Regulation of primary carbon metabolism in Kluyveromyces lactis

Regulation of phosphatidylglycerolphosphate synthase in aerobic yeast Kluyveromyces lactis

The KlPGS1 gene encoding phosphatidylglycerolphosphate synthase (PGPS) is essential for the viability and multiplication of Kluyveromyces lactis. Regulation of PGPS expression by factors affecting mitochondrial development (C source, growth phase) and general phospholipid biosynthesis was followed. PGS1 mRNA levels were not altered as cells progressed from the exponential to the stationary phase of growth in glucose. PGS1 mRNA abundance was nearly identical in cells growing in a medium with glucose or glycerol as the sole C source during the different growth phases. Regulation of PGS1 expression by exogenous myo-inositol and choline was not mediated at the transcriptional level, the PGPS activity dropped to 70 % after myo-inositol addition.

Differential control of isocitrate lyase gene transcription by non-fermentable carbon sources in the milk yeast Kluyveromyces lactis

The KlICL1 gene, encoding isocitrate lyase in Kluyveromyces lactis, is essential for ethanol utilization. Deletion analyses identified two functional promoter elements, CSRE-A and CSRE-B. Transcription is activated on ethanol, but not on glucose, glycerol or lactate. Expression depends on the KlCat8p transcription factor and KlSip4p binds to the promoter elements. Glycerol diminishes KlICL1 expression and a single carbon source responsive element (CSRE) sequence is both necessary and sufficient to mediate this regulation. The glycerol effect is less pronounced in Saccharomyces cerevisiae than in K. lactis. Mutants lacking KlGUT2 (which encodes the glycerol 3-phosphate dehydrogenase) still show reduced expression in glycerol, whereas mutants deficient in glycerol kinase (Klgut1) do not. We conclude that a metabolite of glycerol is required for this regulation.

Amino acids allosterically regulate the thiamine diphosphate-dependent alpha-keto acid decarboxylase from Mycobacterium tuberculosis

The gene rv0853c from Mycobacterium tuberculosis strain H37Rv codes for a thiamine diphosphate-dependent alpha-keto acid decarboxylase (MtKDC), an enzyme involved in the amino acid degradation via the Ehrlich pathway. Steady state kinetic experiments were performed to determine the substrate specificity of MtKDC. The mycobacterial enzyme was found to convert a broad spectrum of branched-chain and aromatic alpha-keto acids. Stopped-flow kinetics showed that MtKDC is allosterically activated by alpha-keto acids. Even more, we demonstrate that also amino acids are potent activators of this thiamine diphosphate-dependent enzyme. Thus, metabolic flow through the Ehrlich pathway can be directly regulated at the decarboxylation step. The influence of amino acids on MtKDC catalysis was investigated, and implications for other thiamine diphosphate-dependent enzymes are discussed.

Functional analysis of Kluyveromyces lactis carboxylic acids permeases: heterologous expression of KlJEN1 and KlJEN2 genes

The present work describes a detailed physiological and molecular characterization of the mechanisms of transport of carboxylic acids in Kluyveromyces lactis. This yeast species presents two homologue genes to JEN1 of Saccharomyces cerevisiae: KlJEN1 encodes a monocarboxylate permease and KlJEN2 encodes a dicarboxylic acid permease. In the strain K. lactis GG1888, expression of these genes does not require an inducer and activity for both transport systems was observed in glucose-grown cells. To confirm their key role for carboxylic acids transport in K. lactis, null mutants were analyzed. Heterologous expression in S. cerevisiae has been performed and chimeric fusions with GFP showed their proper localization in the plasma membrane. S. cerevisiae jen1delta cells transformed with KlJEN1 recovered the capacity to use lactic acid, as well as to transport labeled lactic acid by a mediated mechanism. When KlJEN2 was heterologously expressed, S. cerevisiae transformants gained the ability to transport labeled succinic and malic acids by a mediated mechanism, exhibiting, however, a poor growth in malic acid containing media. The results confirmed the role of KlJen1p and KlJen2p as mono and dicarboxylic acids permeases, respectively, not subjected to glucose repression, being fully functional in S. cerevisiae.

