Cloning and characterization of the genes encoding the malolactic enzyme and the malate permease of Leuconostoc oenos

l-Malate (-2) Protonation State is Required for Efficient Decarboxylation to l-Lactate by the Malolactic Enzyme of Oenococcus oeni

Malolactic fermentation (MLF) is responsible for the decarboxylation of l-malic into lactic acid in most red wines and some white wines. It reduces the acidity of wine, improves flavor complexity and microbiological stability. Despite its industrial interest, the MLF mechanism is not fully understood. The objective of this study was to provide new insights into the role of pH on the binding of malic acid to the malolactic enzyme (MLE) of Oenococcus oeni. To this end, sequence similarity networks and phylogenetic analysis were used to generate an MLE homology model, which was further refined by molecular dynamics simulations. The resulting model, together with quantum polarized ligand docking (QPLD), was used to describe the MLE binding pocket and pose of l-malic acid (MAL) and its l-malate (−1) and (−2) protonation states (MAL⁻ and MAL²⁻, respectively). MAL²⁻ has the lowest ∆G_binding, followed by MAL⁻ and MAL, with values of −23.8, −19.6, and −14.6 kJ/mol, respectively, consistent with those obtained by isothermal calorimetry thermodynamic (ITC) assays. Furthermore, molecular dynamics and MM/GBSA results suggest that only MAL²⁻ displays an extended open conformation at the binding pocket, satisfying the geometrical requirements for Mn²⁺ coordination, a critical component of MLE activity. These results are consistent with the intracellular pH conditions of O. oeni cells—ranging from pH 5.8 to 6.1—where the enzymatic decarboxylation of malate occurs.

Transcriptomic and Proteomic Analysis of Oenococcus oeni Adaptation to Wine Stress Conditions

Oenococcus oeni, the main lactic acid bacteria responsible for malolactic fermentation in wine, has to adapt to stressful conditions, such as low pH and high ethanol content. In this study, the changes in the transcriptome and the proteome of O. oeni PSU-1 during the adaptation period before MLF start have been studied. DNA microarrays were used for the transcriptomic analysis and two complementary proteomic techniques, 2-D DIGE and iTRAQ labeling were used to analyze the proteomic response. One of the most influenced functions in PSU-1 due to inoculation into wine-like medium (WLM) was translation, showing the over-expression of certain ribosomal genes and the corresponding proteins. Amino acid metabolism and transport was also altered and several peptidases were up regulated both at gene and protein level. Certain proteins involved in glutamine and glutamate metabolism showed an increased abundance revealing the key role of nitrogen uptake under stressful conditions. A strong transcriptional inhibition of carbohydrate metabolism related genes was observed. On the other hand, the transcriptional up-regulation of malate transport and citrate consumption was indicative of the use of L-malate and citrate associated to stress response and as an alternative energy source to sugar metabolism. Regarding the stress mechanisms, our results support the relevance of the thioredoxin and glutathione systems in the adaptation of O. oeni to wine related stress. Genes and proteins related to cell wall showed also significant changes indicating the relevance of the cell envelop as protective barrier to environmental stress. The differences found between transcriptomic and proteomic data suggested the relevance of post-transcriptional mechanisms and the complexity of the stress response in O. oeni adaptation. Further research should deepen into the metabolisms mostly altered due to wine conditions to elucidate the role of each mechanism in the O. oeni ability to develop MLF.

Oenococcus alcoholitolerans sp. nov., a lactic acid bacteria isolated from cachaça and ethanol fermentation processes

Four strains of lactic acid bacteria isolated from cachaça and alcohol fermentation vats in Brazil were characterised in order to determine their taxonomic position. Phylogenetic analysis revealed that they belong to the genus Oenococcus and should be distinguished from their closest neighbours. The 16S rRNA gene sequence similarity against the type strains of the other two species of the genus was below 94.76 % (Oenococcus kitaharae) and 94.62 % (Oenococcus oeni). The phylogeny based on pheS gene sequences also confirmed the position of the new taxon. DNA-DNA hybridizations based on in silico genome-to-genome comparison, Average Amino Acid Identity, Average Nucleotide Identity and Karlin genomic signature confirmed the novelty of the taxon. Distinctive phenotypic characteristics are the ability to metabolise sucrose but not trehalose. The name Oenococcus alcoholitolerans sp. nov. is proposed for this taxon, with the type strain UFRJ-M7.2.18(T) ( = CBAS474(T) = LMG27599(T)). In addition, we have determined a draft genome sequence of the type strain.

Implications of new research and technologies for malolactic fermentation in wine

The initial conversion of grape must to wine is an alcoholic fermentation (AF) largely carried out by one or more strains of yeast, typically Saccharomyces cerevisiae. After the AF, a secondary or malolactic fermentation (MLF) which is carried out by lactic acid bacteria (LAB) is often undertaken. The MLF involves the bioconversion of malic acid to lactic acid and carbon dioxide. The ability to metabolise L-malic acid is strain specific, and both individual Oenococcus oeni strains and other LAB strains vary in their ability to efficiently carry out MLF. Aside from impacts on acidity, LAB can also metabolise other precursors present in wine during fermentation and, therefore, alter the chemical composition of the wine resulting in an increased complexity of wine aroma and flavour. Recent research has focused on three main areas: enzymatic changes during MLF, safety of the final product and mechanisms of stress resistance. This review summarises the latest research and technological advances in the rapidly evolving study of MLF and investigates the directions that future research may take.

