clostridia: Revisiting 12 Years' Advances

Clostridium has great potential in industrial application and medical research. But low DNA repair capacity and plasmids transformation efficiency severely delay development and application of genetic tools based on homologous recombination (HR). TargeTron is a gene editing technique dependent on the mobility of group II introns, rather than homologous recombination, which makes it very suitable for gene disruption of Clostridium. The application of TargeTron technology in solventogenic Clostridium is academically reported in 2007 and this tool has been introduced in various clostridia as it is easy to operate, time saving, and reliable. TargeTron has made great progress in solventogenic Clostridium in the aspects of acetone-butanol-ethanol (ABE) fermentation pathway modification, important functional genes identification, and xylose metabolic pathway analysis and reconstruction. In the review, 12 years' advances of TargeTron technology applicable in solventogenic Clostridium, including its principle, technical characteristics, application, and efforts to expand its capabilities, or to avoid potential drawbacks, are revisisted. Some other technologies as putative competitors or collaborators are also discussed. It is believed that TargeTron combined with CRISPR/Cas-assisted gene/base editing and gene-expression regulation system will make a better future for clostridial genetic modification.

Genomic reconstruction of σ54 regulons in Clostridiales

Background

The σ⁵⁴ factor controls unique promoters and interacts with a specialized activator (enhancer binding proteins [EBP]) for transcription initiation. Although σ⁵⁴ is present in many Clostridiales species that have great importance in human health and biotechnological applications, the cellular processes controlled by σ⁵⁴ remain unknown.

Results

For systematic analysis of the regulatory functions of σ⁵⁴, we performed comparative genomic reconstruction of transcriptional regulons of σ⁵⁴ in 57 species from the Clostridiales order. The EBP-binding DNA motifs and regulated genes were identified for 263 EBPs that constitute 39 distinct groups. The reconstructed σ⁵⁴ regulons contain the genes involved in fermentation and amino acid catabolism. The predicted σ⁵⁴ binding sites in the genomes of Clostridiales spp. were verified by in vitro binding assays. To our knowledge, this is the first report about direct regulation of the Stickland reactions and butyrate and alcohols synthesis by σ⁵⁴ and the respective EBPs. Considerable variations were demonstrated in the sizes and gene contents of reconstructed σ⁵⁴ regulons between different Clostridiales species. It is proposed that σ⁵⁴ controls butyrate and alcohols synthesis in solvent-producing species, regulates autotrophic metabolism in acetogenic species, and affects the toxin production in pathogenic species.

Conclusions

This study reveals previously unrecognized functions of σ⁵⁴ and provides novel insights into the regulation of fermentation and amino acid metabolism in Clostridiales species, which could have potential applications in guiding the treatment and efficient utilization of these species.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5918-4) contains supplementary material, which is available to authorized users.

σ54 (σL) plays a central role in carbon metabolism in the industrially relevant Clostridium beijerinckii

The solventogenic C. beijerinckii DSM 6423, a microorganism that naturally produces isopropanol and butanol, was previously modified by random mutagenesis. In this work, one of the resulting mutants was characterized. This strain, selected with allyl alcohol and designated as the AA mutant, shows a dominant production of acids, a severely diminished butanol synthesis capacity, and produces acetone instead of isopropanol. Interestingly, this solvent-deficient strain was also found to have a limited consumption of two carbohydrates and to be still able to form spores, highlighting its particular phenotype. Sequencing of the AA mutant revealed point mutations in several genes including CIBE_0767 (sigL), which encodes the σ⁵⁴ sigma factor. Complementation with wild-type sigL fully restored solvent production and sugar assimilation and RT-qPCR analyses revealed its transcriptional control of several genes related to solventogensis, demonstrating the central role of σ⁵⁴ in C. beijerinckii DSM 6423. Comparative genomics analysis suggested that this function is conserved at the species level, and this hypothesis was further confirmed through the deletion of sigL in the model strain C. beijerinckii NCIMB 8052.

Novel Cysteine Desulfidase CdsB Involved in Releasing Cysteine Repression of Toxin Synthesis in Clostridium difficile

Clostridium difficile, a major cause of nosocomial diarrhea and pseudomembranous colitis, still poses serious health-care challenges. The expression of its two main virulence factors, TcdA and TcdB, is reportedly repressed by cysteine, but molecular mechanism remains unclear. The cysteine desulfidase CdsB affects the virulence and infection progresses of some bacteria. The C. difficile strain 630 genome encodes a homolog of CdsB, and in the present study, we analyzed its role in C. difficile 630Δerm by constructing an isogenic ClosTron-based cdsB mutant. When C. difficile was cultured in TY broth supplemented with cysteine, the cdsB gene was rapidly induced during the exponential growth phase. The inactivation of cdsB not only affected the resistance of C. difficile to cysteine, but also altered the expression levels of intracellular cysteine-degrading enzymes and the production of hydrogen sulfide. This suggests that C. difficile CdsB is a major inducible cysteine-degrading enzyme. The inactivation of the cdsB gene in C. difficile also removed the cysteine-dependent repression of toxin production, but failed to remove the Na₂S-dependent repression, which supports that the cysteine-dependent repression of toxin production is probably attributable to the accumulation of cysteine by-products. We also mapped a δ⁵⁴ (SigL)-dependent promoter upstream from the cdsB gene, and cdsB expression was not induced in response to cysteine in the cdsR::ermB or sigL::ermB strain. Using a reporter gene fusion analysis, we identified the necessary promoter sequence for cysteine-dependent cdsB expression. Taken together, these results indicate that CdsB is a key inducible cysteine desulfidase in C. difficile which is regulated by δ⁵⁴ and CdsR in response to cysteine and that cysteine-dependent regulation of toxin production is closely associated with cysteine degradation.

Sophisticated Regulation of Transcriptional Factors by the Bacterial Phosphoenolpyruvate: Sugar Phosphotransferase System

The phosphoenolpyruvate:sugar phosphotransferase system (PTS) is a carbohydrate transport and phosphorylation system present in bacteria of all different phyla and in archaea. It is usually composed of three proteins or protein complexes, enzyme I, HPr, and enzyme II, which are phosphorylated at histidine or cysteine residues. However, in many bacteria, HPr can also be phosphorylated at a serine residue. The PTS not only functions as a carbohydrate transporter but also regulates numerous cellular processes either by phosphorylating its target proteins or by interacting with them in a phosphorylation-dependent manner. The target proteins can be catabolic enzymes, transporters, and signal transduction proteins but are most frequently transcriptional regulators. In this review, we will describe how PTS components interact with or phosphorylate proteins to regulate directly or indirectly the activity of transcriptional repressors, activators, or antiterminators. We will briefly summarize the well-studied mechanism of carbon catabolite repression in firmicutes, where the transcriptional regulator catabolite control protein A needs to interact with seryl-phosphorylated HPr in order to be functional. We will present new results related to transcriptional activators and antiterminators containing specific PTS regulation domains, which are the phosphorylation targets for three different types of PTS components. Moreover, we will discuss how the phosphorylation level of the PTS components precisely regulates the activity of target transcriptional regulators or antiterminators, with or without PTS regulation domain, and how the availability of PTS substrates and thus the metabolic status of the cell are connected with various cellular processes, such as biofilm formation or virulence of certain pathogens.