Cloning and molecular analysis of a mannitol operon of phosphoenolpyruvate-dependent phosphotransferase (PTS) type from Vibrio cholerae O395

Systematic review on marine carbon source-mannitol: Applications in synthetic biology

Mannitol, one of the most widespread sugar alcohols, has been integral to daily human life for two centuries. Global population growth and competition for freshwater, food, and land have prompted a shift in the fermentation industry from terrestrial to marine raw materials. Mannitol is a readily available carbohydrate in brown seaweed from the ocean and possess a higher reducing power than glucose, making it a promising substrate for biological manufacturing. This has spurred numerous explorations into converting mannitol into high-value chemicals. Researchers have engineered microorganisms to utilize mannitol in various synthetic biological applications, including: (1) employing mannitol as an inducer to control the activation and deactivation of genetic circuits; (2) using mannitol as a carbon source for synthesizing high-value chemicals through biomanufacturing. This review summarizes the latest advances in the application of mannitol in synthetic biology. AIM OF REVIEW: The aim is to present a thorough and in-depth knowledge of mannitol, a marine carbon source, and then use this carbon source in synthetic biology to improve the competitiveness of biosynthetic processes. We outlined the methods and difficulties of utilizing mannitol in synthetic biology with a variety of microbes serving as hosts. Furthermore, future research directions that could alleviate the carbon catabolite repression (CCR) relationship between glucose and mannitol are also covered. EXPECTED CONTRIBUTIONS OF REVIEW: Provide an overview of the current state, drawbacks, and directions for future study on mannitol as a carbon source or genetic circuit inducer in synthetic biology.

Both entry to and exit from diapause arrest in Caenorhabditis elegans are regulated by a steroid hormone pathway

Diapause arrest in animals such as Caenorhabditis elegans is tightly regulated so that animals make appropriate developmental decisions amidst environmental challenges. Fully understanding diapause requires mechanistic insight of both entry and exit from the arrested state. Although a steroid hormone pathway regulates the entry decision into C. elegans dauer diapause, its role in the exit decision is less clear. A complication to understanding steroid hormonal regulation of dauer has been the peculiar fact that steroid hormone mutants such as daf-9 form partial dauers under normal growth conditions. Here, we corroborate previous findings that daf-9 mutants remain capable of forming full dauers under unfavorable growth conditions and establish that the daf-9 partial dauer state is likely a partially exited dauer that has initiated but cannot complete the dauer exit process. We show that the steroid hormone pathway is both necessary for and promotes complete dauer exit, and that the spatiotemporal dynamics of steroid hormone regulation during dauer exit resembles that of dauer entry. Overall, dauer entry and dauer exit are distinct developmental decisions that are both controlled by steroid hormone signaling.

Summary: In animals such as Caenorhabditis elegans, a steroid hormone pathway controls both the entry and exit decisions into and out of the developmentally arrested dauer state in response to environmental signaling.

Membrane Efflux Pumps of Pathogenic Vibrio Species: Role in Antimicrobial Resistance and Virulence

Infectious diseases caused by bacterial species of the Vibrio genus have had considerable significance upon human health for centuries. V. cholerae is the causative microbial agent of cholera, a severe ailment characterized by profuse watery diarrhea, a condition associated with epidemics, and seven great historical pandemics. V. parahaemolyticus causes wound infection and watery diarrhea, while V. vulnificus can cause wound infections and septicemia. Species of the Vibrio genus with resistance to multiple antimicrobials have been a significant health concern for several decades. Mechanisms of antimicrobial resistance machinery in Vibrio spp. include biofilm formation, drug inactivation, target protection, antimicrobial permeability reduction, and active antimicrobial efflux. Integral membrane-bound active antimicrobial efflux pump systems include primary and secondary transporters, members of which belong to closely related protein superfamilies. The RND (resistance-nodulation-division) pumps, the MFS (major facilitator superfamily) transporters, and the ABC superfamily of efflux pumps constitute significant drug transporters for investigation. In this review, we explore these antimicrobial transport systems in the context of Vibrio spp. pathogenesis and virulence.

