Regulation of expression of the Lactobacillus pentosus xylAB operon

Xylose Isomerase Depletion Enhances Virulence of Xanthomonas citri subsp. citri in Citrus aurantifolia

Citrus canker, caused by the bacterium Xanthomonas citri (Xcc), is one of the most devastating diseases for the citrus industry. Xylose is a constituent of the cell wall of plants, and the ability of Xcc to use this carbohydrate may play a role in virulence. Xcc has two genes codifying for xylose isomerase (XI), a bifunctional enzyme that interconverts D-xylose into D-xylulose and D-glucose into D-fructose. The aim of this work was to investigate the functional role of the two putative XI ORFs, XAC1776 (xylA1) and XAC4225 (xylA2), in Xcc pathogenicity. XI-coding genes of Xcc were deleted, and the single mutants (XccΔxylA1 or XccΔxylA2) or the double mutant (XccΔxylA1ΔxylA2) remained viable. The deletion of one or both XI genes (xylA1 and/or xylA2) increased the aggressiveness of the mutants, causing disease symptoms. RT-qPCR analysis of wild strain and xylA deletion mutants grown in vivo and in vitro revealed that the highest expression level of hrpX and xylR was observed in vivo for the double mutant. The results indicate that XI depletion increases the expression of the hrp regulatory genes in Xcc. We concluded that the intracellular accumulation of xylose enhances Xcc virulence.

Macroalgal biomass as a potential resource for lactic acid fermentation

Lactic acid is an essential platform chemical with various applications in the chemicals, food, pharmaceutical, and cosmetic industries. Currently, the demand for lactic acid is driven by the role of lactic acid as the starting material for the production of bioplastic polylactide. Microbial fermentation for lactic acid production is favored due to the production of enantiomerically pure lactic acid required for polylactide synthesis, as opposed to the racemic mixture obtained via chemical synthesis. The utilization of first-generation feedstock for commercial lactic acid production is challenged by feedstock costs and sustainability issues. Macroalgae are photosynthetic benthic aquatic plants that contribute tremendously towards carbon capture with subsequent carbon-rich biomass production. Macroalgae are commercially cultivated to extract hydrocolloids, and recent studies have focused on applying biomass as a fermentation feedstock. This review provides comprehensive information on the design and development of sustainable and cost-effective, algae-based lactic acid production. The central carbon regulation in lactic acid bacteria and the metabolism of seaweed-derived sugars are described. An exhaustive compilation of lactic acid fermentation of macroalgae hydrolysates revealed that lactic acid bacteria can effectively ferment the mixture of sugars present in the hydrolysate with comparable yields. The environmental impacts and economic prospects of macroalgal lactic acid are analyzed. Valorization of the vast amounts of spent macroalgal biomass residue post hydrocolloid extraction in a biorefinery is a viable strategy for cost-effective lactic acid production.

D-Xylose Sensing in Saccharomyces cerevisiae: Insights from D-Glucose Signaling and Native D-Xylose Utilizers

Extension of the substrate range is among one of the metabolic engineering goals for microorganisms used in biotechnological processes because it enables the use of a wide range of raw materials as substrates. One of the most prominent examples is the engineering of baker's yeast Saccharomyces cerevisiae for the utilization of d-xylose, a five-carbon sugar found in high abundance in lignocellulosic biomass and a key substrate to achieve good process economy in chemical production from renewable and non-edible plant feedstocks. Despite many excellent engineering strategies that have allowed recombinant S. cerevisiae to ferment d-xylose to ethanol at high yields, the consumption rate of d-xylose is still significantly lower than that of its preferred sugar d-glucose. In mixed d-glucose/d-xylose cultivations, d-xylose is only utilized after d-glucose depletion, which leads to prolonged process times and added costs. Due to this limitation, the response on d-xylose in the native sugar signaling pathways has emerged as a promising next-level engineering target. Here we review the current status of the knowledge of the response of S. cerevisiae signaling pathways to d-xylose. To do this, we first summarize the response of the native sensing and signaling pathways in S. cerevisiae to d-glucose (the preferred sugar of the yeast). Using the d-glucose case as a point of reference, we then proceed to discuss the known signaling response to d-xylose in S. cerevisiae and current attempts of improving the response by signaling engineering using native targets and synthetic (non-native) regulatory circuits.

Characterization of robust Lactobacillus plantarum and Lactobacillus pentosus starter cultures for environmentally friendly low-salt cucumber fermentations

Seven candidates for starter cultures for cucumber fermentations belonging to the Lactobacillus pentosus and Lactobacillus plantarum species were characterized based on physiological features desired for pickling. The isolates presented variable carbohydrate utilization profile on API^® 50CHL test strips. The L. pentosus strains were unable to utilize d-xylose in MRS broth or the M medium. The lactobacilli were unable to produce histamine, tyramine, putrescine, and cadaverine in biogenic amine broth containing the necessary precursors. Production of d-lactic acid by the lactobacilli, detected enzymatically, was stimulated by growth in MRS broth as compared to cucumber juice medium (CJM). The lactobacilli utilized malic acid in the malate decarboxylase medium. Exopolyssacharide biosynthesis related genes were amplified from the lactobacilli. A sugar type-dependent-ropy phenotype was apparent for all the cultures tested in MRS and CJM. The genes associated with bacteriocin production were detected in the lactobacilli, but not the respective phenotypes. The antibiotic susceptibility profile of the lactobacilli mimics that of other L. plantarum starter cultures. It is concluded that the lactobacilli strains studied here are suitable starter cultures for cucumber fermentation. PRACTICAL APPLICATION: The availability of such starter cultures enables the implementation of low salt cucumber fermentations that can generate products with consistent biochemistry and microbiological profile.

Identification of a xylose-inducible promoter and its application for improving vitamin B12 production in Sinorhizobium meliloti

Sinorhizobium meliloti 320 is a vitamin B₁₂ (VB₁₂ ) high-producing strain that has been isolated and identified in our previous study. Because the regulatory toolbox for S. meliloti is limited, we searched for new genetic components and identified the two xylose-inducible promoters P_A and P_B based on a promoter-probe vector with a green fluorescent protein (GFP) as reporter. Compared with the P_araA promoter from S. meliloti, both promoters exhibited higher induced expression and lower basal expression. Subsequently, the influence of glucose or sucrose on the expression of GFP driven by these three promoters was assayed. Glucose repressed all three promoters, and the expression of P_araA was the lowest in the presence of glucose. Although sucrose repressed the expression of P_A by 35% and improved the expression of P_araA by 16%, the expression level of P_A was the highest and was 13% higher than that of P_araA . Lastly, we overexpressed the hemA gene in the C4 pathway using the P_A promoter in S. meliloti 320, and the VB₁₂ production of the engineered strain increased by 11%. The VB₁₂ production was further increased by 11% by adding 0.1% sodium succinate to the culture medium.