Malic enzyme and malolactic enzyme pathways are functionally linked but independently regulated in Lactobacillus casei BL23

Lactobacillus casei is the only lactic acid bacterium in which two pathways for l-malate degradation have been described: the malolactic enzyme (MLE) and the malic enzyme (ME) pathways. Whereas the ME pathway enables L. casei to grow on l-malate, MLE does not support growth. The mle gene cluster consists of three genes encoding MLE (mleS), the putative l-malate transporter MleT, and the putative regulator MleR. The mae gene cluster consists of four genes encoding ME (maeE), the putative transporter MaeP, and the two-component system MaeKR. Since both pathways compete for the same substrate, we sought to determine whether they are coordinately regulated and their role in l-malate utilization as a carbon source. Transcriptional analyses revealed that the mle and mae genes are independently regulated and showed that MleR acts as an activator and requires internalization of l-malate to induce the expression of mle genes. Notwithstanding, both l-malate transporters were required for maximal l-malate uptake, although only an mleT mutation caused a growth defect on l-malate, indicating its crucial role in l-malate metabolism. However, inactivation of MLE resulted in higher growth rates and higher final optical densities on l-malate. The limited growth on l-malate of the wild-type strain was correlated to a rapid degradation of the available l-malate to l-lactate, which cannot be further metabolized. Taken together, our results indicate that L. casei l-malate metabolism is not optimized for utilization of l-malate as a carbon source but for deacidification of the medium by conversion of l-malate into l-lactate via MLE.

Surface display of malolactic enzyme from Oenococcus oeni on Saccharomyces cerevisiae

In order to display malolactic enzyme (MLE) on the cell surface of Saccharomyces cerevisiae, a yeast cell surface display plasmid pADH1-AGG was constructed by fusing the α-factor signal encoding sequence (267 bp) and the C-terminal half of α-agglutinin encoding sequence (1,645 bp) into the plasmid pADH1. The pADH1-AGG could successfully express and anchor the enhanced green fluorescent protein (EGFP) onto the yeast cell surface when the EGFP was used to verify its function. Then the pADH1-MLE was constructed by inserting the MLE encoding sequence (1,600 bp) into the pADH1-AGG and introduced into S. cerevisiae cells. The positive strain carrying pADH1-MLE was confirmed by use of the 6× His monoclonal antibody and fluorescein isothiocyanate-conjugated goat anti-mouse IgG. All results indicated that the MLE was displayed successfully on the cell surface of positive transformant. The MLE activity of genetically engineered yeast strain could turn 21.11 % L-malate into lactic acid after 12 h reaction with L-malate. The constructed yeast strain might be used to conduct malolactic fermentation (MLF) in wine to solve the important issues of sluggish MLF, microbial spoilage, and adverse metabolic substances produced by the lactic acid bacteria.

Malolactic enzyme from Oenococcus oeni: heterologous expression in Escherichia coli and biochemical characterization

Malolactic enzymes (MLE) are known to directly convert L-malic acid into L-lactic acid with a catalytical requirement of nicotinamide adenine dinucleotide (NAD⁺) and Mn²⁺; however, the reaction mechanism is still unclear. To study a MLE, the structural gene from Oenococcus oeni strain DSM 20255 was heterologously expressed in Escherichia coli, yielding 22.9 kU l⁻¹ fermentation broth. After affinity chromatography and removal of apparently inactive protein by precipitation, purified recombinant MLE had a specific activity of 280 U mg⁻¹ protein with a recovery of approximately 61%. The enzyme appears to be a homodimer with a molecular mass of 128 kDa consisting of two 64 kDa subunits. Characterization of the recombinant enzyme showed optimum activity at pH 6.0 and 45°C, and K_m, V_max and k_cat values of 4.9 mM, 427 U mg⁻¹ and 456 sec⁻¹ for L-malic acid, 91.4 µM, 295 U mg⁻¹ and 315 sec⁻¹ for NAD⁺ and 4.6 µM, 229 U mg⁻¹ and 244 sec⁻¹ for Mn²⁺, respectively. The recombinant MLE retained 95% of its activity after 3 mo at room temperature and 7 mo at 4°C. When using pyruvic acid as substrate, the enzyme showed the conversion of pyruvic acid with detectable L-lactate dehydrogenase (L-LDH) activity and oxidation of NADH. This interesting observation might explain that MLE catalyzes a redox reaction and hence, the requirements for NAD⁺ and Mn²⁺ during the conversion of L-malic to L-lactic acid.

Heterologous expression of Oenococcus oeni malolactic enzyme in Lactobacillus plantarum for improved malolactic fermentation

Lactobacillus plantarum is involved in a multitude of food related industrial fermentation processes including the malolactic fermentation (MLF) of wine. This work is the first report on a recombinant L. plantarum strain successfully conducting MLF. The malolactic enzyme (MLE) from Oenococcus oeni was cloned into the lactobacillal expression vector pSIP409 which is based on the sakacin P operon of Lactobacillus sakei and expressed in the host strain L. plantarum WCFS1. Both recombinant and wild-type L. plantarum strains were tested for MLF using a buffered malic acid solution in absence of glucose. Under the conditions with L-malic acid as the only energy source and in presence of Mn²⁺ and NAD⁺, the recombinant L. plantarum and the wild-type strain converted 85% (2.5 g/l) and 51% (1.5 g/l), respectively, of L-malic acid in 3.5 days. Furthermore, the recombinant L. plantarum cells converted in a modified wine 15% (0.4 g/l) of initial L-malic acid concentration in 2 days. In conclusion, recombinant L. plantarum cells expressing MLE accelerate the malolactic fermentation.