Heterologous expression of cyan and yellow fluorescent proteins from the Kluyveromyces lactis KlMAL21-KlMAL22 bi-directional promoter

We have identified the Kluyveromyces lactis maltase (KlMAL22) and maltose permease (KlMAL21) intergenic region as a candidate bi-directional promoter for heterologous gene expression. The expressions of cyan and yellow fluorescent proteins from, respectively, the KlMAL22 and KlMAL21 orientations of the promoter, were compared between two promoter variants during growth in media containing glucose, galactose or glycerol. Expression from both orientations of the native promoter was repressed during growth in glucose and galactose and was induced during growth in glycerol. Disruption of a putative Mig1p binding site caused some de-repression of the maltase orientation of the promoter by 48 h of growth in glucose. The KlMAL21-KlMAL22 bi-directional promoter can be used to carry out regulated expression of heterologous gene products.

Transcriptomic analysis of extensive changes in metabolic regulation in Kluyveromyces lactis strains

Genome-wide analysis of transcriptional regulation is generally carried out on well-characterized reference laboratory strains; hence, the characteristics of industrial isolates are therefore overlooked. In a previous study on the major cheese yeast Kluyveromyces lactis, we have shown that the reference strain and an industrial strain used in cheese making display a differential gene expression when grown on a single carbon source. Here, we have used more controlled conditions, i.e., growth in a fermentor with pH and oxygen maintained constant, to study how these two isolates grown in glucose reacted to an addition of lactose. The observed differences between sugar consumption and the production of various metabolites, ethanol, acetate, and glycerol, correlated with the response were monitored by the analysis of the expression of 482 genes. Extensive differences in gene expression between the strains were revealed in sugar transport, glucose repression, ethanol metabolism, and amino acid import. These differences were partly due to repression by glucose and another, yet-unknown regulation mechanism. Our results bring to light a new type of K. lactis strain with respect to hexose transport gene content and repression by glucose. We found that a combination of point mutations and variation in gene regulation generates a biodiversity within the K. lactis species that was not anticipated. In contrast to S. cerevisiae, in which there is a massive increase in the number of sugar transporter and fermentation genes, in K. lactis, interstrain diversity in adaptation to a changing environment is based on small changes at the level of key genes and cell growth control.

Why doesKluyveromyces lactisnot grow under anaerobic conditions? Comparison of essential anaerobic genes ofSaccharomyces cerevisiaewith theKluyveromyces lactisgenome

Although some yeast species, e.g. Saccharomyces cerevisiae, can grow under anaerobic conditions, Kluyveromyces lactis cannot. In a systematic study, we have determined which S. cerevisiae genes are required for growth without oxygen. This has been done by using the yeast deletion library. Both aerobically essential and nonessential genes have been tested for their necessity for anaerobic growth. Upon comparison of the K. lactis genome with the genes found to be anaerobically important in S. cerevisiae, which yielded 20 genes that are missing in K. lactis, we hypothesize that lack of import of sterols might be one of the more important reasons that K. lactis cannot grow in the absence of oxygen.

The KlPGS1 gene encoding phosphatidylglycerolphosphate synthase in Kluyveromyces lactis is essential and assigned to chromosome I

The phosphatidylglycerolphosphate synthase (CDP-diacylglycerol:sn-glycerol-3-phosphate 3-phosphatidyltransferase, EC 2.7.8.5) is an essential enzyme in biosynthesis of cardiolipin. In this work we report the isolation, heterological cloning, molecular characterization and physical mapping of the Saccharomyces cerevisiae PEL1/PGS1 homologue from Kluyveromyces lactis. The pel1 mutant strain of S. cerevisiae was used to isolate this homologue by screening a K. lactis genomic library. The novel cloned gene was named KlPGS1. Its coding region was found to consist of 1623 bp. The corresponding protein exhibits 55% amino acid identity to its S. cerevisiae counterpart. The presence of the mitochondrial presequence indicates its mitochondrial localization. Sporulation and ascus dissection of diploids heterozygous for single-copy disruption of KlPGS1 revealed that the KlPGS1 gene, is essential in K. lactis. Using a DIG-dUTP-labeled DNA probe-originated from the KlPGS1 gene and Southern hybridization of contour-clamped homogeneous electric field (CHEF)-separated K. lactis chromosomal DNA, the KlPGS1 gene was assigned to chromosome I. The nucleotide sequence data reported in this paper were submitted to GenBank and assigned the Accession No. AY176328.

