Promoter analysis and transcriptional regulation of Lactobacillus pentosus genes involved in xylose catabolism

Vector Development Timeline for Mucosal Vaccination and Treatment of Disease Using Lactococcus lactis and Design Approaches of Next Generation Food Grade Plasmids

Lactococcus lactis has been used historically in fermentation and food preservation processes as it is considered safe for human consumption (GRAS—Generally Recognized As Safe). Nowadays, in addition to its wide use in the food industry, L. lactis has been used as a bioreactor for the production of molecules of medical interest, as well as vectors for DNA delivery. These applications are possible due to the development of promising genetic tools over the past few decades, such as gene expression, protein targeting systems, and vaccine plasmids. Thus, this review presents some of these genetic tools and their evolution, which allow us to envision new biotechnological and therapeutic uses of L. lactis. Constitutive and inductive expression systems will be discussed, many of which have been used successfully for heterologous production of different proteins, tested on animal models. In addition, advances in the construction of new plasmids to be used as potential DNA vaccines, delivered by this microorganism, will also be viewed. Finally, we will focus on the scene of gene expression systems known as “food-grade systems” based on inducing compounds and safe selection markers, which eliminate the need for the use of compounds harmful to humans or animal health and potential future prospects for their applications.

Genome-Based Genetic Tool Development for Bacillus methanolicus: Theta- and Rolling Circle-Replicating Plasmids for Inducible Gene Expression and Application to Methanol-Based Cadaverine Production

Bacillus methanolicus is a thermophilic methylotroph able to overproduce amino acids from methanol, a substrate not used for human or animal nutrition. Based on our previous RNA-seq analysis a mannitol inducible promoter and a putative mannitol activator gene mtlR were identified. The mannitol inducible promoter was applied for controlled gene expression using fluorescent reporter proteins and a flow cytometry analysis, and improved by changing the -35 promoter region and by co-expression of the mtlR regulator gene. For independent complementary gene expression control, the heterologous xylose-inducible system from B. megaterium was employed and a two-plasmid gene expression system was developed. Four different replicons for expression vectors were compared with respect to their copy number and stability. As an application example, methanol-based production of cadaverine was shown to be improved from 6.5 to 10.2 g/L when a heterologous lysine decarboxylase gene cadA was expressed from a theta-replicating rather than a rolling-circle replicating vector. The current work on inducible promoter systems and compatible theta- or rolling circle-replicating vectors is an important extension of the poorly developed B. methanolicus genetic toolbox, valuable for genetic engineering and further exploration of this bacterium.

Molecular mechanisms of xylose utilization by Pseudomonas fluorescens: overlapping genetic responses to xylose, xylulose, ribose and mannitol

Bacterial degradation of xylose is sequentially mediated by two enzymes - an isomerase (XutA) and a xylulokinase (XutB) - with xylulose as an intermediate. Pseudomonas fluorescens SBW25, though capable of growth on xylose as a sole carbon source, encodes only one degradative enzyme XutA at the xylose utilization (xut) locus. Here, using site-directed mutagenesis and transcriptional assays, we have identified two functional xylulokinase-encoding genes (xutB1 and xutB2) and further show that expression of xutB1 is specifically induced by xylose. Surprisingly, xylose-induced xutB1 expression is mediated by the mannitol-responsive regulator MtlR, using xylulose rather than xylose as the direct inducer. In contrast, expression of the xutA operon is regulated by XutR - a transcriptional activator of the AraC family - in a xylose-, xylulose- and ribose-dependent manner. Detailed genetic and biochemical analyses of XutR, including DNase I footprinting assays, suggest an unconventional model of XutR regulation that does not involve DNA-looping, a mechanism typically found for AraC-type regulators from enteric bacteria. XutR functions as a dimer and recognizes two inverted repeat sequences, but binding to one half site is weak thus requiring an inducer molecule such as xylose for activation.