Genome-scale exploration of transcriptional regulation in the nisin Z producer Lactococcus lactis subsp. lactis IO-1

Transcription is of the most crucial steps of gene expression in bacteria, whose regulation guarantees the bacteria's ability to adapt to varying environmental conditions. Discovering the molecular basis and genomic principles of the transcriptional regulation is thus one of the most important tasks in cellular and molecular biology. Here, a comprehensive phylogenetic footprinting framework was implemented to predict maximal regulons of Lactococcus lactis subsp. lactis IO-1, a lactic acid bacterium known for its high potentials in nisin Z production as well as efficient xylose consumption which have made it a promising biotechnological strain. A total set of 321 regulons covering more than 90% of all the bacterium's operons have been elucidated and validated according to available data. Multiple novel biologically-relevant members were introduced amongst which arsC, mtlA and mtl operon for BusR, MtlR and XylR regulons can be named, respectively. Moreover, the effect of riboflavin on nisin biosynthesis was assessed in vitro and a negative correlation was observed. It is believed that understandings from such networks not only can be useful for studying transcriptional regulatory potentials of the target organism but also can be implemented in biotechnology to rationally design favorable production conditions.

Relationship Between Putative eps Genes and Production of Exopolysaccharide in Lactobacillus casei LC2W

Lactobacillus casei LC2W, a probiotic strain, can produce exopolysaccharide (EPS) with anti-hypertensive bioactivity. The relationship between eps genes and EPS synthesis in LC2W due to unclear regulation mechanism of EPS biosynthesis was investigated. The several relevant genes in EPS biosynthetic gene cluster were deleted, overexpressed and complemented. The results suggested that glucose-1-phosphate thymidyltranseferase gene (LC2W_2179), uncharacterized EPS biosynthesis protein (LC2W_2188), and EPS biosynthesis protein (LC2W_2189) were related to EPS biosynthesis. EPS titer decreased 15, 13, and 21% when the three genes were deleted, respectively. When they were overexpressed, EPS titer increased 16, 10, and 18%. When they were complemented, EPS titer was similar to the wild-type strain. This work showed the three eps genes from LC2W played important roles on EPS production.

Characterization of a cryptic plasmid isolated from Lactobacillus casei CP002616 and construction of shuttle vectors based on its replicon

The cryptic plasmid pLC2W was isolated from Lactobacillus casei CP002616. Nucleotide sequence analysis revealed that 4 putative open reading frames (ORF) were responsible for DNA replication. Four Escherichia coli-Lactobacillus shuttle vectors were constructed using different lengths of the pLC2W replicon to identify the shortest functional replicon. The length of the pLC2W replicon did not affect the stability of the plasmids. Green fluorescent protein (GFP) as a reporter was expressed successfully in several lactobacilli using our constructed vectors. The results suggested that the expression vectors pUE-F0GFP and pUE-F1GFP are potential molecular tools for heterologous gene cloning and expression in lactobacilli. Moreover, 2 plasmid-curing methods were used to eliminate pLC2W from L. casei. We detected no difference between L. casei CP002616 and L. casei CP002616 pLC2WΔ-IC (mutant strain cured by plasmid incompatibility method) in production of exopolysaccharide (EPS) or acid. However, EPS and acid production were both reduced in L. casei CP002616 pLC2WΔ-HT (mutant strain cured by high-temperature heat treatment method), demonstrating a difference between these 2 curing methods. Sequence analysis of pLC2W and plasmid curing data suggest that plasmid pLC2W is not involved in EPS synthesis.

CRISPR-Cas9D10A Nickase-Assisted Genome Editing in Lactobacillus casei

Lactobacillus casei has drawn increasing attention as a health-promoting probiotic, while effective genetic manipulation tools are often not available, e.g., the single-gene knockout in L. casei still depends on the classic homologous recombination-dependent double-crossover strategy, which is quite labor-intensive and time-consuming. In the present study, a rapid and precise genome editing plasmid, pLCNICK, was established for L. casei genome engineering based on CRISPR-Cas9^D10A In addition to the P₂₃-Cas9^D10A and P_ldh-sgRNA (single guide RNA) expression cassettes, pLCNICK includes the homologous arms of the target gene as repair templates. The ability and efficiency of chromosomal engineering using pLCNICK were evaluated by in-frame deletions of four independent genes and chromosomal insertion of an enhanced green fluorescent protein (eGFP) expression cassette at the LC2W_1628 locus. The efficiencies associated with in-frame deletions and chromosomal insertion is 25 to 62%. pLCNICK has been proved to be an effective, rapid, and precise tool for genome editing in L. casei, and its potential application in other lactic acid bacteria (LAB) is also discussed in this study.IMPORTANCE The lack of efficient genetic tools has limited the investigation and biotechnological application of many LAB. The CRISPR-Cas9^D10A nickase-based genome editing in Lactobacillus casei, an important food industrial microorganism, was demonstrated in this study. This genetic tool allows efficient single-gene deletion and insertion to be accomplished by one-step transformation, and the cycle time is reduced to 9 days. It facilitates a rapid and precise chromosomal manipulation in L. casei and overcomes some limitations of previous methods. This editing system can serve as a basic technological platform and offers the possibility to start a comprehensive investigation on L. casei As a broad-host-range plasmid, pLCNICK has the potential to be adapted to other Lactobacillus species for genome editing.

Natural DNA Transformation Is Functional in Lactococcus lactis subsp. cremoris KW2

Lactococcus lactis is one of the most commonly used lactic acid bacteria in the dairy industry. Activation of competence for natural DNA transformation in this species would greatly improve the selection of novel strains with desired genetic traits. Here, we investigated the activation of natural transformation in L. lactis subsp. cremoris KW2, a strain of plant origin whose genome encodes the master competence regulator ComX and the complete set of proteins usually required for natural transformation. In the absence of knowledge about competence regulation in this species, we constitutively overproduced ComX in a reporter strain of late competence phase activation and showed, by transcriptomic analyses, a ComX-dependent induction of all key competence genes. We further demonstrated that natural DNA transformation is functional in this strain and requires the competence DNA uptake machinery. Since constitutive ComX overproduction is unstable, we alternatively expressed comX under the control of an endogenous xylose-inducible promoter. This regulated system was used to successfully inactivate the adaptor protein MecA and subunits of the Clp proteolytic complex, which were previously shown to be involved in ComX degradation in streptococci. In the presence of a small amount of ComX, the deletion of mecA, clpC, or clpP genes markedly increased the activation of the late competence phase and transformability. Altogether, our results report the functionality of natural DNA transformation in L. lactis and pave the way for the identification of signaling mechanisms that trigger the competence state in this species.IMPORTANCE Lactococcus lactis is a lactic acid bacterium of major importance, which is used as a starter species for milk fermentation, a host for heterologous protein production, and a delivery platform for therapeutic molecules. Here, we report the functionality of natural transformation in L. lactis subsp. cremoris KW2 by the overproduction of the master competence regulator ComX. The developed procedure enables a flexible approach to modify the chromosome with single point mutation, sequence insertion, or sequence replacement. These results represent an important step for the genetic engineering of L. lactis that will facilitate the design of strains optimized for industrial applications. This will also help to discover natural regulatory mechanisms controlling competence in the genus Lactococcus.