The inactivation of KlNOT4, a Kluyveromyces lactis gene encoding a component of the CCR4-NOT complex, reveals new regulatory functions

We have isolated the KlNOT4 gene of the yeast Kluyveromyces lactis, which encodes a component of the evolutionarily conserved CCR4-NOT complex. We show that inactivation of the gene leads to pleiotropic defects that were differentially suppressed by the NOT4 gene of S. cerevisiae, indicating that these genes have overlapping, but not identical, functions. K. lactis strains lacking Not4p are defective in fermentation and show reduced transcription of glucose transporter and glycolytic genes, which are phenotypes that are not found in the corresponding mutant of S. cerevisiae. We also show that Not4 proteins control the respiratory pathway in both yeasts, although with some differences. They activate transcription of KlACS2 and KlCYC1, but repress KlICL1, ScICL1, ScACS1, and ScCYC1. Altogether, our results indicate that Not4p is a pivotal factor involved in the regulation of carbon metabolism in yeast.

Role of Snf1p in regulation of intracellular sorting of the lactose and galactose transporter Lac12p in Kluyveromyces lactis

The protein kinase Snf1/AMPK plays a central role in carbon and energy homeostasis in yeasts and higher eukaryotes. To work out which aspects of the Snf1-controlled regulatory network are conserved in evolution, the Snf1 requirement in galactose metabolism was analyzed in the yeast Kluyveromyces lactis. Whereas galactose induction was only delayed, K. lactis snf1 mutants failed to accumulate the lactose/galactose H+ symporter Lac12p in the plasma membran,e as indicated by Lac12-green fluorescent protein fusions. In contrast to wild-type cells, the fusion protein was mostly intracellular in the mutant. Growth on galactose and galactose uptake could be restored by the KHT3 gene, which encodes a new transporter of the HXT subfamily of major facilitators These findings indicate a new role of Snf1p in regulation of sugar transport in K. lactis.

Genome-wide analysis of Kluyveromyces lactis in wild-type and rag2 mutant strains

The use of heterologous DNA arrays from Saccharomyces cerevisiae has been tested and revealed as a suitable tool to compare the transcriptomes of S. cerevisiae and Kluyveromyces lactis, two yeasts with notable differences in their respirofermentative metabolism. The arrays have also been applied to study the changes in the K. lactis transcriptome owing to mutation in the RAG2 gene coding for the glycolytic enzyme phosphoglucose isomerase. Comparison of the rag2 mutant growing in 2% glucose versus 2% fructose has been used as a model to elucidate the importance of transcriptional regulation of metabolic routes, which may be used to reoxidize the NADPH produced in the pentose phosphate pathway. At this transcriptional level, routes related to the oxidative stress response become an interesting alternative for NADPH use.

Carboxylic acids permeases in yeast: two genes in Kluyveromyces lactis

Two new genes KlJEN1 and KlJEN2 were identified in Kluyveromyces lactis. The deduced structure of their products is typical of membrane-bound carriers and displays high similarity to Jen1p, the monocarboxylate permease of Saccharomyces cerevisiae. Both KlJEN1 and KlJEN2 are under the control of glucose repression mediated by FOG1 and FOG2, corresponding to S. cerevisiae GAL83 and SNF1 respectively, and KlCAT8, proteins involved in glucose signalling cascade in K. lactis. KlJEN1, but not KlJEN2, is induced by lactate. KlJEN2 in contrast is expressed at high level in ethanol and succinate. The physiological characterization of null mutants showed that KlJEN1 is the functional homologue of ScJEN1, whereas KlJEN2 encodes a dicarboxylic acids transporter. In fact, KlJen1p [transporter classification (TC) number: 2.A.1.12.2.] is required for lactate uptake and therefore for growth on lactate. KlJen2p is required for succinate transport, as demonstrated by succinate uptake experiments and by inability of Kljen2 mutant to grow on succinate. This carrier appears to transport also malate and fumarate because the Kljen2 mutant cannot grow on these substrates and the succinate uptake is competed by these carboxylic acids. We conclude that KlJEN2 is the first yeast gene shown to encode a dicarboxylic acids permease.