Characterization of temperature inducible promoters from a novel rolling circle replicating plasmid of Enterococcus faecium DJ1

Limited studies have been performed on the characterization of small size plasmids of Enterococcus faecium with the intention of evaluating the strength of their promoters in Escherichia coli. The complete nucleotide sequence (3.825 Kb) and structural organization of E. faecium DJ1 cryptic plasmid pNJAKD is presented. Seven promoter sequences from the pNJAKD plasmid of E. faecium have been identified. The regions coding for the putative promoters were either amplified using PCR based techniques or chemically synthesized as oligonucleotides of different sizes. These were subsequently cloned in the pEGFP vector at the Pvu II site. The efficiency of putative promoter fragments were measured using the intensity of eGFP fluorescence in E. coli JM101, DH5α and BL21(DE3), among which AKD3 exhibited moderate to strongest promoter activity at temperatures of 30, 37, and 42°C.

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.

Functional expression of the thiolase gene thl from Clostridium beijerinckii P260 in Lactococcus lactis and Lactobacillus buchneri

The first step of the butanol pathway involves an acetyl-CoA acetyltransferase (ACoAAT), which controls the key branching point from acetyl-CoA to butanol. ACoAAT, also known as thiolase (EC 2.3.1.9), is encoded by the thl gene and catalyzes ligation of two acetyl-CoA into acetoacetyl-CoA. Bioinformatics analyses suggest there are no thl in the genomes of lactic acid bacteria (LAB), in this study we aimed to introduce the thl gene into selected LAB strains and analyze the fermentation products. The thl gene from Clostridium beijerinckii P260 was amplified by genomic PCR using gene-specific primers designed from the published genome sequences of C. beijerinckii NCIMB 8025. The 1.2 kb thl gene was cloned into the pETBlue vector and overexpressed in Escherichia coli Tuner (DE3) pLacI cells. Functional enzyme activity was detected spectrophotometrically by measuring the decrease in absorbance at 303 nm, which reflects the change in acetoacetyl-CoA concentrations. The thl gene was subsequently introduced into Lactococcus lactis and Lactobacillus buchneri strains, and GC analysis indicated about 28 mg/L and 66 mg/L of butanol was produced in the recombinant strains, respectively. This study reports the first step toward developing a butanolgenic LAB through the introduction of the butanol pathway into butanol-tolerant strains of LAB.

Isolation and characterization of promoters from Lactococcus lactis ssp. cremoris LM0230

DNA fragments showing promoter activity were obtained from the chromosomal DNA of Lactococcus lactis ssp. cremoris LM0230 by using a promoter-screening vector pBV5030, which contains a promoterless chloramphenicol acetyltransferase gene. Ten fragments were identified based on their ability to confer resistance against chloramphenicol in Escherichia coli. DNA sequencing revealed that all the fragments had a consensus region recognized by the sigma factor and only the nucleotide sequence of fragment 15C had the identical consensus region with the promoter P2 from L. lactis ssp. lactis MG1614. To compare their promoter strengths, an E. coli-lactococcal shuttle vector pWL1 containing a luciferase gene as the reporter gene was constructed based on lactococcal plasmid pMG36e. The putative promoter regions of 10 fragments exhibiting promoter activity were characterized in E. coli and L. lactis by measuring the luciferase activity, among which the putative promoter P6C exhibited the highest promoter activity both in E. coli JM109 and L. lactis ssp. cremoris MG1363. The luciferase system endowed significantly different expression levels enough to compare promoter strengths in E. coli and lactococcal host. The transcription-initiation sites of P6C and P13C were mapped by primer extension, which showed that they corresponded to a purine residue. The characterized promoters could be useful for the industrial production of heterologous proteins in L. lactis in case the proteins require a high safety level.