Enhanced D-lactic acid production from renewable resources using engineered Lactobacillus plantarum

D-lactic acid is used as a monomer in the production of poly-D-lactic acid (PDLA), which is used to form heat-resistant stereocomplex poly-lactic acid. To produce cost-effective D-lactic acid by using all sugars derived from biomass efficiently, xylose-assimilating genes encoding xylose isomerase and xylulokinase were cloned into an L-lactate-deficient strain, Lactobacillus plantarum. The resulting recombinant strain, namely L. plantarum NCIMB 8826 ∆ldhL1-pLEM-xylAB, was able to produce D-lactic acid (at optical purity >99 %) from xylose at a yield of 0.53 g g(-1). Simultaneous utilization of glucose and xylose to produce D-lactic acid was also achieved by this strain, and 47.2 g L(-1) of D-lactic acid was produced from 37.5 g L(-1) glucose and 19.7 g L(-1) xylose. Corn stover and soybean meal extract (SBME) were evaluated as cost-effective medium components for D-lactic acid production. Optimization of medium composition using response surface methodology resulted in 30 % reduction in enzyme loading and 70 % reduction in peptone concentration. In addition, we successfully demonstrated D-lactic acid fermentation from corn stover and SBME in a fed-batch fermentation, which yielded 61.4 g L(-1) D-lactic acid with an overall yield of 0.77 g g(-1). All these approaches are geared to attaining high D-lactic acid production from biomass sugars to produce low-cost, highly thermostable biodegradable plastics.

A versatile mini-mazF-cassette for marker-free targeted genetic modification in Bacillus subtilis

There are some drawbacks for MazF-cassette constructed in previous reports for marker-free genetic manipulation in Bacillus subtilis, including cloning-dependent methodology and non-strictly controlled expression system. In our study, the modifications on mazF-cassette are carried out, such as using mini Zeocin resistance gene as positive-selectable marker and strictly controlled xyl promoter from the B. subtilis to replace non-strictly controlled IPTG-inducible Pspac or xyl promoter from Bacillus megaterium. Then the mini-mazF-cassette was successfully applied to knock-out the amyE gene, to delete a 90-kb gene cluster, and to knock-in a green fluorescent protein expression cassette employing a cloning-independent methodology, without introducing undesirable redundant sequences at the modified locus in the B. subtilis 1A751. Besides, the mini-mazF-cassette could be used repeatedly to delete multiple genes or gene clusters with only a 2- to 2.5-kb PCR-fused fragment, which largely reduced the frequency of nucleic acid mutations generated by PCR compared to previous reports. We further demonstrated that the frequency of spontaneous mazF-resistant mutants was lower, and the frequency of generating desired clones was nearly 100%. The entire procedure for marker-free genetic manipulation using the mini-mazF-cassette can be finished in about 3days. This modified cassette has remarkable improvement compared to existing approaches and is applicable for available manipulating Bacillus species chromosomes.

Atypical ethanol production by carbon catabolite derepressed lactobacilli

Cost effective use of lignocellulosic biomass for bio-based chemical production requires the discovery of novel strains and processes. Lactobacillus pentosus JH5XP5 is a carbon catabolite repression negative mutant which utilizes glucose and pentoses derived from lignocellulosic biomass in the media simultaneously. With a broad range of carbon substrates, L. pentosus JH5XP5 produced a significant amount of ethanol without acetate formation. The yields of ethanol were 2.0- to 2.5-fold higher than those of lactate when glucose, galactose or maltose was used either as a single carbon source or simultaneously with glucose. L. pentosus JH5XP5 was successfully used in an integrated process of simultaneous saccharification and mixed sugar fermentation of rice straw hydrolysate. During the fermentation, the enzyme activities for the saccharification of cellulose were not diminished. Moreover glucose, xylose, and arabinose sugars derived from rice straw hyrolysate were consumed concurrently as if a single carbon source existed and no sugars or cellulosic fiber remained after the fermentation.

Combined transcriptome and proteome analysis of Bifidobacterium animalis subsp. lactis BB-12 grown on xylo-oligosaccharides and a model of their utilization

Recent studies have demonstrated that xylo-oligosaccharides (XOS), which are classified as emerging prebiotics, selectively enhance the growth of bifidobacteria in general and of Bifidobacterium animalis subsp. lactis strains in particular. To elucidate the metabolism of XOS in the well-documented and widely used probiotic strain B. animalis subsp. lactis BB-12, a combined proteomic and transcriptomic approach was applied, involving DNA microarrays, real-time quantitative PCR (qPCR), and two-dimensional difference gel electrophoresis (2D-DIGE) analyses of samples obtained from cultures grown on either XOS or glucose. The analyses show that 9 of the 10 genes that encode proteins predicted to play a role in XOS catabolism (i.e., XOS-degrading and -metabolizing enzymes, transport proteins, and a regulatory protein) were induced by XOS at the transcriptional level, and the proteins encoded by three of these (β-d-xylosidase, sugar-binding protein, and xylose isomerase) showed higher abundance on XOS. Based on the obtained results, a model for the catabolism of XOS in BB-12 is suggested, according to which the strain utilizes an ABC (ATP-binding cassette) transport system (probably for oligosaccharides) to bind XOS on the cell surface and transport them into the cell. XOS are then degraded intracellularly through the action of xylanases and xylosidases to d-xylose, which is subsequently metabolized by the d-fructose-6-P shunt. The findings obtained in this study may have implications for the design of a synbiotic application containing BB-12 and the XOS used in the present study.

Cloning Vectors Based on Cryptic Plasmids Isolated from Lactic Acid Bacteria:Their Characteristics and Potential Applications in Biotechnology

Lactic acid bacteria (LAB) are Gram positive bacteria, widely distributed in nature, and industrially important as they are used in a variety of industrial food fermentations. The use of genetic engineering techniques is an effective means of enhancing the industrial applicability of LAB. However, when using genetic engineering technology, safety becomes an essential factor for the application of improved LAB to the food industry. Cloning and expression systems should be derived preferably from LAB cryptic plasmids that generally encode genes for which functions can be proposed, but no phenotypes can be observed. However, some plasmid-encoded functions have been discovered in cryptic plasmids originating from Lactobacillus, Streptococcus thermophilus, and Pediococcus spp. and can be used as selective marker systems in vector construction. This article presents information concerning LAB cryptic plasmids, and their structures, functions, and applications. A total of 134 cryptic plasmids collated are discussed.

Regulation of sugar catabolism in Lactococcus lactis

The increasing number of genomic and post-genomic studies on Gram-positive organisms and especially on lactic acid bacteria brings a lot of information on sugar catabolism in these bacteria. Like for many other bacteria, glucose is the most preferred source of carbon and energy for Lactococcus lactis. Other carbon sources can induce their own utilization in the absence of well-metabolized sugar. These processes engage numbers of genes and undergo complex mechanisms of regulation. In this review, we discuss various biochemical and genetic control mechanisms involved in sugar catabolism, like regulation by repressors, activators, antiterminators or carbon catabolite repression control.