Expression of the malolactic enzyme gene (mle) from Lactobacillus plantarum under winemaking conditions

Malolactic fermentation (MLF) plays an important role in the production of wine, especially red wines, resulting in microbial stability, deacidification, as well as contributing to the aroma profile. MLF can be influenced by a number of factors. In this study, the influence of pH and ethanol on expression of the structural malolactic enzyme gene (mle) from Lactobacillus plantarum was investigated in a synthetic wine media, as well as in wine using quantitative PCR. Expression of mle was shown to be inducible by the presence of malic acid, with increased expression in the middle of MLF. Expression of mle was also shown to be increased at low pH values and decreased in the presence of ethanol. This indicates the role of MLF in acid tolerance and the negative impact of ethanol on the completion of MLF. The results therefore provide further evidence that L. plantarum should be applied as co-inoculation for MLF where alcohol will initially not have a negative impact on the malic acid degradation.

Alkali production associated with malolactic fermentation by oral streptococci and protection against acid, oxidative, or starvation damage

Alkali production by oral streptococci is considered important for dental plaque ecology and caries moderation. Recently, malolactic fermentation (MLF) was identified as a major system for alkali production by oral streptococci, including Streptococcus mutans. Our major objectives in the work described in this paper were to further define the physiology and genetics of MLF of oral streptococci and its roles in protection against metabolic stress damage. L-Malic acid was rapidly fermented to L-lactic acid and CO(2) by induced cells of wild-type S. mutans, but not by deletion mutants for mleS (malolactic enzyme) or mleP (malate permease). Mutants for mleR (the contiguous regulator gene) had intermediate capacities for MLF. Loss of capacity to catalyze MLF resulted in loss of capacity for protection against lethal acidification. MLF was also found to be protective against oxidative and starvation damage. The capacity of S. mutans to produce alkali from malate was greater than its capacity to produce acid from glycolysis at low pH values of 4 or 5. MLF acted additively with the arginine deiminase system for alkali production by Streptococcus sanguinis, but not with urease of Streptococcus salivarius. Malolactic fermentation is clearly a major process for alkali generation by oral streptococci and for protection against environmental stresses.

Improvement of Saccharomyces yeast strains used in brewing, wine making and baking

Yeast was the first microorganism domesticated by mankind. Indeed, the production of bread and alcoholic beverages such as beer and wine dates from antiquity, even though the fact that the origin of alcoholic fermentation is a microorganism was not known until the nineteenth century. The use of starter cultures in yeast industries became a common practice after methods for the isolation of pure yeast strains were developed. Moreover, effort has been undertaken to improve these strains, first by classical genetic methods and later by genetic engineering. In general, yeast strain development has aimed at improving the velocity and efficiency of the respective production process and the quality of the final products. This review highlights the achievements in genetic engineering of Saccharomyces yeast strains applied in food and beverage industry.

Dual effect of organic acids as a function of external pH in Oenococcus oeni

In this study we analyzed under various pH conditions including low pH, the effects of L-malic acid and citric acid, combined or not, on the growth, the proton motive force components and the transcription level of selected genes of the heterolactic bacterium Oenococcus oeni. It is shown here that L-malate enhanced the growth yield at pH equal or below 4.5 while the presence of citrate in media led to a complete and unexpected inhibition of the growth at pH 3.2. Nevertheless, whatever the growth conditions, both L-malate and citrate participated in the enhancement of the transmembrane pH gradient, whereas the membrane potential decreased with the pH. These results suggested that it was not citrate that was directly responsible for the inhibition observed in cultures done at low pH, but probably its end products. This was confirmed since, in media containing L-malate, the addition of acetate substantially impaired the growth rate of the bacterium and slightly the membrane potential and pH gradient. Finally, study of the expression of genes involved in the metabolism of organic acids showed that at pH 4.5 and 3.2 the presence of L-malate led to an increased amount of mRNA of mleP encoding a malate transporter.

New insight into the biochemical mechanisms regulating auxin transport in plants

The transport of the plant hormone auxin has been under intense investigation since its identification 80 years ago. Studies have gradually refined our understanding of the importance of auxin transport in many aspects of plant signalling and development, and the focus has intensified in recent years towards the identification of the proteins involved in auxin transport and their functional mechanism. Within the past 18 months, the field has progressed rapidly, with confirmation that several distinct classes of proteins, previously dubbed as 'putative auxin permeases' or 'auxin transport facilitators', are bona fide transporters of IAA (indol-3-ylacetic acid). In this review we will appraise the recent transport data and highlight likely future research directions, including the characterization of auxiliary proteins necessary for the regulation of auxin transporters.