Transcriptional regulation of the mannitol phosphotransferase system operon by the ferric uptake regulator (Fur) in Vibrio cholerae El Tor serogroup O1

The phosphoenolpyruvate (PEP): carbohydrate phosphotransferase system (PTS) allows bacteria to use various carbohydrates as energy resources including mannitol. The mannitol-specific PTS transporter in Vibrio cholerae is encoded by the mtlADR operon. Expression of the mtl operon has been shown to be strictly regulated by CRP, MtlS, and MtlR. In the present study, we investigated the regulation of mtlADR by the ferric uptake regulator (Fur). The results showed that Fur binds to the promoter-proximal DNA region of mtlADR to repress its transcription independent of iron, in mannitol-containing growth medium. The capacity for mannitol fermentation was significantly increased in Δfur relative to that of WT for normal and iron-replete growth media. The level of organic acids produced by Δfur was significantly enhanced relative to that produced by the WT strain in the normal and iron-replete media but not in an iron-starved medium. The results provided for a deeper understanding of the regulation of mtlADR in V. cholerae.

Targeting Mannitol Metabolism as an Alternative Antimicrobial Strategy Based on the Structure-Function Study of Mannitol-1-Phosphate Dehydrogenase in Staphylococcus aureus

Mannitol-1-phosphate dehydrogenase (M1PDH) is a key enzyme in Staphylococcus aureus mannitol metabolism, but its roles in pathophysiological settings have not been established. We performed comprehensive structure-function analysis of M1PDH from S. aureus USA300, a strain of community-associated methicillin-resistant S. aureus, to evaluate its roles in cell viability and virulence under pathophysiological conditions. On the basis of our results, we propose M1PDH as a potential antibacterial target. In vitro cell viability assessment of ΔmtlD knockout and complemented strains confirmed that M1PDH is essential to endure pH, high-salt, and oxidative stress and thus that M1PDH is required for preventing osmotic burst by regulating pressure potential imposed by mannitol. The mouse infection model also verified that M1PDH is essential for bacterial survival during infection. To further support the use of M1PDH as an antibacterial target, we identified dihydrocelastrol (DHCL) as a competitive inhibitor of S. aureus M1PDH (SaM1PDH) and confirmed that DHCL effectively reduces bacterial cell viability during host infection. To explain physiological functions of SaM1PDH at the atomic level, the crystal structure of SaM1PDH was determined at 1.7-Å resolution. Structure-based mutation analyses and DHCL molecular docking to the SaM1PDH active site followed by functional assay identified key residues in the active site and provided the action mechanism of DHCL. Collectively, we propose SaM1PDH as a target for antibiotic development based on its physiological roles with the goals of expanding the repertory of antibiotic targets to fight antimicrobial resistance and providing essential knowledge for developing potent inhibitors of SaM1PDH based on structure-function studies.IMPORTANCE Due to the shortage of effective antibiotics against drug-resistant Staphylococcus aureus, new targets are urgently required to develop next-generation antibiotics. We investigated mannitol-1-phosphate dehydrogenase of S. aureus USA300 (SaM1PDH), a key enzyme regulating intracellular mannitol levels, and explored the possibility of using SaM1PDH as a target for developing antibiotic. Since mannitol is necessary for maintaining the cellular redox and osmotic potential, the homeostatic imbalance caused by treatment with a SaM1PDH inhibitor or knockout of the gene encoding SaM1PDH results in bacterial cell death through oxidative and/or mannitol-dependent cytolysis. We elucidated the molecular mechanism of SaM1PDH and the structural basis of substrate and inhibitor recognition by enzymatic and structural analyses of SaM1PDH. Our results strongly support the concept that targeting of SaM1PDH represents an alternative strategy for developing a new class of antibiotics that cause bacterial cell death not by blocking key cellular machinery but by inducing cytolysis and reducing stress tolerance through inhibition of the mannitol pathway.

Transcription of cis Antisense Small RNA MtlS in Vibrio cholerae Is Regulated by Transcription of Its Target Gene, mtlA

Vibrio cholerae is a bacterial pathogen that relies on genetic tools, such as regulatory RNAs, to adapt to changing extracellular conditions. While many studies have focused on how these regulatory RNAs function, fewer have focused on how they are themselves modulated. V. cholerae expresses the noncoding RNA MtlS, which can regulate mannitol transport and use, and here we demonstrate that MtlS levels are controlled by the level of transcription occurring in the antisense direction. Our findings provide a model of regulation describing how bacteria like V. cholerae can modulate the levels of an important regulatory RNA. Our work contributes to knowledge of how bacteria deploy regulatory RNAs as an adaptive mechanism to buffer against environmental flux.