How phosphotransferase system-related protein phosphorylation regulates carbohydrate metabolism in bacteria

The phosphoenolpyruvate(PEP):carbohydrate phosphotransferase system (PTS) is found only in bacteria, where it catalyzes the transport and phosphorylation of numerous monosaccharides, disaccharides, amino sugars, polyols, and other sugar derivatives. To carry out its catalytic function in sugar transport and phosphorylation, the PTS uses PEP as an energy source and phosphoryl donor. The phosphoryl group of PEP is usually transferred via four distinct proteins (domains) to the transported sugar bound to the respective membrane component(s) (EIIC and EIID) of the PTS. The organization of the PTS as a four-step phosphoryl transfer system, in which all P derivatives exhibit similar energy (phosphorylation occurs at histidyl or cysteyl residues), is surprising, as a single protein (or domain) coupling energy transfer and sugar phosphorylation would be sufficient for PTS function. A possible explanation for the complexity of the PTS was provided by the discovery that the PTS also carries out numerous regulatory functions. Depending on their phosphorylation state, the four proteins (domains) forming the PTS phosphorylation cascade (EI, HPr, EIIA, and EIIB) can phosphorylate or interact with numerous non-PTS proteins and thereby regulate their activity. In addition, in certain bacteria, one of the PTS components (HPr) is phosphorylated by ATP at a seryl residue, which increases the complexity of PTS-mediated regulation. In this review, we try to summarize the known protein phosphorylation-related regulatory functions of the PTS. As we shall see, the PTS regulation network not only controls carbohydrate uptake and metabolism but also interferes with the utilization of nitrogen and phosphorus and the virulence of certain pathogens.

Carbohydrate, peptide and lipid metabolism of lactic acid bacteria in sourdough

The metabolic pathways of lactic acid bacteria that influence bread quality are coupled to the central carbon flux by the availability of cofactors influencing the cellular and environmental redox potential. Homo- and heterofermentative metabolism differ fundamentally with respect to the requirement for regeneration of reduced cofactors, NADH or NADPH. The utilization of co-substrates such as oxygen or fructose as electron acceptors by obligate heterofermentative lactobacilli is coupled to an increased production of acetate in dough. Recently, several oxidoreductases involved in cofactor regeneration were characterized and glutathione and short-chain aldehydes derived from lipid oxidation were identified as substrates for cofactor regeneration by Lactobacillus sanfranciscensis. Based on the different metabolic requirements for cofactor regeneration, homo- and heterofermentative lactobacilli exert divergent effects on redox-reactions in sourdough that influence bread quality beyond the formation of acetate. Proteolysis, followed by peptide or amino acid metabolism by LAB is one of the key routes of flavour formation in bread flavour, and enables the strain-specific formation of antifungal metabolites. Peptide metabolism as well as the metabolism of cysteine, arginine, and phenylalanine in Lactobacillus plantarum, L. sanfranciscensis, and Lactobacillus pontis is increasingly understood and these insights provide new opportunities for the directed application of sourdough LAB for improved bread quality.

Engineering of a xylose metabolic pathway in Corynebacterium glutamicum

The aerobic microorganism Corynebacterium glutamicum was metabolically engineered to broaden its substrate utilization range to include the pentose sugar xylose, which is commonly found in agricultural residues and other lignocellulosic biomass. We demonstrated the functionality of the corynebacterial xylB gene encoding xylulokinase and constructed two recombinant C. glutamicum strains capable of utilizing xylose by cloning the Escherichia coli gene xylA encoding xylose isomerase, either alone (strain CRX1) or in combination with the E. coli gene xylB (strain CRX2). These genes were provided on a high-copy-number plasmid and were under the control of the constitutive promoter trc derived from plasmid pTrc99A. Both recombinant strains were able to grow in mineral medium containing xylose as the sole carbon source, but strain CRX2 grew faster on xylose than strain CRX1. We previously reported the use of oxygen deprivation conditions to arrest cell replication in C. glutamicum and divert carbon source utilization towards product production rather than towards vegetative functions (M. Inui, S. Murakami, S. Okino, H. Kawaguchi, A. A. Vertès, and H. Yukawa, J. Mol. Microbiol. Biotechnol. 7:182-196, 2004). Under these conditions, strain CRX2 efficiently consumed xylose and produced predominantly lactic and succinic acids without growth. Moreover, in mineral medium containing a sugar mixture of 5% glucose and 2.5% xylose, oxygen-deprived strain CRX2 cells simultaneously consumed both sugars, demonstrating the absence of diauxic phenomena relative to the new xylA-xylB construct, albeit glucose-mediated regulation still exerted a measurable influence on xylose consumption kinetics.

A xylose-inducible expression system forLactococcus lactis

A new controlled production system to target heterologous proteins to cytoplasm or extracellular medium is described for Lactococcus lactis NCDO2118. It is based on the use of a xylose-inducible lactococcal promoter, P(xylT). The capacities of this system to produce cytoplasmic and secreted proteins were tested using the Staphylococcus aureus nuclease gene (nuc) fused or not to the lactococcal Usp45 signal peptide. Xylose-inducible nuc expression is tightly controlled and resulted in high-level and long-term protein production, and correct targeting either to the cytoplasm or to the extracellular medium. Furthermore, this expression system is versatile and can be switched on or off easily by adding either xylose or glucose, respectively. These results confirm the potential of this expression system as an alternative and useful tool for the production of proteins of interest in L. lactis.

Effect of adaptation to ethanol on cytoplasmic and membrane protein profiles of Oenococcus oeni

The practical application of commercial malolactic starter cultures of Oenococcus oeni surviving direct inoculation in wine requires insight into mechanisms of ethanol toxicity and of acquired ethanol tolerance in this organism. Therefore, the site-specific location of proteins involved in ethanol adaptation, including cytoplasmic, membrane-associated, and integral membrane proteins, was investigated. Ethanol triggers alterations in protein patterns of O. oeni cells stressed with 12% ethanol for 1 h and those of cells grown in the presence of 8% ethanol. Levels of inosine-5'-monophosphate dehydrogenase and phosphogluconate dehydrogenase, which generate reduced nicotinamide nucleotides, were decreased during growth in the presence of ethanol, while glutathione reductase, which consumes NADPH, was induced, suggesting that maintenance of the redox balance plays an important role in ethanol adaptation. Phosphoenolpyruvate:mannose phosphotransferase system (PTS) components of mannose PTS, including the phosphocarrier protein HPr and EII(Man), were lacking in ethanol-adapted cells, providing strong evidence that mannose PTS is absent in ethanol-adapted cells, and this represents a metabolic advantage to O. oeni cells during malolactic fermentation. In cells grown in the presence of ethanol, a large increase in the number of membrane-associated proteins was observed. Interestingly, two of these proteins, dTDT-glucose-4,6-dehydratase and D-alanine:D-alanine ligase, are known to be involved in cell wall biosynthesis. Using a proteomic approach, we provide evidence for an active ethanol adaptation response of O. oeni at the cytoplasmic and membrane protein levels.

Complementation of a Delta ccpA mutant of Lactobacillus casei with CcpA mutants affected in the DNA- and cofactor-binding domains

In low-G+C Gram-positive bacteria, the regulatory protein CcpA has been shown to play a major part in the so-called carbon catabolite repression (CCR) process, as well as in the induction of basic metabolic genes, for which it is considered a global regulator. A strain of Lactobacillus casei that carried a complete deletion of ccpA has been constructed and used to test the effect of CCR on N-acetylglucosaminidase activity and growth performance of a collection of seven CcpA mutations obtained by site-directed mutagenesis. The replaced amino acids were located in the DNA- and cofactor (P-Ser-HPr)-binding domains. Mutations in the DNA-binding domain lacked CCR, as found in Bacillus megaterium. However, mutations in the cofactor-binding domain of L. casei CcpA had a different phenotype to that observed in the previous studies with B. megaterium. Two of them, S80L and T307I, displayed a significant hyper-repression, an effect never reported before for CcpA. Comparison of growth capabilities provided by the different mutants and their ability to sustain CCR demonstrated that CCR, at least on the enzymic activity tested, and the growth defect caused by the CcpA mutations are unrelated features.