Functional expression of bacterial Zymobacter palmae pyruvate decarboxylase gene in Lactococcus lactis

A pyruvate decarboxylase (PDC) gene from bacterial Zymobacter palmae (Zymopdc) was cloned, characterized, and introduced into Lactococcus lactis via a shuttle vector pAK80 as part of a research strategy to develop an efficient ethanol-producing lactic acid bacteria (LAB). The expression levels of Zymopdc gene in the host, as measured by a colorimetric assay based on PDC catalyzed formation of (R)-phenylacetylcarbinol ((R)-PAC), appeared to be dependent on the strength of corresponding Gram-positive promoters. A constitutive, highly expressed promoter conferred the greatest PDC activity, and an acid-inducible promoter demonstrated acid-inducible expression. The metabolic production of ethanol and other products was examined in flask fermentations. More than eightfold increases in acetaldehyde concentrations were detected in two recombinant strains. However, no detectable differences for ethanol fermentation in these engineered strains were observed compared with that of the strain carrying lacZ reporter.

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.

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.

Genetically modified lactic acid bacteria: applications to food or health and risk assessment

Lactic acid bacteria have a long history of use in fermented food products. Progress in gene technology allows their modification by introducing new genes or by modifying their metabolic functions. These modifications may lead to improvements in food technology (bacteria better fitted to technological processes, leading to improved organoleptic properties em leader ), or to new applications including bacteria producing therapeutic molecules that could be delivered by mouth. Examples in these two fields will be discussed, at the same time evaluating their potential benefit to society and the possible risks associated with their use. Risk assessment and expected benefits will determine the future use of modified bacteria in the domains of food technology and health.

Regulation and adaptive evolution of lactose operon expression in Lactobacillus delbrueckii

Lactobacillus delbrueckii subsp. bulgaricus and L. delbrueckii subsp. lactis are both used in the dairy industry as homofermentative lactic acid bacteria in the production of fermented milk products. After selective pressure for the fast fermentation of milk in the manufacture of yogurts, L. delbrueckii subsp. bulgaricus loses its ability to regulate lac operon expression. A series of mutations led to the constitutive expression of the lac genes. A complex of insertion sequence (IS) elements (ISL4 inside ISL5), inserted at the border of the lac promoter, induced the loss of the palindromic structure of one of the operators likely involved in the binding of regulatory factors. A lac repressor gene was discovered downstream of the beta-galactosidase gene of L. delbrueckii subsp. lactis and was shown to be inactivated by several mutations in L. delbrueckii subsp. bulgaricus. Regulatory mechanisms of the lac gene expression of L. delbrueckii subsp. bulgaricus and L. delbrueckii subsp. lactis were compared by heterologous expression in Lactococcus lactis of the two lac promoters in front of a reporter gene (beta-glucuronidase) in the presence or absence of the lac repressor gene. Insertion of the complex of IS elements in the lac promoter of L. delbrueckii subsp. bulgaricus increased the promoter's activity but did not prevent repressor binding; rather, it increased the affinity of the repressor for the promoter. Inactivation of the lac repressor by mutations was then necessary to induce the constitutive expression of the lac genes in L. delbrueckii subsp. bulgaricus.

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.

Genetic evidence for a defective xylan degradation pathway in Lactococcus lactis

Genetic and biochemical evidence for a defective xylan degradation pathway was found linked to the xylose operon in three lactococcal strains, Lactococcus lactis 210, L. lactis IO-1, and L. lactis NRRL B-4449. Immediately downstream of the xylulose kinase gene (xylB) (K. A. Erlandson, J.-H. Park, W. El Khal, H.-H. Kao, P. Basaran, S. Brydges, and C. A. Batt, Appl. Environ. Microbiol. 66:3974-3980, 1999) are two open reading frames encoding a mutarotase (xylM) and a xyloside transporter (xynT) and a partial open reading frame encoding a beta-xylosidase (xynB). These are functions previously unreported for lactococci or lactobacilli. The mutarotase activity of the putative xylM gene product was confirmed by overexpression of the L. lactis enzyme in Escherichia coli and purification of recombinant XylM. We hypothesize that the mutarotase links xylan degradation to xylose metabolism due to the anomeric preference of xylose isomerase. In addition, Northern hybridization experiments suggested that the xylM and xynTB genes are cotranscribed with the xylRAB genes, responsible for xylose metabolism. Although none of the three strains appeared to metabolize xylan or xylobiose, they exhibited xylosidase activity, and L. lactis IO-1 and L. lactis NRRL B-4449 had functional mutarotases.