Vibrio cholerae, the facultative pathogen responsible for cholera disease, continues to pose a global health burden. Its persistence can be attributed to a flexible genetic tool kit that allows for adaptation to different environments with distinct carbon sources, including the six-carbon sugar alcohol mannitol. V. cholerae takes up mannitol through the transporter protein MtlA, whose production is downregulated at the posttranscriptional level by MtlS, a cis antisense small RNA (sRNA) whose promoter lies within the mtlA open reading frame. Though it is known that mtlS expression is robust under growth conditions lacking mannitol, it has remained elusive as to what factors govern the steady-state levels of MtlS. Here, we show that manipulating mtlA transcription is sufficient to drive inverse changes in MtlS levels, likely through transcriptional interference. This work has uncovered a cis-acting sRNA whose expression pattern is predominantly controlled by transcription of the sRNA’s target gene.

IMPORTANCEVibrio cholerae is a bacterial pathogen that relies on genetic tools, such as regulatory RNAs, to adapt to changing extracellular conditions. While many studies have focused on how these regulatory RNAs function, fewer have focused on how they are themselves modulated. V. cholerae expresses the noncoding RNA MtlS, which can regulate mannitol transport and use, and here we demonstrate that MtlS levels are controlled by the level of transcription occurring in the antisense direction. Our findings provide a model of regulation describing how bacteria like V. cholerae can modulate the levels of an important regulatory RNA. Our work contributes to knowledge of how bacteria deploy regulatory RNAs as an adaptive mechanism to buffer against environmental flux.

Strategies for discovery of new molecular targets for anti-infective drugs

Multidrug resistant bacterial pathogens as causative agents of infectious disease are a primary public health concern. Clinical efficacy of antimicrobial chemotherapy toward bacterial infection has been compromised in cases where causative agents are resistant to multiple structurally distinct antimicrobial agents. Modification of extant antimicrobial agents that exploit conventional bacterial targets have been developed since the advent of the antimicrobial era. This approach, while successful in certain cases, nonetheless suffers overall from the costs of development and rapid emergence of bacterial variants with confounding resistances to modified agents. Thus, additional strategies toward discovery of new molecular targets have been developed based on bioinformatics analyses and comparative genomics. These and other strategies meant to identify new molecular targets represent promising avenues for reducing emergence of bacterial infections. This short review considers these strategies for discovery of new molecular targets within bacterial pathogens.

MtlR negatively regulates mannitol utilization by Vibrio cholerae

The phosphoenopyruvate:carbohydrate phosphotransferase system (PTS) enables Vibrio cholerae – and other bacteria – to recognize and transport exogenous carbon sources for energy, including the six-carbon sugar alcohol, mannitol. The mannitol-specific PTS transporter is encoded by mtlA and its expression is expected to be regulated by the putative repressor encoded by the mtlR gene. Here, we show that mtlR overexpression inhibits V. cholerae growth in medium supplied with mannitol as the sole carbon source and represses MtlA-mediated biofilm formation. We demonstrate that when V. cholerae is grown in non-mannitol medium, knocking out mtlR leads to both increased MtlA protein and mtlA mRNA levels, with these increases being especially pronounced in non-glucose sugars. We propose that in non-mannitol, non-glucose growth conditions, MtlR is a major regulator of mtlA transcription. Surprisingly, with regard to mtlR expression, transcript and protein levels are highest in mannitol medium, conditions where mtlA expression should not be repressed. We further show that MtlR levels increase during growth of the bacteria and linger in cells switched from mannitol to non-mannitol medium. Our data suggests an expression paradigm for mtlA where MtlR acts as a transcriptional repressor responsible for calibrating MtlA levels during environmental transitions.