Isolation and transcriptional regulation of the Kluyveromyces lactis FBA1 (fructose-1,6-bisphosphate aldolase) gene

Cloning and transcriptional regulation of the KlFBA1 gene that codes for the class II fructose-1,6-bisphosphate aldolase of the yeast Kluyveromyces lactis are described. KlFBA1 mRNA diminishes transiently during the shift from hypoxic to fully aerobic conditions and increases in the reversal shift. This regulation is mediated by heme since expression was higher in a mutant defective in heme biosynthesis. KlFBA1 transcription is not induced by calcium-shortage, low temperature, or at stationary phase. These data suggest that KlFBA1 plays a role in the balance between oxidative and fermentative metabolism and that this gene is differentially regulated in K. lactis and Saccharomyces cerevisiae, i.e., a respiratory vs. fermentative yeast.

Improved production of heterologous proteins by a glucose repression-defective mutant of Kluyveromyces lactis

The secreted production of heterologous proteins in Kluyveromyces lactis was studied. A glucoamylase (GAA) from the yeast Arxula adeninivorans was used as a reporter protein for the study of the secretion efficiencies of several wild-type and mutant strains of K. lactis. The expression of the reporter protein was placed under the control of the strong promoter of the glyceraldehyde-3-phosphate dehydrogenase of Saccharomyces cerevisiae. Among the laboratory strains tested, strain JA6 was the best producer of GAA. Since this strain is known to be highly sensitive to glucose repression and since this is an undesired trait for biomass-oriented applications, we examined heterologous protein production by using glucose repression-defective mutants isolated from this strain. One of them, a mutant carrying a dgr151-1 mutation, showed a significantly improved capability of producing heterologous proteins such as GAA, human serum albumin, and human interleukin-1beta compared to the parent strain. dgr151-1 is an allele of RAG5, the gene encoding the only hexokinase present in K. lactis (a homologue of S. cerevisiae HXK2). The mutation in this strain was mapped to nucleotide position +527, resulting in a change from glycine to aspartic acid within the highly conserved kinase domain. Cells carrying the dgr151-1 allele also showed a reduction in N- and O-glycosylation. Therefore, the dgr151 strain may be a promising host for the production of heterologous proteins, especially when the hyperglycosylation of recombinant proteins must be avoided.

Ethanol formation and enzyme activities around glucose-6-phosphate in Kluyveromyces marxianus CBS 6556 exposed to glucose or lactose excess

The aim of this work was to investigate the physiology of Kluyveromyces marxianus CBS 6556 in terms of its low tendency to form ethanol under exposure to sugar excess, and the split of carbon flux which takes place at the level of glucose-6-phosphate. Measurements were performed in batch cultivations, and after a glucose or a lactose pulse applied to chemostat-grown respiring cells (with a dilution rate of 0.1 h(-1)). No ethanol formation was observed in batch cultivations or during pulse experiments, unless the oxygen supply was shut down, indicating that this organism is more strictly Crabtree-negative than its close relative K. lactis and other known Crabtree-negative yeasts. During the pulse experiments, activities of phosphoglucoisomerase, glucose-6-phosphate dehydrogenase and phosphoglucomutase in cell-free extracts remained rather constant, at higher levels than those of Saccharomyces cerevisiae grown at similar conditions. When cells were exposed to glucose concentrations as high as 26 gl(-1), the activity of phosphoglucomutase was higher than that in cells exposed to 14 gl(-1) glucose, whereas the activities of phosphoglucoisomerase and glucose-6-phosphate dehydrogenase did not change. Our results suggest that the low tendency for ethanol formation in K. marxianus might be a consequence of this yeast's capacity of keeping the glycolytic flux constant, due at least in part to the diversion of carbon flux towards the biosynthesis of carbohydrates and towards the pentose phosphate pathway.