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.

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.

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.

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.

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.

Functional expression in Lactobacillus plantarum of xylP encoding the isoprimeverose transporter of Lactobacillus pentosus

The xylP gene of Lactobacillus pentosus, the first gene of the xylPQR operon, was recently found to be involved in isoprimeverose metabolism. By expression of xylP on a multicopy plasmid in Lactobacillus plantarum 80, a strain which lacks active isoprimeverose and D-xylose transport activities, it was shown that xylP encodes a transporter. Functional expression of the XylP transporter was shown by uptake of isoprimeverose in L. plantarum 80 cells, and this transport was driven by the proton motive force generated by malolactic fermentation. XylP was unable to catalyze transport of D-xylose.

The D-xylose-binding protein, XylF, from Thermoanaerobacter ethanolicus 39E: cloning, molecular analysis, and expression of the structural gene

Immediately downstream from the Thermoanaerobacter ethanolicus xylAB operon, comprising genes that encode D-xylose isomerase and D-xylulose kinase, lies a 1,101-bp open reading frame that exhibits 61% amino acid sequence identity to the Escherichia coli D-xylose binding periplasmic receptor, XylF, a component of the high-affinity binding-protein-dependent D-xylose transport. The 25-residue N-terminal fragment of the deduced T. ethanolicus XylF has typical features of bacterial leader peptides. The C-terminal portion of this leader sequence matches the cleavage consensus for lipoproteins and is followed by a 22-residue putative linker sequence rich in serine, threonine, and asparagine. The putative mature 341-amino-acid-residue XylF (calculated molecular mass of 37,069 Da) appears to be a lipoprotein attached to the cell membrane via a lipid anchor covalently linked to the N-terminal cysteine, as demonstrated by metabolic labelling of the recombinant XylF with [14C]palmitate. The induced E. coli avidly bound D-[14C]xylose, yielding additional evidence that T. ethanolicus XylF is the D-xylose-binding protein. On the basis of sequence comparison of XylFs to other monosaccharide-binding proteins, we propose that the sequence signature of binding proteins specific for hexoses and pentoses be refined as (KDQ)(LIVFAG)3IX3(DN)(SGP)X3(GS)X(LIVA) 2X2A. Transcription of the monocistronic 1.3-kb xylF mRNA is inducible by xylose and unaffected by glucose. Primer extension analysis indicated that xylF transcription initiates from two +1 sites, both situated within the xylAB operon. Unlike in similar transport systems in other bacteria, the genes specifying the membrane components (e.g., ATP-binding protein and permease) of the high-affinity D-xylose uptake system are not located in the vicinity of xylF in T. ethanolicus. This is the first report of a gene encoding a xylose-binding protein in a gram-positive or thermophilic bacterium.

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.

Cloning and characterization of transcription of the xylAB operon in Thermoanaerobacter ethanolicus

The genes encoding xylose isomerase (xylA) and xylulose kinase (xylB) from the thermophilic anaerobe Thermoanaerobacter ethanolicus were found to constitute an operon with the transcription initiation site 169 nucleotides upstream from the previously assigned (K. Dekker, H. Yamagata, K. Sakaguchi, and S. Udaka, Agric. Biol. Chem. 55:221-227, 1991) promoter region. The bicistronic xylAB mRNA was processed by cleavage within the 5'-terminal portion of the XylB-coding sequence. Transcription of xylAB was induced in the presence of xylose, and, unlike in all other xylose-utilizing bacteria studied, was not repressed by glucose. The existence of putative xyl operator sequences suggested that xylose utilization is controlled by a repressor-operator mechanism. The T. ethanolicus xylB gene coded for a 500-amino-acid-residue protein with a deduced amino acid sequence highly homologous to those of other XylBs. This is the first report of an xylB nucleotide sequence and an xyLAB operon from a thermophilic anaerobic bacterium.