Systematic genetic dissection of PTS in Vibrio cholerae uncovers a novel glucose transporter and a limited role for PTS during infection of a mammalian host

A common mechanism for high affinity carbohydrate uptake in microbial species is the phosphoenolpyruvate-dependent phosphotransferase system (PTS). This system consists of a shared component, EI, which is required for all PTS transport, and numerous carbohydrate uptake transporters. In Vibrio cholerae, there are 13 distinct PTS transporters. Due to genetic redundancy within this system, the carbohydrate specificity of each of these transporters is not currently defined. Here, using multiplex genome editing by natural transformation (MuGENT), we systematically dissect PTS transport in V. cholerae. Specifically, we generated a mutant strain that lacks all 13 PTS transporters, and from this strain, we created a panel of mutants where each expresses a single transporter. Using this panel, we have largely defined the carbohydrate specificities of each PTS transporter. In addition, this analysis uncovered a novel glucose transporter. We have further defined the mechanism of this transporter and characterized its regulation. Using our 13 PTS transporter mutant, we also provide the first clear evidence that carbohydrate transport by the PTS is not essential during infection in an infant mouse model of cholera. In summary, this study shows how multiplex genome editing can be used to rapidly dissect complex biological systems and genetic redundancy in microbial systems.

Bacterial genome engineering and synthetic biology: combating pathogens

BACKGROUND

The emergence and prevalence of multidrug resistant (MDR) pathogenic bacteria poses a serious threat to human and animal health globally. Nosocomial infections and common ailments such as pneumonia, wound, urinary tract, and bloodstream infections are becoming more challenging to treat due to the rapid spread of MDR pathogenic bacteria. According to recent reports by the World Health Organization (WHO) and Centers for Disease Control and Prevention (CDC), there is an unprecedented increase in the occurrence of MDR infections worldwide. The rise in these infections has generated an economic strain worldwide, prompting the WHO to endorse a global action plan to improve awareness and understanding of antimicrobial resistance. This health crisis necessitates an immediate action to target the underlying mechanisms of drug resistance in bacteria.

RESEARCH

The advent of new bacterial genome engineering and synthetic biology (SB) tools is providing promising diagnostic and treatment plans to monitor and treat widespread recalcitrant bacterial infections. Key advances in genetic engineering approaches can successfully aid in targeting and editing pathogenic bacterial genomes for understanding and mitigating drug resistance mechanisms. In this review, we discuss the application of specific genome engineering and SB methods such as recombineering, clustered regularly interspaced short palindromic repeats (CRISPR), and bacterial cell-cell signaling mechanisms for pathogen targeting. The utility of these tools in developing antibacterial strategies such as novel antibiotic production, phage therapy, diagnostics and vaccine production to name a few, are also highlighted.

CONCLUSIONS

The prevalent use of antibiotics and the spread of MDR bacteria raise the prospect of a post-antibiotic era, which underscores the need for developing novel therapeutics to target MDR pathogens. The development of enabling SB technologies offers promising solutions to deliver safe and effective antibacterial therapies.

A cis-regulatory antisense RNA represses translation in Vibrio cholerae through extensive complementarity and proximity to the target locus

As with all facultative pathogens, Vibrio cholerae must optimize its cellular processes to adapt to different environments with varying carbon sources and to environmental stresses. More specifically, in order to metabolize mannitol, V. cholerae must regulate the synthesis of MtlA, a mannitol transporter protein produced exclusively in the presence of mannitol. We previously showed that a cis-acting small RNA (sRNA) expressed by V. cholerae, MtlS, appears to post-transcriptionally downregulate the expression of mtlA and is produced in the absence of mannitol. We hypothesized that since it is complementary to the 5′ untranslated region (UTR) of mtlA mRNA, MtlS may affect synthesis of MtlA by forming an mtlA-MtlS complex that blocks translation of the mRNA through occlusion of its ribosome binding site. To test this hypothesis, we used in vitro translation assays in order to examine the role MtlS plays in mtlA regulation and found that MtlS is sufficient to suppress translation of transcripts harboring the 5′ UTR of mtlA. However, in a cellular context, the 5′ UTR of mtlA is not sufficient for targeted repression by endogenous MtlS; additional segments from the coding region of mtlA play a role in the ability of the sRNA to regulate translation of mtlA mRNA. Additionally, proximity of transcription sites between the sRNA and mRNA significantly affects the efficacy of MtlS.