Metabolic engineering of malolactic wine yeast

Malolactic fermentation is essential for the deacidification of high acid grape must. We have constructed a genetically stable industrial strain of Saccharomyces cerevisiae by integrating a linear cassette containing the Schizosaccharomyces pombe malate permease gene (mae1) and the Oenococcus oeni malolactic gene (mleA) under control of the S. cerevisiae PGK1 promoter and terminator sequences into the URA3 locus of an industrial wine yeast. The malolactic yeast strain, ML01, fully decarboxylated 5.5 g/l of malate in Chardonnay grape must during the alcoholic fermentation. Analysis of the phenotype, genotype, transcriptome, and proteome revealed that the ML01 yeast is substantially equivalent to the parental industrial wine yeast. The ML01 yeast enjoys 'Generally Regarded As Safe' status from the FDA and is the first genetically enhanced yeast that has been commercialized. Its application will prevent the formation of noxious biogenic amines produced by lactic acid bacteria in wine.

Real-time PCR for characterizing the stress response of Oenococcus oeni in a wine-like medium

The tolerance of the lactic acid bacterium Oenococcus oeni to hostile wine conditions is essential for the success of malolactic fermentation (MLF). In this study, reverse transcription quantitative PCR (RT-qPCR) was used to quantify the transcript level of 13 genes that could play a role in adaptation of O. oeni in wine. To optimize survival and growth in wine, cells were adapted during growth at low pH (3.5) prior to inoculation into wine. The level of gene expression was analyzed after growth at pH 3.5 in a rich medium and during MLF in a wine-like medium. RT-qPCR analyses exhibited different expression ratios of stress genes. The data obtained showed that determination of mRNA levels could constitute a new approach to studying the stress response of O. oeni after adaptation at low pH and during growth in a wine-like medium.

The 2-hydroxycarboxylate transporter family: physiology, structure, and mechanism

The 2-hydroxycarboxylate transporter family is a family of secondary transporters found exclusively in the bacterial kingdom. They function in the metabolism of the di- and tricarboxylates malate and citrate, mostly in fermentative pathways involving decarboxylation of malate or oxaloacetate. These pathways are found in the class Bacillales of the low-CG gram-positive bacteria and in the gamma subdivision of the Proteobacteria. The pathways have evolved into a remarkable diversity in terms of the combinations of enzymes and transporters that built the pathways and of energy conservation mechanisms. The transporter family includes H+ and Na+ symporters and precursor/product exchangers. The proteins consist of a bundle of 11 transmembrane helices formed from two homologous domains containing five transmembrane segments each, plus one additional segment at the N terminus. The two domains have opposite orientations in the membrane and contain a pore-loop or reentrant loop structure between the fourth and fifth transmembrane segments. The two pore-loops enter the membrane from opposite sides and are believed to be part of the translocation site. The binding site is located asymmetrically in the membrane, close to the interface of membrane and cytoplasm. The binding site in the translocation pore is believed to be alternatively exposed to the internal and external media. The proposed structure of the 2HCT transporters is different from any known structure of a membrane protein and represents a new structural class of secondary transporters.

Cloning and expression of the malolactic gene of Pediococcus damnosus NCFB1832 in Saccharomyces cerevisiae

Wine production is characterized by a primary alcoholic fermentation, conducted by Saccharomyces cerevisiae, followed by a secondary malolactic fermentation (MLF). Although most lactic acid bacteria (LAB) have the ability to metabolize L-malate, only a few species survive the high ethanol and SO2 levels in wine. Wines produced in colder viticultural regions have a lower pH than wines produced in warmer regions. The decarboxylation of L-malate in these wines leads to an increase in pH, more organoleptic complexity and microbiological stability. MLF is, however, difficult to control and problems often occur during filtering of such wines. Pediococcus spp. are known to occur in high pH wines and have strong malolactic activity. However, some pediococci synthesize exocellular polysaccharides, which may lead to abnormal viscosity in wine. In this study, the malolactic gene from Pediococcus damnosus NCFB1832 (mleD) was cloned into S. cerevisiae and co-expressed with the malate permease gene (mae1) of Schizosaccharomyces pombe. Expression of the mleD gene was compared to the expression of two other malolactic genes, mleS from Lactococcus lactis MG1363 and mleA from Oenococcus oeni Lal1. The genetically modified strain of S. cerevisiae decreased the level of L-malate in grape must to less than 0.3 gl(-1) within 3 days. This is the first expression of a malolactic gene from Pediococcus in S. cerevisiae.

Winemaking Biochemistry and Microbiology: Current Knowledge and Future Trends

The fermentation of grape must and the production of premium quality wines are a complex biochemical process that involves the interactions of enzymes from many different microbial species, but mainly yeasts and lactic acid bacteria. Yeasts are predominant in wine and carry out the alcoholic fermentation, while lactic acid bacteria are responsible for malolactic fermentation. Moreover, several optional winemaking techniques involve the use of technical enzyme preparations. Considerable progress has been made recently in understanding the biochemistry and interactions of enzymes during the winemaking process. In this study, some of these recent contributions in the biochemistry of winemaking are reviewed. This article intends to provide an updated overview (including works published until December, 2003) on the main biochemical and microbiological contributions of the different techniques that can be used in winemaking. As well as considering the transformations that take place in traditional winemaking, the production of special wines, such as sparkling wines, 'sur lie' wines, and biologically aged wines, are also studied.