Key role of Ser562/661 in Snf1-dependent regulation of Cat8p in Saccharomyces cerevisiae and Kluyveromyces lactis

Utilization of nonfermentable carbon sources by Kluyveromyces lactis and Saccharomyces cerevisiae requires the Snf1p kinase and the Cat8p transcriptional activator, which binds to carbon source-responsive elements of target genes. We demonstrate that KlSnf1p and KlCat8p from K. lactis interact in a two-hybrid system and that the interaction is stronger with a kinase-dead mutant form of KlSnf1p. Of two putative phosphorylation sites in the KlCat8p sequence, serine 661 was identified as a key residue governing KlCat8p regulation. Serine 661 is located in the middle homology region, a regulatory domain conserved among zinc cluster transcription factors, and is part of an Snf1p consensus phosphorylation site. Single mutations at this site are sufficient to completely change the carbon source regulation of the KlCat8p transactivation activity observed. A serine-to-glutamate mutant form mimicking constitutive phosphorylation results in a nearly constitutively active form of KlCat8p, while a serine-to-alanine mutation has the reverse effect. Furthermore, it is shown that KlCat8p phosphorylation depends on KlSNF1. The Snf1-Cat8 connection is evolutionarily conserved: mutation of corresponding serine 562 of ScCat8p gave similar results in S. cerevisiae. The enhanced capacity of ScCat8S562E to suppress the phenotype caused by snf1 strengthens the hypothesis of direct phosphorylation of Cat8p by Snf1p. Unlike that of S. cerevisiae ScCAT8, KlCAT8 transcription is not carbon source regulated, illustrating the prominent role of posttranscriptional regulation of Cat8p in K. lactis.

Genetic aspects of targeted insertion mutagenesis in yeasts

Targeted insertion mutagenesis is a main molecular tool of yeast science initially applied in Saccharomyces cerevisiae. The method was extended to fission yeast Schizosaccharomyces pombe and to "non-conventional" yeast species, which show specific properties of special interest to both basic and applied research. Consequently, the behaviour of such non-Saccharomyces yeasts is reviewed against the background of the knowledge of targeted insertion mutagenesis in S. cerevisiae. Data of homologous integration efficiencies obtained with circular, ends-in or ends-out vectors in several yeasts are compared. We follow details of targeted insertion mutagenesis in order to recognize possible rate-limiting steps. The route of the vector to the target and possible mechanisms of its integration into chromosomal genes are considered. Specific features of some yeast species are discussed. In addition, similar approaches based on homologous recombination that have been established for the mitochondrial genome of S. cerevisiae are described.

Isocitrate lyase of the yeast Kluyveromyces lactis is subject to glucose repression but not to catabolite inactivation

KlICL1, encoding the isocitrate lyase of Kluyveromyces lactis, was isolated by complementation of the Saccharomyces cerevisiae icl1 deletion mutant. Sequence analysis revealed an open reading frame of 1626 nucleotides encoding a protein with 542 amino acids. The deduced protein shows extensive homologies to isocitrate lyases from various organisms, with an overall identity of 69% to the enzyme from S. cerevisiae. The KlICL1 gene has two major transcription start-points, located at -113 bp and -95 bp relative to the ATG translation start codon. The gene is expressed on ethanol medium only in respiratory-competent cells. Transcription is repressed by glucose. Mutants carrying a Klcat8 deletion lack the ability to derepress KlICL1 transcription. A Klicl1 deletion mutant does not grow on ethanol medium and lacks any isocitrate lyase activity. A strain lacking the gene KlFBP1, which encodes the gluconeogenic enzyme fructose 1,6-bisphosphatase, lacks the ability to grow on non-fermentable carbon sources. This implies that K. lactis does not contain additional isoenzymes catalyzing either of the reactions. Enzyme assays revealed that neither KlIcl1p nor KlFbp1p are subject to catabolite inactivation. However, the respective enzymes from S. cerevisiae are efficiently inactivated when expressed in K. lactis. Thus, despite the extensive sequence similarities of the enzymes involved, non-fermentative carbohydrate metabolism in the two yeasts displays distinct regulatory properties.