PTS-Mediated Regulation of the Transcription Activator MtlR from Different Species: Surprising Differences despite Strong Sequence Conservation

The hexitol D-mannitol is transported by many bacteria via a phosphoenolpyruvate (PEP):carbohydrate phosphotransferase system (PTS). In most Firmicutes, the transcription activator MtlR controls the expression of the genes encoding the D-mannitol-specific PTS components and D-mannitol-1-P dehydrogenase. MtlR contains an N-terminal helix-turn-helix motif followed by an Mga-like domain, two PTS regulation domains (PRDs), an EIIB(Gat)- and an EIIA(Mtl)-like domain. The four regulatory domains are the target of phosphorylation by PTS components. Despite strong sequence conservation, the mechanisms controlling the activity of MtlR from Lactobacillus casei, Bacillus subtilis and Geobacillus stearothermophilus are quite different. Owing to the presence of a tyrosine in place of the second conserved histidine (His) in PRD2, L. casei MtlR is not phosphorylated by Enzyme I (EI) and HPr. When the corresponding His in PRD2 of MtlR from B. subtilis and G. stearothermophilus was replaced with alanine, the transcription regulator was no longer phosphorylated and remained inactive. Surprisingly, L. casei MtlR functions without phosphorylation in PRD2 because in a ptsI (EI) mutant MtlR is constitutively active. EI inactivation prevents not only phosphorylation of HPr, but also of the PTS(Mtl) components, which inactivate MtlR by phosphorylating its EIIB(Gat)- or EIIA(Mtl)-like domain. This explains the constitutive phenotype of the ptsI mutant. The absence of EIIB(Mtl)-mediated phosphorylation leads to induction of the L. caseimtl operon. This mechanism resembles mtlARFD induction in G. stearothermophilus, but differs from EIIA(Mtl)-mediated induction in B. subtilis. In contrast to B. subtilis MtlR, L. casei MtlR activation does not require sequestration to the membrane via the unphosphorylated EIIB(Mtl) domain.

The mannitol utilization system of the marine bacterium Zobellia galactanivorans

Mannitol is a polyol that occurs in a wide range of living organisms, where it fulfills different physiological roles. In particular, mannitol can account for as much as 20 to 30% of the dry weight of brown algae and is likely to be an important source of carbon for marine heterotrophic bacteria. Zobellia galactanivorans (Flavobacteriia) is a model for the study of pathways involved in the degradation of seaweed carbohydrates. Annotation of its genome revealed the presence of genes potentially involved in mannitol catabolism, and we describe here the biochemical characterization of a recombinant mannitol-2-dehydrogenase (M2DH) and a fructokinase (FK). Among the observations, the M2DH of Z. galactanivorans was active as a monomer, did not require metal ions for catalysis, and featured a narrow substrate specificity. The FK characterized was active on fructose and mannose in the presence of a monocation, preferentially K(+). Furthermore, the genes coding for these two proteins were adjacent in the genome and were located directly downstream of three loci likely to encode an ATP binding cassette (ABC) transporter complex, suggesting organization into an operon. Gene expression analysis supported this hypothesis and showed the induction of these five genes after culture of Z. galactanivorans in the presence of mannitol as the sole source of carbon. This operon for mannitol catabolism was identified in only 6 genomes of Flavobacteriaceae among the 76 publicly available at the time of the analysis. It is not conserved in all Bacteroidetes; some species contain a predicted mannitol permease instead of a putative ABC transporter complex upstream of M2DH and FK ortholog genes.

Biotechnological applications of mobile group II introns and their reverse transcriptases: gene targeting, RNA-seq, and non-coding RNA analysis

Mobile group II introns are bacterial retrotransposons that combine the activities of an autocatalytic intron RNA (a ribozyme) and an intron-encoded reverse transcriptase to insert site-specifically into DNA. They recognize DNA target sites largely by base pairing of sequences within the intron RNA and achieve high DNA target specificity by using the ribozyme active site to couple correct base pairing to RNA-catalyzed intron integration. Algorithms have been developed to program the DNA target site specificity of several mobile group II introns, allowing them to be made into ‘targetrons.’ Targetrons function for gene targeting in a wide variety of bacteria and typically integrate at efficiencies high enough to be screened easily by colony PCR, without the need for selectable markers. Targetrons have found wide application in microbiological research, enabling gene targeting and genetic engineering of bacteria that had been intractable to other methods. Recently, a thermostable targetron has been developed for use in bacterial thermophiles, and new methods have been developed for using targetrons to position recombinase recognition sites, enabling large-scale genome-editing operations, such as deletions, inversions, insertions, and ‘cut-and-pastes’ (that is, translocation of large DNA segments), in a wide range of bacteria at high efficiency. Using targetrons in eukaryotes presents challenges due to the difficulties of nuclear localization and sub-optimal magnesium concentrations, although supplementation with magnesium can increase integration efficiency, and directed evolution is being employed to overcome these barriers. Finally, spurred by new methods for expressing group II intron reverse transcriptases that yield large amounts of highly active protein, thermostable group II intron reverse transcriptases from bacterial thermophiles are being used as research tools for a variety of applications, including qRT-PCR and next-generation RNA sequencing (RNA-seq). The high processivity and fidelity of group II intron reverse transcriptases along with their novel template-switching activity, which can directly link RNA-seq adaptor sequences to cDNAs during reverse transcription, open new approaches for RNA-seq and the identification and profiling of non-coding RNAs, with potentially wide applications in research and biotechnology.