A nodule-specific dicarboxylate transporter from alder is a member of the peptide transporter family

Alder (Alnus glutinosa) and more than 200 angiosperms that encompass 24 genera are collectively called actinorhizal plants. These plants form a symbiotic relationship with the nitrogen-fixing actinomycete Frankia strain HFPArI3. The plants provide the bacteria with carbon sources in exchange for fixed nitrogen, but this metabolite exchange in actinorhizal nodules has not been well defined. We isolated an alder cDNA from a nodule cDNA library by differential screening with nodule versus root cDNA and found that it encoded a transporter of the PTR (peptide transporter) family, AgDCAT1. AgDCAT1 mRNA was detected only in the nodules and not in other plant organs. Immunolocalization analysis showed that AgDCAT1 protein is localized at the symbiotic interface. The AgDCAT1 substrate was determined by its heterologous expression in two systems. Xenopus laevis oocytes injected with AgDCAT1 cRNA showed an outward current when perfused with malate or succinate, and AgDCAT1 was able to complement a dicarboxylate uptake-deficient Escherichia coli mutant. Using the E. coli system, AgDCAT1 was shown to be a dicarboxylate transporter with a K(m) of 70 microm for malate. It also transported succinate, fumarate, and oxaloacetate. To our knowledge, AgDCAT1 is the first dicarboxylate transporter to be isolated from the nodules of symbiotic plants, and we suggest that it may supply the intracellular bacteria with dicarboxylates as carbon sources.

Molecular characterization of Oenococcus oeni genes encoding proteins involved in arginine transport

AIMS

This work was carried out to complete the sequence of the arc cluster involved in arginine catabolism in Oenococcus oeni, and particularly to characterize the genes encoding proteins involved in arginine transport.

METHODS AND RESULTS

Using molecular cloning, two loci encoding proteins involved in the arginine-ornithine antiport were isolated. Their expression patterns were monitored by RT-PCR to study the influence of arginine on their transcription. Polycistronic mRNAs were detected. PCR performed directly on colonies with primer pairs specific of arc genes was used to discriminate strains able/unable to degrade arginine.

CONCLUSIONS

Oenococcus oeni contains two arcD loci encoding similar proteins. Their expression is not influenced by arginine and polycistronic messengers were detected. The inability to use arginine is due to a lack of genetic information encoding proteins of the arginine deiminase pathway.

SIGNIFICANCE AND IMPACT OF THE STUDY

The constitutive expression of arcD genes points to the positive role of arginine on O. oeni cell growth. The occasional presence of all the arc ABCD genes together in O. oeni strains might provide insights into the growth rate variability within this species.

Induction of Oenococcus oeni H+-ATPase activity and mRNA transcription under acidic conditions

The profiles of Oenococcus oeni IOB84.13 H(+)-ATPase activity under various conditions of growth were studied. Cells growing at low pH 3.5 had a 1.6-fold higher H(+)-ATPase activity compared to control cells grown at pH 5.3. While the pH of the growth medium was shown to be stable in the presence of malic acid, a drastic decrease in pH from 5.3 down to 3.9 during growth in the absence of malic acid induced an increase in H(+)-ATPase activity by 1.5-fold. This induction was even greater when the initial pH was 3.5. Partial cloning of the genes encoding the beta-subunit and the epsilon-subunit of the H(+)-ATPase suggested a typical F(1)F(0)-ATPase genetic organization in O. oeni. The atp mRNA was detected by slot blots. Cells shocked at acidic pH were shown to contain higher levels of atp mRNA compared to the control cells grown at pH 5.3. Taken together, these results indicate that the H(+)-ATPase of O. oeni is induced at low pH and that regulation seems to occur at the level of transcription. This agrees with the role of this enzyme in the regulation of the cytoplasmic pH and in the acid tolerance of O. oeni.

Absence of malolactic activity is a characteristic of H+-ATPase-deficient mutants of the lactic acid bacterium Oenococcus oeni

The lack of malolactic activity in H(+)-ATPase-deficient mutants of Oenococcus oeni selected previously was analyzed at the molecular level. Western blot experiments revealed a spot at 60 kDa corresponding to the malolactic enzyme only in the parental strain. Moreover, the mleA transcript encoding the malolactic enzyme was not detected by reverse transcription (RT)-PCR analysis of mutants. These results suggest that the malolactic operon was not transcribed in ATPase-deficient mutants. The mleR gene encoding a LysR-type regulatory protein which should be involved in expression of the malolactic genes was described previously for O. oeni. Results obtained in this study show that the mleR transcript was not detected in the mutants by RT-PCR. No mutation in the nucleotide sequences of the mleR gene and the malolactic operon was found. The effect of a reduction in H(+)-ATPase activity on L-malate metabolism was then investigated by using other malolactic bacteria. Spontaneous H(+)-ATPase-deficient mutant strains of Lactococcus lactis and Leuconostoc mesenteroides were isolated by using neomycin resistance. Two mutants were selected. These mutants exhibited ATPase activities that were reduced to 54 and 70% of the activities obtained for the L. lactis and L. mesenteroides parental strains, respectively. These mutants were also acid sensitive. However, in contrast to the ATPase-deficient mutants of O. oeni, activation of L-malate metabolism was observed with the L. lactis and L. mesenteroides mutants under optimal or acidic growth conditions. These data support the suggestion that expression of the genes encoding malolactic enzymes in O. oeni is regulated by the mleR product, as it is in L. lactis. Nevertheless, our results strongly suggest that there is a difference between the regulation of expression of the malolactic locus in O. oeni and the regulation of expression of this locus in less acidophilic lactic acid bacteria.