The unique hexokinase of Kluyveromyces lactis. Molecular and functional characterization and evaluation of a role in glucose signaling

The Crabtree-negative yeast Kluyveromyces lactis is capable of adjusting its glycolytic flux to the requirements of respiration by tightly regulating glucose uptake. RAG5 encoding the only glucose and fructose phosphorylating enzyme present in K. lactis is required for the up-regulation of glucose transport and also for glucose repression. To understand the significance of the molecular identity and specific function(s) of the corresponding kinase to glucose signaling, RAG5 was overexpressed and its gene product KlHxk1 (Rag5p) isolated and characterized. Stopped-flow kinetics and sedimentation analysis indicated a monomer-homodimer equilibrium of KlHxk1 in a condition of catalysis, i.e. in the presence of substrates and products. The kinetic constants of ATP-dependent glucose phosphorylation identified a 53-kDa monomer as the high affinity/high activity form of the novel enzyme for both glycolytic substrates suggesting a control of glucose phosphorylation at the level of dimer formation and dissociation. In contrast to the highly homologous hexokinase isoenzyme 2 of Saccharomyces cerevisiae (ScHxk2), KlHxk1 was not inhibited by free ATP in a physiological range of nucleotide concentration. Mass spectrometric sequencing of tryptic peptides of KlHxk1 identified unmodified serine at amino acid position 156. The corresponding amino acid in ScHxk2 is serine 157, which represents the autophosphorylation-inactivation site. KlHxk1 did not display, however, the typical pattern of inactivation under the respective in vitro conditions and maintained a high residual glucose phosphorylating activity. The biophysical and functional data are discussed with respect to a possible regulatory role of KlHxk1 in glucose metabolism and signaling in K. lactis.

Functional characterization of the Frt1 sugar transporter and of fructose uptake in Kluyveromyces lactis

Most yeast hexose transporters studied so far at the molecular level mediate facilitated diffusion of glucose and fructose. Here, we report that a novel Kluyveromyces lactis gene, FRT1, encodes a proton-coupled fructose-uptake transporter. Frt1, when expressed in a Saccharomyces cerevisiae hxt null mutant strain that is unable to take up monosaccharides, restored growth on fructose. Determination of substrate specificities and kinetic parameters revealed Frt1 as a fructose transporter with a K(m) of 0.16+/-0.02 mM. Uptake of fructose was accompanied by an initial alkalization of the medium, indicating a proton-coupled uptake mechanism. Deletion of the FRT1 gene in a K. lactis strain already deleted for its RAG1 and HGT1 hexose transporter genes completely prevented uptake of and growth with fructose but not with glucose. Kinetic parameters of Frt1 in K. lactis, as assessed in a rag1 hgt1 mutant strain, were comparable with those obtained after heterologous expression in S. cerevisiae. Transcription of the FRT1 gene, which was undetectable when cells were grown in ethanol, was induced by various sugars. Our results indicate that, unlike S. cerevisiae, K. lactis exhibits proton symport systems for the uptake of hexoses, in addition to their facilitated diffusion.

KlROM2 encodes an essential GEF homologue in Kluyveromyces lactis

Cellular integrity in yeasts is ensured by a rigid cell wall whose synthesis is controlled by a MAP kinase signal transduction cascade. In Saccharomyces cerevisiae upstream regulatory components of this MAP kinase pathway involve a single protein kinase C, which is regulated in part by interaction with the small GTPase Rho1p. This small G protein is in turn rendered inactive (GDP-bound) or is activated (GTP-bound) by the influence of GTPase activating proteins (GAPs) and the GDP/GTP exchange factors (GEFs), respectively. We report here on the isolation of a gene from Kluyveromyces lactis, KlROM2, which encodes a member of the latter protein family. The nucleotide sequence contains an open reading frame of 1227 amino acids, with an overall identity of 57% to the Rom2 protein of S. cerevisiae. Four conserved sequence motifs could be identified: a RhoGEF domain, a DEP sequence, a CNH domain and a less conserved pleckstrin homology (PH) sequence. Klrom2 null mutants show a lethal phenotype, which indicates that the gene may encode the only functional GEF regulating the cellular integrity pathway in K. lactis. Conditional genomic expression of KlROM2 resulted in sensitivity towards caffeine and Calcofluor white as typical phenotypes of mutants defective in this pathway. Overexpression of KIROM2 from multicopy plasmids under the control of the ScGAL1 promoter severely impaired growth in both S. cerevisiae and in K. lactis. The fact that the lethal phenotype was not prevented in mpk1 deletion mutants indicates that growth inhibition is not simply caused by hyperactivation of the Pkc1p signal transduction pathway.