Comparative genome analysis of non-toxigenic non-O1 versus toxigenic O1 Vibrio cholerae

Pathogenic strains of Vibrio cholerae are responsible for endemic and pandemic outbreaks of the disease cholera. The complete toxigenic mechanisms underlying virulence in Vibrio strains are poorly understood. The hypothesis of this work was that virulent versus non-virulent strains of V. cholerae harbor distinctive genomic elements that encode virulence. The purpose of this study was to elucidate genomic differences between the O1 serotypes and non-O1 V. cholerae PS15, a non-toxigenic strain, in order to identify novel genes potentially responsible for virulence. In this study, we compared the whole genome of the non-O1 PS15 strain to the whole genomes of toxigenic serotypes at the phylogenetic level, and found that the PS15 genome was distantly related to those of toxigenic V. cholerae. Thus we focused on a detailed gene comparison between PS15 and the distantly related O1 V. cholerae N16961. Based on sequence alignment we tentatively assigned chromosome numbers 1 and 2 to elements within the genome of non-O1 V. cholerae PS15. Further, we found that PS15 and O1 V. cholerae N16961 shared 98% identity and 766 genes, but of the genes present in N16961 that were missing in the non-O1 V. cholerae PS15 genome, 56 were predicted to encode not only for virulence-related genes (colonization, antimicrobial resistance, and regulation of persister cells) but also genes involved in the metabolic biosynthesis of lipids, nucleosides and sulfur compounds. Additionally, we found 113 genes unique to PS15 that were predicted to encode other properties related to virulence, disease, defense, membrane transport, and DNA metabolism. Here, we identified distinctive and novel genomic elements between O1 and non-O1 V. cholerae genomes as potential virulence factors and, thus, targets for future therapeutics. Modulation of such novel targets may eventually enhance eradication efforts of endemic and pandemic disease cholera in afflicted nations.

Modulation of Bacterial Multidrug Resistance Efflux Pumps of the Major Facilitator Superfamily

Bacterial infections pose a serious public health concern, especially when an infectious disease has a multidrug resistant causative agent. Such multidrug resistant bacteria can compromise the clinical utility of major chemotherapeutic antimicrobial agents. Drug and multidrug resistant bacteria harbor several distinct molecular mechanisms for resistance. Bacterial antimicrobial agent efflux pumps represent a major mechanism of clinical resistance. The major facilitator superfamily (MFS) is one of the largest groups of solute transporters to date and includes a significant number of bacterial drug and multidrug efflux pumps. We review recent work on the modulation of multidrug efflux pumps, paying special attention to those transporters belonging primarily to the MFS.

Mannitol and the mannitol-specific enzyme IIB subunit activate Vibrio cholerae biofilm formation

Vibrio cholerae is a halophilic, Gram-negative rod found in marine environments. Strains that produce cholera toxin cause the diarrheal disease cholera. V. cholerae use a highly conserved, multicomponent signal transduction cascade known as the phosphoenolpyruvate phosphotransferase system (PTS) to regulate carbohydrate uptake and biofilm formation. Regulation of biofilm formation by the PTS is complex, involving many different regulatory pathways that incorporate distinct PTS components. The PTS consists of the general components enzyme I (EI) and histidine protein (HPr) and carbohydrate-specific enzymes II. Mannitol transport by V. cholerae requires the mannitol-specific EII (EII(Mtl)), which is expressed only in the presence of mannitol. Here we show that mannitol activates V. cholerae biofilm formation and transcription of the vps biofilm matrix exopolysaccharide synthesis genes. This regulation is dependent on mannitol transport. However, we show that, in the absence of mannitol, ectopic expression of the B subunit of EII(Mtl) is sufficient to activate biofilm accumulation. Mannitol, a common compatible solute and osmoprotectant of marine organisms, is a main photosynthetic product of many algae and is secreted by algal mats. We propose that the ability of V. cholerae to respond to environmental mannitol by forming a biofilm may play an important role in habitat selection.