Identification of Listeria monocytogenes genes expressed in response to growth at low temperature

Listeria monocytogenes is a food-borne bacterial pathogen that is able to grow at refrigeration temperatures. To investigate microbial gene expression associated with cold acclimation, we used a differential cDNA cloning procedure known as selective capture of transcribed sequences (SCOTS) to identify bacterial RNAs that were expressed at elevated levels in bacteria grown at 10 degrees C compared to those grown at 37 degrees C. A total of 24 different cDNA clones corresponding to open reading frames in the L. monocytogenes strain EGD-e genome were obtained by SCOTS. These included cDNAs for L. monocytogenes genes involved in previously described cold-adaptive responses (flaA and flp), regulatory adaptive responses (rpoN, lhkA, yycJ, bglG, adaB, and psr), general microbial stress responses (groEL, clpP, clpB, flp, and trxB), amino acid metabolism (hisJ, trpG, cysS, and aroA), cell surface alterations (fbp, psr, and flaA), and degradative metabolism (eutB, celD, and mleA). Four additional cDNAs were obtained corresponding to genes potentially unique to L. monocytogenes and showing no significant similarity to any other previously described genes. Northern blot analyses confirmed increased steady-state levels of RNA for all members of a subset of genes examined during growth at a low temperature. These results indicated that L. monocytogenes acclimation to growth at 10 degrees C likely involves amino acid starvation, oxidative stress, aberrant protein synthesis, cell surface remodeling, alterations in degradative metabolism, and induction of global regulatory responses.

Bacillus subtilis YqkI is a novel malic/Na+-lactate antiporter that enhances growth on malate at low protonmotive force

Bacillus subtilis yheL encodes a Na(+)/H(+) antiporter, whereas its paralogue, yqkI, encodes a novel antiporter that achieves a simultaneous Na(+)/H(+) and malolactate antiport. B. subtilis yufR, a control in some experiments, encodes a Na(+)/malate symporter. YqkI complemented a malate transport mutant of Escherichia coli if Na(+) and lactate were present. YheL conferred Na(+) uptake capacity on everted membrane vesicles from an antiporter-deficient E. coli mutant that was consistent with a secondary Na(+)/H(+) antiport, but YqkI-dependent Na(+) uptake depended on intravesicular malate and extravesicular lactate. YqkI-dependent lactate uptake depended on intravesicular malate and extravesicular Na(+). YqkI mediated an electroneutral exchange, which is proposed to be a malic(-2)-2H(+) (or fully protonated malate)/Na(+)-lactate(-1) antiport. Because the composite YqkI-mediated exchanges could be driven by gradients of the malate-lactate pair, this transporter could play a role in growth of B. subtilis on malate at low protonmotive force. A mutant with a disruption of yqkI exhibited an abrupt arrest in the mid-logarithmic phase of growth on malate when low concentrations of protonophore were present. Thus growth of B. subtilis to high density on a putatively nonfermentative dicarboxylic acid substrate depends on a malolactate exchange at suboptimal protonmotive force.

Tailoring wine yeast for the new millennium: novel approaches to the ancient art of winemaking

Yeasts are predominant in the ancient and complex process of winemaking. In spontaneous fermentations, there is a progressive growth pattern of indigenous yeasts, with the final stages invariably being dominated by the alcohol-tolerant strains of Saccharomyces cerevisiae. This species is universally known as the 'wine yeast' and is widely preferred for initiating wine fermentations. The primary role of wine yeast is to catalyze the rapid, complete and efficient conversion of grape sugars to ethanol, carbon dioxide and other minor, but important, metabolites without the development of off-flavours. However, due to the demanding nature of modern winemaking practices and sophisticated wine markets, there is an ever-growing quest for specialized wine yeast strains possessing a wide range of optimized, improved or novel oenological properties. This review highlights the wealth of untapped indigenous yeasts with oenological potential, the complexity of wine yeasts' genetic features and the genetic techniques often used in strain development. The current status of genetically improved wine yeasts and potential targets for further strain development are outlined. In light of the limited knowledge of industrial wine yeasts' complex genomes and the daunting challenges to comply with strict statutory regulations and consumer demands regarding the future use of genetically modified strains, this review cautions against unrealistic expectations over the short term. However, the staggering potential advantages of improved wine yeasts to both the winemaker and consumer in the third millennium are pointed out.

Regulation of stress response in Oenococcus oeni as a function of environmental changes and growth phase

Oenococcus oeni is a lactic acid bacterium which is able to grow in wine and perform malolactic fermentation. To survive and grow in such a harsh environment as wine, O. oeni uses several mechanisms of resistance including stress protein synthesis. The molecular characterisation of three stress genes hsp18, clpX, trxA encoding for a small heat shock protein, an ATPase regulation component of ClpP protease and a thioredoxin, respectively, allow us to suggest the existence in O. oeni of multiple regulation mechanisms as is the case in Bacillus subtilis. One common feature of these genes is that they are expressed under the control of housekeeping promoters. The expression of these genes as a function of growth is significantly different. Surprisingly, the clpX gene, which is induced by heat shock, was highly expressed in the early phase of growth. In addition to stress protein synthesis, adaptation to the acid pH of wine requires efficient cellular systems to extrude protons. Using inhibitors specific for different types of ATPases, we demonstrated the existence of H+-ATPase and P-type ATPase.