Isolation, heterological cloning and sequencing of the RPL28 gene in Kluyveromyces lactis

By virtue of heterologous functional complementation of the Saccharomyces cerevisiae Delta pdr5 mutant strain, using a Kluyveromyces lactis genomic library, three different K. lactis chromosomal inserts were obtained. Transformation of the S. cerevisiae Delta pdr1 Delta pdr3 mutant strain, hypersensitive to drugs, with isolated plasmids resulted in resistance to cycloheximide and fluconazole. Transformation of K. lactis host strains, using the cloned chromosomal fragments, led to an increased level of resistance to some mitochondrial inhibitors and azole antifungals. The nucleotide sequence of the cloned inserts revealed that two of them contain the drug efflux transporter gene Kl-PDR5 and the third contains a DNA segment homologous to chromosome VII of S. cerevisiae. Along with three novel ORFs, encoding two proteins of unknown molecular function and one putative hexose transporter, this segment also contained the Kl-RPL28 gene, found to be responsible for the cycloheximide resistance of heterologous transformants. This gene codes for the large subunit ribosomal protein (149 amino acids) that shares 89.9% identity with its S. cerevisiae counterpart. The coding region of Kl-RPL28 was found to be interrupted with one intron near the 5' end. The nucleotide sequence data reported in this paper were submitted to GenBank and assigned the accession number AF493565.

Pyruvate decarboxylase from Kluyveromyces lactis. An enzyme with an extraordinary substrate activation behaviour

Pyruvate decarboxylase (EC 4.1.1.1) was isolated and purified from the yeast Kluyveromyces lactis. The properties of this enzyme relating to the native oligomeric state, the subunit size, the nucleotide sequence of the coding gene(s), the catalytic activity, and protein fluorescence as well as circular dichroism are very similar to those of the well characterized pyruvate decarboxylase species from yeast. Remarkable differences were found in the substrate activation behaviour of the two pyruvate decarboxylases using three independent methods: steady-state kinetics, stopped-flow measurements, and kinetic dilution experiments. The dependence of the observed activation rate constant on the substrate concentration of pyruvate decarboxylase from K. lactis showed a minimum at a pyruvate concentration of 1.5 mm. According to the mechanism of substrate activation suggested this local minimum occurs due to the big ratio of the dissociation constants for the binding of the first (regulatory) and the second (catalytic) substrate molecule. The microscopic rate constants of the substrate activation could be determined by a refined fit procedure. The influence of the artificial activator pyruvamide on the activation of the enzyme was studied.

Functional analysis of structural genes for NAD(+)-dependent formate dehydrogenase in Saccharomyces cerevisiae

Co-consumption of formate by aerobic, glucose-limited chemostat cultures of Saccharomyces cerevisiae CEN.PK 113-7D led to an increased biomass yield relative to cultures grown on glucose as the sole carbon and energy substrate. In this respect, this strain differed from two previously investigated S. cerevisiae strains, in which formate oxidation did not lead to an increased biomass yield on glucose. Enzyme assays confirmed the presence of a formate-inducible, cytosolic and NAD(+)-dependent formate dehydrogenase. To investigate whether this enzyme activity was entirely encoded by the previously reported FDH1 gene, an fdh1Delta null mutant was constructed. This mutant strain still contained formate dehydrogenase activity and remained capable of co-consumption of formate. The formate dehydrogenase activity in the mutant was demonstrated to be encoded by a second structural gene for formate dehydrogenase (FDH2) in S. cerevisiae CEN.PK 113-7D. FDH2 was highly homologous to FDH1 and consisted of a fusion of two open reading frames (ORFs) (YPL275w and YPL276w) reported in the S. cerevisiae genome databases. Sequence analysis confirmed that, in the database genetic background, the presence of two single-nucleotide differences led to two truncated ORFs rather than the full-length FDH2 gene present in strain CEN.PK 113-7D. In the latter strain background an fdh1Deltafdh2Delta double mutant lacked formate dehydrogenase activity and was unable to co-consume formate. Absence of formate dehydrogenase activity did not affect growth on glucose as sole carbon source, but led to a reduced biomass yield on glucose-formate mixtures. These findings are consistent with a role of formate dehydrogenase in the detoxification of exogenous formate.