Identification of Lactobacillus reuteri genes specifically induced in the mouse gastrointestinal tract

Lactobacilli are common inhabitants of the gastrointestinal tracts of mammals and have received considerable attention due to their putative health-promoting properties. Little is known about the traits that enhance the ability of these bacteria to inhabit the gastrointestinal tract. In this paper we describe the development and application of a strategy based on in vivo expression technology (IVET) that enables detection of Lactobacillus reuteri genes specifically induced in the murine gut. A plasmid-based system was constructed containing 'ermGT (which confers lincomycin resistance) as the primary reporter gene for selection of promoters active in the gastrointestinal tract of mice treated with lincomycin. A second reporter gene, 'bglM (beta-glucanase), allowed differentiation between constitutive and in vivo inducible promoters. The system was successfully tested in vitro and in vivo by using a constitutive promoter. Application of the IVET system with chromosomal DNA of L. reuteri 100-23 and reconstituted lactobacillus-free mice revealed three genes induced specifically during colonization. Two of the sequences showed homology to genes encoding xylose isomerase (xylA) and peptide methionine sulfoxide reductase (msrB), which are involved in nutrient acquisition and stress responses, respectively. The third locus showed homology to the gene encoding a protein whose function is not known. Our IVET system has the potential to identify genes of lactobacilli that have not previously been functionally characterized but which may be essential for growth of these bacteria in the gastrointestinal ecosystem.

Lactobacilli as live vaccine delivery vectors: progress and prospects

Evidence is accumulating that lactobacilli influence the immune response in a strain-dependent manner. This immunomodulatory capacity is important for the development of the immune response, and also identifies Lactobacillus as a potent oral vaccine carrier. Most of our current knowledge of the use of lactobacilli for vaccination purposes has been obtained with tetanus toxin fragment C (TTFC) as the model antigen. This knowledge, together with our ever-increasing understanding of the immune system and recent developments in cloning and expression techniques, should enable the utilisation of antigens other than TTFC and has made the development of lactobacilli as live vaccines a realistic prospect.

Expression of the xylulose 5-phosphate phosphoketolase gene, xpkA, from Lactobacillus pentosus MD363 is induced by sugars that are fermented via the phosphoketolase pathway and is repressed by glucose mediated by CcpA and the mannose phosphoenolpyruvate phosphotransferase system

Purification of xylulose 5-phosphate phosphoketolase (XpkA), the central enzyme of the phosphoketolase pathway (PKP) in lactic acid bacteria, and cloning and sequence analysis of the encoding gene, xpkA, from Lactobacillus pentosus MD363 are described. xpkA encodes a 788-amino-acid protein with a calculated mass of 88,705 Da. Expression of xpkA in Escherichia coli led to an increase in XpkA activity, while an xpkA knockout mutant of L. pentosus lost XpkA activity and was not able to grow on energy sources that are fermented via the PKP, indicating that xpkA encodes an enzyme with phosphoketolase activity. A database search revealed that there are high levels of similarity between XpkA and a phosphoketolase from Bifidobacterium lactis and between XpkA and a (putative) protein present in a number of evolutionarily distantly related organisms (up to 54% identical residues). Expression of xpkA in L. pentosus was induced by sugars that are fermented via the PKP and was repressed by glucose mediated by carbon catabolite protein A (CcpA) and by the mannose phosphoenolpyruvate phosphotransferase system. Most of the residues involved in correct binding of the cofactor thiamine pyrophosphate (TPP) that are conserved in transketolase, pyruvate decarboxylase, and pyruvate oxidase were also conserved at a similar position in XpkA, implying that there is a similar TPP-binding fold in XpkA.

Xyloside transport by XylP, a member of the galactoside-pentoside-hexuronide family

This paper describes the functional characterization of the xyloside transporter, XylP, of Lactobacillus pentosus with the aid of a spectroscopy-based assay system. In order to monitor the transport reaction, the natural xyloside isoprimeverose, a building block of hemicellulose, and the analogue methyl-isoprimeverose were chemically synthesized by a new and efficient procedure. The XylP protein was purified by metal affinity chromatography, following high level expression in Lactococcus lactis from the nisin-inducible promoter. The purified XylP protein was incorporated into liposomes, in which the glucose dehydrogenase from Acinetobacter calcoaceticus (sGDH) was entrapped. sGDH can oxidize aldose sugars in the presence of dichlorophenol-indophenol as electron acceptor. The coupled assay thus involves XylP-mediated isoprimeverose uptake followed by internal oxidation of the sugar by sGDH, which can be monitored from the reduction of 2,6-dichlorophenol-indophenol at 600 nm. The uptake of isoprimeverose was stimulated by the presence of the non-oxidizable methyl-isoprimeverose on the trans-side of the membrane, indicating that exchange transport is faster than unidirectional downhill uptake. Unlike other members of the galactoside-pentoside-hexuronide family, XylP does not transport monosaccharides (xylose) but requires a glycosidic linkage at the anomeric carbon position. Consistent with a proton motive force-driven mechanism, the uptake was stimulated by a membrane potential (inside negative relative to outside) and inhibited by a pH gradient (inside acidic relative to outside). The advantages of the here-described transport assay for studies of carbohydrate transport are discussed.

Contribution of the phosphoenolpyruvate:mannose phosphotransferase system to carbon catabolite repression in Lactobacillus pentosus

The role of the Lactobacillus pentosus phosphoenolpyruvate:mannose phosphotransferase system (mannose PTS) in sugar transport and control of sugar utilization was investigated. Growth experiments and measurements of PEP-dependent phosphorylation of sugars, of sugar transport and of catabolic enzyme activity were performed, to compare a wild-type strain with an EIIB(Man) mutant, LPE6, and a ccpA mutant, LPE4. Fructose uptake in wild-type bacteria demonstrated the presence of two fructose-specific PTSs: a high-affinity system, EII(Fru) (K:(m)=52 microM) which is inducible by fructose, and a low-affinity system (K:(m)=300 microM). The latter system was lacking in LPE6 and therefore corresponds to EII(Man). LPE6 was unable to phosphorylate glucose, mannose, N:-acetylglucosamine and 2-deoxyglucose in a PEP-dependent reaction, indicating that these sugars are substrates of EII(Man). Transport and phosphorylation of these compounds was the same in LPE4 and in wild-type bacteria, although growth of LPE4 on these sugars was impaired. In wild-type bacteria and in LPE4 the activity of EII(Fru) was lowered by the presence of EII(Man) substrates in the growth medium, but this decrease was not observed in LPE6. These results indicate that EII(Man) but not CcpA regulates the synthesis of EII(Fru). Mutations in EII(Man) or CcpA resulted in a relief of catabolite repression exerted by EII(Man) substrates on the activity of beta-galactosidase and beta-glucosidase, indicating that EII(Man) and CcpA are important components in catabolite repression in L. pentosus. Fructose-mediated repression of these two enzymes appeared to be correlated with the activity of EII(Fru).