Generalized bacterial genome editing using mobile group II introns and Cre-lox

Efficient bacterial genetic engineering approaches with broad-host applicability are rare. We combine two systems, mobile group II introns ('targetrons') and Cre/lox, which function efficiently in many different organisms, into a versatile platform we call GETR (Genome Editing via Targetrons and Recombinases). The introns deliver lox sites to specific genomic loci, enabling genomic manipulations. Efficiency is enhanced by adding flexibility to the RNA hairpins formed by the lox sites. We use the system for insertions, deletions, inversions, and one-step cut-and-paste operations. We demonstrate insertion of a 12-kb polyketide synthase operon into the lacZ gene of Escherichia coli, multiple simultaneous and sequential deletions of up to 120 kb in E. coli and Staphylococcus aureus, inversions of up to 1.2 Mb in E. coli and Bacillus subtilis, and one-step cut-and-pastes for translocating 120 kb of genomic sequence to a site 1.5 Mb away. We also demonstrate the simultaneous delivery of lox sites into multiple loci in the Shewanella oneidensis genome. No selectable markers need to be placed in the genome, and the efficiency of Cre-mediated manipulations typically approaches 100%.

The Vibrio cholerae mannitol transporter is regulated posttranscriptionally by the MtlS small regulatory RNA

Vibrio cholerae continues to pose a health threat in many developing nations and regions of the world struck by natural disasters. It is a pathogen that rapidly adapts to aquatic environments and the human small intestine. Small regulatory RNAs (sRNAs) may contribute to this adaptability. Specifically, the mannitol operon sRNA (MtlS sRNA; previously designated the IGR7 sRNA) is transcribed antisense to the 5' untranslated region of the mtl operon, encoding the mannitol-specific phosphotransferase system. Mannitol is a six-carbon sugar alcohol that accumulates in the human small intestine, the primary site of V. cholerae colonization. To better understand the V. cholerae mtl operon at a molecular level, we investigated mtlA expression in the presence of various carbon sources and the role of the MtlS sRNA. We observed that MtlA protein is present only in cells grown on mannitol sugar, whereas MtlS sRNA is expressed during growth on all sugars other than mannitol. In contrast, mtlA mRNA is expressed in similar amounts regardless of the carbon source used for bacterial growth. These observations suggest that the regulation of MtlA protein expression is a posttranscriptional event. We further demonstrate that MtlS sRNA overexpression repressed MtlA synthesis without affecting the stability of the messenger and that this process is largely independent of Hfq. We propose a model in which, when carbon sources other than mannitol are present, MtlS sRNA is transcribed, base pairs with the 5' untranslated region of the mtlA mRNA, occluding the ribosome binding site, and inhibits the synthesis of the mannitol-specific phosphotransferase system.

Molecular cloning and characterization of mannitol-1-phosphate dehydrogenase from Vibrio cholerae

Vibrio cholerae utilizes mannitol through an operon of the phosphoenolpyruvate-dependent phosphotransferase (PTS) type. A gene, mtlD, encoding mannitol-1-phosphate dehydrogenase was identified within the 3.9 kb mannitol operon of V. cholerae. The mtlD gene was cloned from V. cholerae O395, and the recombinant enzyme was functionally expressed in E. coli as a 6×His-tagged protein and purified to homogeneity. The recombinant protein is a monomer with a molecular mass of 42.35 kDa. The purified recombinant MtlD reduced fructose 6-phosphate (F6P) using NADH as a cofactor with a K(m) of 1.54 +/- 0.1 mM and V(max) of 320.8 +/- 7.81 micronmol/min/mg protein. The pH and temperature optima for F6P reduction were determined to be 7.5 and 37°C, respectively. Using quantitative real-time PCR analysis, mtlD was found to be constitutively expressed in V. cholerae, but the expression was up-regulated when grown in the presence of mannitol. The MtlD expression levels were not significantly different between V. cholerae O1 and non-O1 strains.