Respiration-dependent utilization of sugars in yeasts: a determinant role for sugar transporters

In many yeast species, including Kluyveromyces lactis, growth on certain sugars (such as galactose, raffinose, and maltose) occurs only under respiratory conditions. If respiration is blocked by inhibitors, mutation, or anaerobiosis, growth does not take place. This apparent dependence on respiration for the utilization of certain sugars has often been suspected to be associated with the mechanism of the sugar uptake step. We hypothesized that in many yeast species, the permease activities for these sugars are not sufficient to ensure the high substrate flow that is necessary for fermentative growth. By introducing additional sugar permease genes, we have obtained K. lactis strains that were capable of growing on galactose and raffinose in the absence of respiration. High dosages of both the permease and maltase genes were indeed necessary for K. lactis cells to grow on maltose in the absence of respiration. These results strongly suggest that the sugar uptake step is the major bottleneck in the fermentative assimilation of certain sugars in K. lactis and probably in many other yeasts.

Feedback regulation of glucose transporter gene transcription in Kluyveromyces lactis by glucose uptake

In the respirofermentative yeast Kluyveromyces lactis, only a single genetic locus encodes glucose transporters that can support fermentative growth. This locus is polymorphic in wild-type isolates carrying either KHT1 and KHT2, two tandemly arranged HXT-like genes, or RAG1, a low-affinity transporter gene that arose by recombination between KHT1 and KHT2. Here we show that KHT1 is a glucose-induced gene encoding a low-affinity transporter very similar to Rag1p. Kht2p has a lower K(m) (3.7 mM) and a more complex regulation. Transcription is high in the absence of glucose, further induced by low glucose concentrations, and repressed at higher glucose concentrations. The response of KHT1 and KHT2 gene regulation to high but not to low concentrations of glucose depends on glucose transport. The function of either Kht1p or Kht2p is sufficient to mediate the characteristic response to high glucose, which is impaired in a kht1 kht2 deletion mutant. Thus, the KHT genes are subject to mutual feedback regulation. Moreover, glucose repression of the endogenous beta-galactosidase (LAC4) promoter and glucose induction of pyruvate decarboxylase were abolished in the kht1 kht2 mutant. These phenotypes could be partially restored by HXT gene family members from Saccharomyces cerevisiae. The results indicate that the specific responses to high but not to low glucose concentrations require a high rate of glucose uptake.

RAG4 gene encodes a glucose sensor in Kluyveromyces lactis

The rag4 mutant of Kluyveromyces lactis was previously isolated as a fermentation-deficient mutant, in which transcription of the major glucose transporter gene RAG1 was affected. The wild-type RAG4 was cloned by complementation of the rag4 mutation and found to encode a protein homologous to Snf3 and Rgt2 of Saccharomyces cerevisiae. These two proteins are thought to be sensors of low and high concentrations of glucose, respectively. Rag4, like Snf3 and Rgt2, is predicted to have the transmembrane structure of sugar transporter family proteins as well as a long C-terminal cytoplasmic tail possessing a characteristic 25-amino-acid sequence. Rag4 may therefore be expected to have a glucose-sensing function. However, the rag4 mutation was fully complemented by one copy of either SNF3 or RGT2. Since K. lactis appears to have no other genes of the SNF3/RGT2 type, we suggest that Rag4 of K. lactis may have a dual function of signaling high and low concentrations of glucose. In rag4 mutants, glucose repression of several inducible enzymes is abolished.

Genetics and molecular physiology of the yeast Kluyveromyces lactis

With the recent development of powerful molecular genetic tools, Kluyveromyces lactis has become an excellent alternative yeast model organism for studying the relationships between genetics and physiology. In particular, comparative yeast research has been providing insights into the strikingly different physiological strategies that are reflected by dominance of respiration over fermentation in K. lactis versus Saccharomyces cerevisiae. Other than S. cerevisiae, whose physiology is exceptionally affected by the so-called glucose effect, K. lactis is adapted to aerobiosis and its respiratory system does not underlie glucose repression. As a consequence, K. lactis has been successfully established in biomass-directed industrial applications and large-scale expression of biotechnically relevant gene products. In addition, K. lactis maintains species-specific phenomena such as the "DNA-killer system, " analyses of which are promising to extend our knowledge about microbial competition and the fundamentals of plasmid biology.