Identification and cloning of gusA, encoding a new beta-glucuronidase from Lactobacillus gasseri ADH

The gusA gene, encoding a new beta-glucuronidase enzyme, has been cloned from Lactobacillus gasseri ADH. This is the first report of a beta-glucuronidase gene cloned from a bacterial source other than Escherichia coli. A plasmid library of L. gasseri chromosomal DNA was screened for complementation of an E. coli gus mutant. Two overlapping clones that restored beta-glucuronidase activity in the mutant strain were sequenced and revealed three complete and two partial open reading frames. The largest open reading frame, spanning 1,797 bp, encodes a 597-amino-acid protein that shows 39% identity to beta-glucuronidase (GusA) of E. coli K-12 (EC 3.2.1.31). The other two complete open reading frames, which are arranged to be separately transcribed, encode a putative bile salt hydrolase and a putative protein of unknown function with similarities to MerR-type regulatory proteins. Overexpression of GusA was achieved in a beta-glucuronidase-negative L. gasseri strain by expressing the gusA gene, subcloned onto a low-copy-number shuttle vector, from the strong Lactobacillus P6 promoter. GusA was also expressed in E. coli from a pET expression system. Preliminary characterization of the GusA protein from crude cell extracts revealed that the enzyme was active across an acidic pH range and a broad temperature range. An analysis of other lactobacilli identified beta-glucuronidase activity and gusA homologs in other L. gasseri isolates but not in other Lactobacillus species tested.

Control of lactose transport, beta-galactosidase activity, and glycolysis by CcpA in Streptococcus thermophilus: evidence for carbon catabolite repression by a non-phosphoenolpyruvate-dependent phosphotransferase system sugar

Streptococcus thermophilus, unlike many other gram-positive bacteria, prefers lactose over glucose as the primary carbon and energy source. Moreover, lactose is not taken up by a phosphoenolpyruvate-dependent phosphotransferase system (PTS) but by the dedicated transporter LacS. In this paper we show that CcpA plays a crucial role in the fine-tuning of lactose transport, beta-galactosidase (LacZ) activity, and glycolysis to yield optimal glycolytic flux and growth rate. A catabolite-responsive element (cre) was identified in the promoter of the lacSZ operon, indicating a possible role for regulation by CcpA. Transcriptional analysis showed a sevenfold relief of repression in the absence of a functional CcpA when cells were grown on lactose. This CcpA-mediated repression of lacSZ transcription did not occur in wild-type cells during growth on galactose, taken up by the same LacS transport system. Lactose transport during fermentation was increased significantly in strains carrying a disrupted ccpA gene. Moreover, a ccpA disruption strain was found to release substantial amounts of glucose into the medium when grown on lactose. Transcriptional analysis of the ldh gene showed that expression was induced twofold during growth on lactose compared to glucose or galactose, in a CcpA-dependent manner. A reduced rate of glycolysis concomitant with an increased lactose transport rate could explain the observed expulsion of glucose in a ccpA disruption mutant. We propose that CcpA in S. thermophilus acts as a catabolic regulator during growth on the preferred non-PTS sugar lactose. In contrast to other bacteria, S. thermophilus possesses an overcapacity for lactose uptake that is repressed by CcpA to match the rate-limiting glycolytic flux.

Dissolution of xylose metabolism in Lactococcus lactis

Xylose metabolism, a variable phenotype in strains of Lactococcus lactis, was studied and evidence was obtained for the accumulation of mutations that inactivate the xyl operon. The xylose metabolism operon (xylRAB) was sequenced from three strains of lactococci. Fragments of 4.2, 4.2, and 5.4 kb that included the xyl locus were sequenced from L. lactis subsp. lactis B-4449 (formerly Lactobacillus xylosus), L. lactis subsp. lactis IO-1, and L. lactis subsp. lactis 210, respectively. The two environmental isolates, L. lactis B-4449 and L. lactis IO-1, produce active xylose isomerases and xylulokinases and can metabolize xylose. L. lactis 210, a dairy starter culture strain, has neither xylose isomerase nor xylulokinase activity and is Xyl(-). Xylose isomerase and xylulokinase activities are induced by xylose and repressed by glucose in the two Xyl(+) strains. Sequence comparisons revealed a number of point mutations in the xylA, xylB, and xylR genes in L. lactis 210, IO-1, and B-4449. None of these mutations, with the exception of a premature stop codon in xylB, are obviously lethal, since they lie outside of regions recognized as critical for activity. Nevertheless, either cumulatively or because of indirect affects on the structures of catalytic sites, these mutations render some strains of L. lactis unable to metabolize xylose.

Enzyme I and HPr from Lactobacillus casei: their role in sugar transport, carbon catabolite repression and inducer exclusion

We have cloned and sequenced the Lactobacillus casei ptsH and ptsI genes, which encode enzyme I and HPr, respectively, the general components of the phosphoenolpyruvate-carbohydrate phosphotransferase system (PTS). Northern blot analysis revealed that these two genes are organized in a single-transcriptional unit whose expression is partially induced. The PTS plays an important role in sugar transport in L. casei, as was confirmed by constructing enzyme I-deficient L. casei mutants, which were unable to ferment a large number of carbohydrates (fructose, mannose, mannitol, sorbose, sorbitol, amygdaline, arbutine, salicine, cellobiose, lactose, tagatose, trehalose and turanose). Phosphorylation of HPr at Ser-46 is assumed to be important for the regulation of sugar metabolism in Gram-positive bacteria. L. casei ptsH mutants were constructed in which phosphorylation of HPr at Ser-46 was either prevented or diminished (replacement of Ser-46 of HPr with Ala or Thr respectively). In a third mutant, Ile-47 of HPr was replaced with a threonine, which was assumed to reduce the affinity of P-Ser-HPr for its target protein CcpA. The ptsH mutants exhibited a less pronounced lag phase during diauxic growth in a mixture of glucose and lactose, two PTS sugars, and diauxie was abolished when cells were cultured in a mixture of glucose and the non-PTS sugars ribose or maltose. The ptsH mutants synthesizing Ser-46-Ala or Ile-47-Thr mutant HPr were partly or completely relieved from carbon catabolite repression (CCR), suggesting that the P-Ser-HPr/CcpA-mediated mechanism of CCR is common to most low G+C Gram-positive bacteria. In addition, in the three constructed ptsH mutants, glucose had lost its inhibitory effect on maltose transport, providing for the first time in vivo evidence that P-Ser-HPr participates also in inducer exclusion.

Carbon catabolite repression in Lactobacillus pentosus: analysis of the ccpA region

The catabolite control protein CcpA is a central regulator in low-G+C-content gram-positive bacteria. It confers carbon catabolite repression to numerous genes required for carbon utilization. It also operates as a transcriptional activator of genes involved in diverse phenomena, such as glycolysis and ammonium fixation. We have cloned the ccpA region of Lactobacillus pentosus. ccpA encodes a protein of 336 amino acids exhibiting similarity to CcpA proteins of other bacteria and to proteins of the LacI/GalR family of transcriptional regulators. Upstream of ccpA was found an open reading frame with similarity to the pepQ gene, encoding a prolidase. Primer extension experiments revealed two start sites of transcription for ccpA. In wild-type cells grown on glucose, mRNA synthesis occurred only from the promoter proximal to ccpA. In a ccpA mutant strain, both promoters were used, with increased transcription from the distant promoter, which overlaps a presumptive CcpA binding site called cre (for catabolite responsive element). This suggests that expression of ccpA is autoregulated. Determination of the expression levels of CcpA in cells grown on repressing and nonrepressing carbon sources revealed that the amounts of CcpA produced did not change significantly, leading to the conclusion that the arrangement of two promoters may ensure constant expression of ccpA under various environmental conditions. A comparison of the genetic structures of ccpA regions revealed that lactic acid bacteria possess the gene order pepQ-ccpA-variable while the genetic structure in bacilli and Staphylococcus xylosus is aroA-ccpA-variable-acuC.