The Oenococcus oeni clpX homologue is a heat shock gene preferentially expressed in exponential growth phase

Using degenerated primers from conserved regions of previously studied clpX gene products, we cloned the clpX gene of the malolactic bacterium Oenococcus oeni. The clpX gene was sequenced, and the deduced protein of 413 amino acids (predicted molecular mass of 45,650 Da) was highly similar to previously analyzed clpX gene products from other organisms. An open reading frame located upstream of the clpX gene was identified as the tig gene by similarity of its predicted product to other bacterial trigger factors. ClpX was purified by using a maltose binding protein fusion system and was shown to possess an ATPase activity. Northern analyses indicated the presence of two independent 1.6-kb monocistronic clpX and tig mRNAs and also showed an increase in clpX mRNA amount after a temperature shift from 30 to 42 degrees C. The clpX transcript is abundant in the early exponential growth phase and progressively declines to undetectable levels in the stationary phase. Thus, unlike hsp18, the gene encoding one of the major small heat shock proteins of Oenococcus oeni, clpX expression is related to the exponential growth phase and requires de novo protein synthesis. Primer extension analysis identified the 5' end of clpX mRNA which is located 408 nucleotides upstream of a putative AUA start codon. The putative transcription start site allowed identification of a predicted promoter sequence with a high similarity to the consensus sequence found in the housekeeping gene promoter of gram-positive bacteria as well as Escherichia coli.

Acid sensitivity of neomycin-resistant mutants of Oenococcus oeni: a relationship between reduction of ATPase activity and lack of malolactic activity

Mutants of Oenococcus oeni were isolated as spontaneous neomycin-resistant mutants. Three of these mutants harbored a significantly reduced ATPase activity that represented 50% of that of the wild-type strain. Their growth rates were also impaired at pH 5.3 (46-86% of the wild-type level). However, the profiles of sugar consumption appeared identical to those of the parental strain. At pH 3.2, all the mutant strains failed to grow and a drastic decrease in viability was observed after an acid shock. Surprisingly, all the isolated mutants were devoid of malolactic activity. These results suggest that the ATPase and malolactic activities of O. oeni are linked to each other and play a crucial role in the mechanism of resistance to an acid stress.

Physical map of the genome of Oenococcus oeni PSU-1 and localization of genetic markers

A physical map of the chromosome of Oenococcus oeni PSU-1 was constructed. This represents the first map for a strain of this species. A total of 37 restriction sites for the rare-cutting endonucleases Ascl, Fsel, Notl and Sfil were mapped on the chromosome, which was found to be circular with an estimated size of 1857 kb. Fragment order was determined using several approaches: analysis of partial and double digestions, two-dimensional pulsed-field gel electrophoresis, isolation of linking clones, and Southern hybridization with labelled restriction fragments both from PSU-1 and from O. oeni strain GM. Oenococcal genes alsS/alsD, mleA and mir, two phage attachment sites and recurrent sequences such as IS1165-like elements and rrn loci were located on the physical map. Specific fragments hybridizing with gene probes from Lactococcus lactis, Leuconostoc mesenteroides and Bacillus subtilis were also identified. The two ribosomal operons have been precisely located and their transcription direction determined.

Biochemical basis for glucose-induced inhibition of malolactic fermentation in Leuconostoc oenos

The sugar-induced inhibition of malolactic fermentation in cell suspensions of Leuconostoc oenos, recently reclassified as Oenococcus oeni (L. M. T. Dicks, F. Dellaglio, and M. D. Collins, Int. J. Syst. Bacteriol. 45:395-397, 1995) was investigated by in vivo and in vitro nuclear magnetic resonance (NMR) spectroscopy and manometric techniques. At 2 mM, glucose inhibited malolactic fermentation by 50%, and at 5 mM or higher it caused a maximum inhibitory effect of ca. 70%. Galactose, trehalose, maltose, and mannose caused inhibitory effects similar to that observed with glucose, but ribose and 2-deoxyglucose did not affect the rate of malolactic activity. The addition of fructose or citrate completely relieved the glucose-induced inhibition. Glucose was not catabolized by permeabilized cells, and inhibition of malolactic fermentation was not observed under these conditions. 31P NMR analysis of perchloric acid extracts of cells obtained during glucose-malate cometabolism showed high intracellular concentrations of glucose-6-phosphate, 6-phosphogluconate, and glycerol-3-phosphate. Glucose-6-phosphate, 6-phosphogluconate, and NAD(P)H inhibited the malolactic activity in permeabilized cells or cell extracts, whereas NADP+ had no inhibitory effect. The purified malolactic enzyme was strongly inhibited by NADH, whereas all the other above-mentioned metabolites exerted no inhibitory effect, showing that NADH was responsible for the inhibition of malolactic activity in vivo. The concentration of NADH required to inhibit the activity of the malolactic enzyme by 50% was ca. 25 microM. The data provide a coherent biochemical basis to understand the glucose-induced inhibition of malolactic fermentation in L. oenos.