Regulation of expression of the Lactobacillus pentosus xylAB operon

The xylose cluster of Lactobacillus pentosus consists of five genes, two of which, xylAB, form an operon and code for the enzymes involved in the catabolism of xylose, while a third encodes a regulatory protein, XylR. By introduction of a multicopy plasmid carrying the xyl operator and by disruption of the chromosomal xylR gene, it was shown that L. pentosus xylR encodes a repressor. Constitutive expression of xylAB in the xylR mutant is repressed by glucose, indicating that glucose repression does not require XylR. The xylR mutant displayed a prolonged lag phase compared to wild-type bacteria when bacteria were shifted from glucose to xylose medium. Differences in the growth rate in xylose medium at different stages of growth are not correlated with differences in levels of xylAB transcription in L. pentosus wild-type or xylR mutant bacteria but are positively correlated in Lactobacillus casei with a plasmid containing xylAB. Glucose repression was further investigated with a ccpA mutant. An 875-bp internal fragment of the ccpA gene of L. pentosus was isolated by PCR and used to construct a ccpA knockout mutant. Transcription analysis of L. pentosus xylA showed that CcpA is involved in glucose repression. CcpA was also shown to be involved in glucose repression of the alpha-amylase promoter of Lactobacillus amylovorus by demonstrating that glucose repression of the chloramphenicol acetyltransferase gene under control of the alpha-amylase promoter is strongly reduced in the L. pentosus ccpA mutant strain.

Controlled overproduction of proteins by lactic acid bacteria

Lactic acid bacteria are widely used in industrial food fermentations, contributing to flavour, texture and preservation of the fermented products. Here we describe recent advances in the development of controlled gene expression systems, which allow the regulated overproduction of any desirable protein by lactic acid bacteria. Some systems benefit from the fact that the expression vectors, marker genes and inducing factors can be used directly in food applications since they are all derived from food-grade lactic acid bacteria. These systems have also been employed for the development of autolytic bacteria, suitable for various industrial applications.

Inducible gene expression and environmentally regulated genes in lactic acid bacteria

Relatively recently, a number of genes and operons have been identified in lactic acid bacteria that are inducible and respond to environmental factors. Some of these genes/operons had been isolated and analysed because of their importance in the fermentation industry and, consequently, their transcription was studied and found to be regulatable. Examples are the lactose operon, the operon for nisin production, and genes in the proteolytic pathway of Lactococcus lactis, as well as xylose metabolism in Lactobacillus pentosus. Some other operons were specifically targetted with the aim to compare their mode of regulation with known regulatory mechanisms in other well-studied bacteria. These studies, dealing with the biosynthesis of histidine, tryptophan, and of the branched chain amino acids in L. lactis, have given new insights in gene regulation and in the occurrence of auxotrophy in these bacteria. Also, nucleotide sequence analyses of a number of lactococcal bacteriophages was recently initiated to, among other things, specifically learn more about regulation of the phage life cycle. Yet another approach in the analysis of regulated genes is the 'random' selection of genetic elements that respond to environmental stimuli and the first of such sequences from lactic acid bacteria have been identified and characterized. The potential of these regulatory elements in fundamental research and practical (industrial) applications will be discussed.

Metabolic engineering of sugar catabolism in lactic acid bacteria

Lactic acid bacteria are characterized by a relatively simple sugar fermentation pathway that, by definition, results in the formation of lactic acid. The extensive knowledge of traditional pathways and the accumulating genetic information on these and novel ones, allows for the rerouting of metabolic processes in lactic acid bacteria by physiological approaches, genetic methods, or a combination of these two. This review will discuss past and present examples and future possibilities of metabolic engineering of lactic acid bacteria for the production of important compounds, including lactic and other acids, flavor compounds, and exopolysaccharides.