Mutations in catabolite control protein CcpA showing glucose-independent regulation in Bacillus megaterium

We identified five single amino acid exchanges in CcpA that lead to permanent repression of the xylose utilization genes in the absence of glucose. Other proteins from the CcpA regulon also show glucose-independent regulation in the mutants. The mutant CcpA proteins bind to the DNA target catabolite responsive elements without the corepressor HPr-Ser-P.

Evidence of a glucose proton motive force-dependent permease and a fructose phosphoenolpyruvate:phosphotransferase transport system in Lactobacillus reuteri CRL 1098

Sugar uptake and phosphoenolpyruvate phosphorylation assays have shown that the heterofermentative strain Lactobacillus reuteri CRL 1098, of likely probiotic value, can transport D-fructose through an inducible fructose-specific phosphotransferase system (K(m) 95 microM) and D-glucose mainly through a proton motive force-driven permease. These data open new perspectives for metabolic and regulatory studies in this bacterium.

Transport of D-xylose in Lactobacillus pentosus, Lactobacillus casei, and Lactobacillus plantarum: evidence for a mechanism of facilitated diffusion via the phosphoenolpyruvate:mannose phosphotransferase system

We have identified and characterized the D-xylose transport system of Lactobacillus pentosus. Uptake of D-xylose was not driven by the proton motive force generated by malolactic fermentation and required D-xylose metabolism. The kinetics of D-xylose transport were indicative of a low-affinity facilitated-diffusion system with an apparent K(m) of 8.5 mM and a V(max) of 23 nmol min(-1) mg of dry weight(-1). In two mutants of L. pentosus defective in the phosphoenolpyruvate:mannose phosphotransferase system, growth on D-xylose was absent due to the lack of D-xylose transport. However, transport of the pentose was not totally abolished in a third mutant, which could be complemented after expression of the L. curvatus manB gene encoding the cytoplasmic EIIB(Man) component of the EII(Man) complex. The EII(Man) complex is also involved in D-xylose transport in L. casei ATCC 393 and L. plantarum 80. These two species could transport and metabolize D-xylose after transformation with plasmids which expressed the D-xylose-catabolizing genes of L. pentosus, xylAB. L. casei and L. plantarum mutants resistant to 2-deoxy-D-glucose were defective in EII(Man) activity and were unable to transport D-xylose when transformed with plasmids containing the xylAB genes. Finally, transport of D-xylose was found to be the rate-limiting step in the growth of L. pentosus and of L. plantarum and L. casei ATCC 393 containing plasmids coding for the D-xylose-catabolic enzymes, since the doubling time of these bacteria on D-xylose was proportional to the level of EII(Man) activity.

Identification of a gene in Staphylococcus xylosus encoding a novel glucose uptake protein

By transposon Tn917 mutagenesis, two mutants of Staphylococcus xylosus were isolated that showed higher levels of beta-galactosidase activity in the presence of glucose than the wild type. Both transposons integrated in a gene, designated glcU, encoding a protein involved in glucose uptake in S. xylosus, which is followed by a glucose dehydrogenase gene (gdh). Glucose-mediated repression of beta-galactosidase, alpha-glucosidase, and beta-glucuronidase activities was partially relieved in the mutant strains, while repression by sucrose or fructose remained as strong as in the wild type. In addition to the pleiotropic regulatory effect, integration of the transposons into glcU reduced glucose dehydrogenase activity, suggesting cotranscription of glcU and gdh. Insertional inactivation of the gdh gene and deletion of the glcU gene without affecting gdh expression showed that loss of GlcU function is exclusively responsible for the regulatory defect. Reduced glucose repression is most likely the consequence of impaired glucose uptake in the glcU mutant strains. With cloned glcU, an Escherichia coli mutant deficient in glucose transport could grow with glucose as sole carbon source, provided a functional glucose kinase was present. Therefore, glucose is internalized by glcU in nonphosphorylated form. A gene from Bacillus subtilis, ycxE, that is homologous to glcU, could substitute for glcU in the E. coli glucose growth experiments and restored glucose repression in the S. xylosus glcU mutants. Three more proteins with high levels of similarity to GlcU and YcxE are currently in the databases. It appears that these proteins constitute a novel family whose members are involved in bacterial transport processes. GlcU and YcxE are the first examples whose specificity, glucose, has been determined.

Mutations in catabolite control protein CcpA separating growth effects from catabolite repression

Carbon catabolite repression in Bacillus megaterium is mediated by the transcriptional regulator CcpA. A chromosomal deletion of ccpA eliminates catabolite repression and reduces the growth rate on glucose. We describe four single-amino-acid mutations in CcpA which separate the growth effect from catabolite repression, suggesting distinct regulatory pathways for these phenotypes.

Gene expression systems for lactic acid bacteria

Considerable advances have been made in the genetics and molecular biology of lactic acid bacteria, including Lactococcus, Lactobacillus, Leuconostoc, Pediococcus and Streptococcus spp. These have resulted in the construction of constitutive gene expression cassettes, inducible gene expression systems, and specific protein targeting systems for these bacteria. These developments are important in the food industry where lactic acid bacteria can be exploited as food-grade cell factories.

Carbon catabolite repression in bacteria

Carbon catabolite repression (CCR) is a regulatory mechanism by which the expression of genes required for the utilization of secondary sources of carbon is prevented by the presence of a preferred substrate. This enables bacteria to increase their fitness by optimizing growth rates in natural environments providing complex mixtures of nutrients. In most bacteria, the enzymes involved in sugar transport and phosphorylation play an essential role in signal generation leading through different transduction mechanisms to catabolite repression. The actual mechanisms of regulation are substantially different in various bacteria. The mechanism of lactose-glucose diauxie in Escherichia coli has been reinvestigated and was found to be caused mainly by inducer exclusion. In addition, the gene encoding HPr kinase, a key component of CCR in many bacteria, was discovered recently.