Metabolic analysis of S. cerevisiae strains engineered for malolactic fermentation

A complete malolactic fermentation was achieved using Saccharomyces cerevisiae strains coexpressing the genes mleS and mae1 coding for the Lactococcus lactis malolactic enzyme and the Schizosaccharomyces pombe malate permease under the control of yeast promoters. The expression level of mae1 greatly influences the kinetics of the reaction by controlling the rate of malate uptake meanwhile a high expression level of mleS induces a partial consumption of malate derived from glucose by the malolactic enzyme. A strain expressing several copies of mae1 and one copy of mleS degrades 3 g/l of malate almost exclusively through the malolactic pathway in 4 days under enological conditions, without metabolic side effects.

Characterization of the L-malate permease gene (maeP) of Streptococcus bovis ATCC 15352

A gene which was shown to be cotranscribed with the NAD+-dependent malic enzyme gene (maeE) of Streptococcus bovis ATCC 15352 was revealed to encode L-malate-specific permease (MaeP), which showed high activity at low pHs (pH 5.1 to 5.9). MaeP was strongly inhibited by the ionophores nigericin and valinomycin.

Molecular characterization of an inducible p-coumaric acid decarboxylase from Lactobacillus plantarum: gene cloning, transcriptional analysis, overexpression in Escherichia coli, purification, and characterization

By using degenerate primers designed from the first 19 N-terminal amino acids of Lactobacillus plantarum p-coumaric acid decarboxylase (PDC), a 56-bp fragment was amplified from L. plantarum in PCRs and used as a probe for screening an L. plantarum genomic bank. Of the 2,880 clones in the genomic bank, one was isolated by colony hybridization and contained a 519-bp open reading frame (pdc gene) followed by a putative terminator structure. The pdc gene is expressed on a monocistronic transcriptional unit, which is transcribed from promoter sequences homologous to Lactococcus promoter sequences. No mRNA from pdc and no PDC activity were detected in uninduced cell extracts, indicating that the expression is transcriptionally regulated by p-coumaric acid, which corresponds to an activation factor up to 6,000. The pdc gene was overexpressed constitutively in Escherichia coli, and the recombinant enzyme was purified and characterized.

Molecular characterization of the gene encoding an 18-kilodalton small heat shock protein associated with the membrane of Leuconostoc oenos

In Leuconostoc oenos, different stresses such as heat, ethanol, and acid shocks dramatically induce the expression of an 18-kDa small heat shock protein called Lo 18. The corresponding gene (hsp18) was cloned from a genomic library of L. oenos constructed in Escherichia coli. A 2.3-kb DNA fragment carrying the hsp18 gene was sequenced. The hsp18 gene encodes a polypeptide of 148 amino acids with a calculated molecular mass of 16,938 Da. The Lo18 protein has a significant identity with small heat shock proteins of the alpha-crystallin family. The transcriptional start site was determined by primer extension. This experiment allowed us to identify the promoter region exhibiting high similarity to consensus promoter sequences of gram-positive bacteria, as well as E. coli. Northern blot analysis showed that hsp18 consists of a unique transcription unit of 0.6 kb. Moreover, hsp18 expression seemed to be controlled at the transcriptional level. This small heat shock protein was found to be peripherally associated with the membrane of L. oenos.

Genetic organization of the mle locus and identification of a mleR-like gene from Leuconostoc oenos

Characterization of the mle locus harboring the malolactic enzyme gene mleA and malate permease gene mleP from Leuconostoc oenos was completed in this study by mRNA analysis. Northern (RNA) blot experiments revealed a 2.6-kb transcript, suggesting an operon structure harboring mleA and mleP genes. Primer extension analysis showed that the mle operon has a single transcription start site located 17 nucleotides upstream of the ATG translation start site for the mleA gene. We found sequences, TTGACT and TATGAT (which are separated by 18 bp), that are closely related to the gram-positive and Escherichia coli consensus promoter sequences. Upstream of the mleA gene, an 894-bp open reading frame that transcribed divergently from the mle operon was found. Sequence analysis and expression in E. coli minicells suggest that this open reading frame encodes a polypeptide with an apparent molecular mass of 34 kDa belonging to the LysR-type regulatory protein family. Protein comparisons showed the highest level of identity with the MleR regulatory protein from Lactococcus lactis, which is involved in the expression of the malolactic genes in the presence of L-malate. However, the MleR-like protein of L. oenos seems different from the protein of Lactococcus lactis, since no regulation of the malolactic enzyme by L-malate was effective under our experimental conditions.

13C nuclear magnetic resonance analysis of glucose and citrate end products in an ldhL-ldhD double-knockout strain of Lactobacillus plantarum

We have examined the metabolic consequences of knocking out the two ldh genes in Lactobacillus plantarum using 13C nuclear magnetic resonance. Unlike its wild-type isogenic progenitor, which produced lactate as the major metabolite under all conditions tested, ldh null strain TF103 mainly produced acetoin. A variety of secondary end products were also found, including organic acids (acetate, succinate, pyruvate, and lactate), ethanol, 2,3-butanediol, and mannitol.