Molecular cloning and functional expression in lactobacillus plantarum 80 of xylT, encoding the D-xylose-H+ symporter of Lactobacillus brevis

A 3-kb region, located downstream of the Lactobacillus brevis xylA gene (encoding D-xylose isomerase), was cloned in Escherichia coli TG1. The sequence revealed two open reading frames which could code for the D-xylulose kinase gene (xylB) and another gene (xylT) encoding a protein of 457 amino acids with significant similarity to the D-xylose-H+ symporters of E. coli, XylE (57%), and Bacillus megaterium, XylT (58%), to the D-xylose-Na+ symporter of Tetragenococcus halophila, XylE (57%), and to the L-arabinose-H+ symporter of E. coli, AraE (60%). The L. brevis xylABT genes showed an arrangement similar to that of the B. megaterium xylABT operon and the T. halophila xylABE operon. Southern hybridization performed with the Lactobacillus pentosus xylR gene (encoding the D-xylose repressor protein) as a probe revealed the existence of a xylR homologue in L. brevis which is not located with the xyABT locus. The existence of a functional XylR was further suggested by the presence of xylO sequences upstream of xylA and xylT and by the requirement of D-xylose for the induction of D-xylose isomerase, D-xylulose kinase, and D-xylose transport activities in L. brevis. When L. brevis was cultivated in a mixture of D-glucose and D-xylose, the D-xylose isomerase and D-xylulose kinase activities were reduced fourfold and the D-xylose transport activity was reduced by sixfold, suggesting catabolite repression by D-glucose of D-xylose assimilation. The xylT gene was functionally expressed in Lactobacillus plantarum 80, a strain which lacks proton motive force-linked D-xylose transport activity. The role of the XylT protein was confirmed by the accumulation of D-xylose in L. plantarum 80 cells, and this accumulation was dependent on the proton motive force generated by either malolactic fermentation or by the metabolism of D-glucose. The apparent affinity constant of XylT for D-xylose was approximately 215 microM, and the maximal initial velocity of transport was 35 nmol/min per mg (dry weight). Furthermore, of a number of sugars tested, only 6-deoxy-D-glucose inhibited the transport of D-xylose by XylT competitively, with a Ki of 220 microM.

A homolog of CcpA mediates catabolite control in Listeria monocytogenes but not carbon source regulation of virulence genes

Readily utilizable sugars down-regulate virulence gene expression in Listeria monocytogenes, which has led to the proposal that this regulation may be an aspect of global catabolite regulation (CR). We recently demonstrated that the metabolic enzyme alpha-glucosidase is under CR in L. monocytogenes. Here, we report the cloning and characterization from L. monocytogenes of an apparent ortholog of ccpA, which encodes an important mediator of CR in several low-G+C-content gram-positive bacteria. L. monocytogenes ccpA (ccpALm) is predicted to encode a 335-amino-acid protein with nearly 65% identity to the gene product of Bacillus subtilis ccpA (ccpABs). Southern blot analysis with a probe derived from ccpALm revealed a single strongly hybridizing band and also a second band of much lower intensity, suggesting that there may be other closely related sequences in the L. monocytogenes chromosome, as is the case in B. subtilis. Disruption of ccpALm resulted in the inability of the mutant to grow on glucose-containing minimal medium or increase its growth rate in the presence of preferred sugars, and it completely eliminated CR of alpha-glucosidase activity in liquid medium. However, alpha-glucosidase activity was only partially relieved from CR on solid medium. These results suggest that ccpA is an important element of carbon source regulation in L. monocytogenes. Nevertheless, utilizable sugars still down-regulate the expression of hly, which encodes the virulence factor hemolysin, in a ccpALm mutant, indicating that CcpA is not involved in carbon source regulation of virulence genes.

Transcriptional activation of the glycolytic las operon and catabolite repression of the gal operon in Lactococcus lactis are mediated by the catabolite control protein CcpA

The Lactococcus lactis ccpA gene, encoding the global regulatory protein CcpA, was identified and characterized. Northern blot and primer extension analyses showed that the L. lactis ccpA gene is constitutively transcribed from a promoter that does not contain a cre sequence. Inactivation of the ccpA gene resulted in a twofold reduction in the growth rate compared with the wild type on glucose, sucrose and fructose, while growth on galactose was almost completely abolished. The observed growth defects could be complemented by the expression of either the L. lactis or the Bacillus subtilis ccpA gene. The disruption of the ccpA gene reduced the catabolite repression of the gal operon, which contains a cre site at the transcription start site and encodes enzymes involved in galactose catabolism. In contrast, CcpA activates the transcription of the cre-containing promoter of the las operon, encoding the glycolytic enzymes phosphofructokinase, pyruvate kinase and L-lactate dehydrogenase, because its transcription level was fourfold reduced in the ccpA mutant strain compared with the wild-type strain. The lower activities of pyruvate kinase and L-lactate dehydrogenase in the ccpA mutant strain resulted in the production of metabolites characteristic of a mixed-acid fermentation, whereas the fermentation pattern of the wild-type strain was essentially homolactic.

NADP, corepressor for the Bacillus catabolite control protein CcpA

Expression of the alpha-amylase gene (amyE) of Bacillus subtilis is subject to CcpA (catabolite control protein A)-mediated catabolite repression, a global regulatory mechanism in Bacillus and other Gram-positive bacteria. To determine effectors of CcpA, we tested the ability of glycolytic metabolites, nucleotides, and cofactors to affect CcpA binding to the amyE operator, amyO. Those that stimulated the DNA-binding affinity of CcpA were tested for their effect on transcription. HPr-P (Ser-46), proposed as an effector of CcpA, also was tested. In DNase I footprint assays, the affinity of CcpA for amyO was stimulated 2-fold by fructose-1,6-diphosphate (FDP), 1.5-fold by oxidized or reduced forms of NADP, and 10-fold by HPr-P (Ser-46). However, the triple combinations, CcpA/NADP/HPr-P (Ser-46) and CcpA/FDP/HPr-P (Ser-46) synergistically stimulated DNA-binding affinity by 120- and 300-fold, respectively. NADP added to CcpA specifically stimulated transcription inhibition of the amyE promoter by 120-fold. CcpA combined with HPr (Ser-46) inhibited transcription from the amyE promoter, but it also inhibited several control promoters. FDP did not stimulate transcription inhibition by CcpA nor did the triple combinations. The finding that NADP had little effect on CcpA DNA binding but increased the ability of CcpA to inhibit transcription suggests that catabolite repression is not simply caused by CcpA binding amyO but rather a result of interactions with the transcription machinery enhanced by NADP.

Cloning, sequence analysis, and characterization of the genes involved in isoprimeverose metabolism in Lactobacillus pentosus

Two genes, xylP and xylQ, from the xylose regulon of Lactobacillus pentosus were cloned and sequenced. Together with the repressor gene of the regulon, xylR, the xylPQ genes form an operon which is inducible by xylose and which is transcribed from a promoter located 145 bp upstream of xylP. A putative xylR binding site (xylO) and a cre-like element, mediating CcpA-dependent catabolite repression, were found in the promoter region. L. pentosus mutants in which both xylP and xylQ (LPE1) or only xylQ (LPE2) was inactivated retained the ability to ferment xylose but were impaired in their ability to ferment isoprimeverose (alpha-D-xylopyranosyl-(1,6)-D-glucopyranose). Disruption of xylQ resulted specifically in the loss of a membrane-associated alpha-xylosidase activity when LPE1 or LPE2 cells were grown on xylose. In the membrane fraction of wild-type bacteria, alpha-xylosidase could catalyze the hydrolysis of isoprimeverose and p-nitrophenyl-alpha-D-xylopyranoside with apparent Km and Vmax values of 0.2 mM and 446 nmol/min/mg of protein, and 1.3 mM and 54 nmol/min/mg of protein, respectively. The enzyme could also hydrolyze the alpha-xylosidic linkage in xyloglucan oligosaccharides, but neither methyl-alpha-D-xylopyranoside nor alpha-glucosides were substrates. Glucose repressed the synthesis of alpha-xylosidase fivefold, and 80% of this repression was released in an L. pentosus delta ccpA mutant. The alpha-xylosidase gene was also expressed in the absence of xylose when xylR was disrupted.