The HMGB Protein KlIxr1, a DNA Binding Regulator of Kluyveromyces lactis Gene Expression Involved in Oxidative Metabolism, Growth, and dNTP Synthesis

In the traditional fermentative model yeast Saccharomyces cerevisiae, ScIxr1 is an HMGB (High Mobility Group box B) protein that has been considered as an important regulator of gene transcription in response to external changes like oxygen, carbon source, or nutrient availability. Kluyveromyces lactis is also a useful eukaryotic model, more similar to many human cells due to its respiratory metabolism. We cloned and functionally characterized by different methodologies KlIXR1, which encodes a protein with only 34.4% amino acid sequence similarity to ScIxr1. Our data indicate that both proteins share common functions, including their involvement in the response to hypoxia or oxidative stress induced by hydrogen peroxide or metal treatments, as well as in the control of key regulators for maintenance of the dNTP (deoxyribonucleotide triphosphate) pool and ribosome synthesis. KlIxr1 is able to bind specific regulatory DNA sequences in the promoter of its target genes, which are well conserved between S. cerevisiae and K. lactis. Oppositely, we found important differences between ScIrx1 and KlIxr1 affecting cellular responses to cisplatin or cycloheximide in these yeasts, which could be dependent on specific and non-conserved domains present in these two proteins.

Construction of engineered RuBisCO Kluyveromyces marxianus for a dual microbial bioethanol production system

Ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) genes play important roles in CO₂ fixation and redox balancing in photosynthetic bacteria. In the present study, the kefir yeast Kluyveromyces marxianus 4G5 was used as host for the transformation of form I and form II RubisCO genes derived from the nonsulfur purple bacterium Rhodopseudomonas palustris using the Promoter-based Gene Assembly and Simultaneous Overexpression (PGASO) method. Hungateiclostridium thermocellum ATCC 27405, a well-known bacterium for its efficient solubilization of recalcitrant lignocellulosic biomass, was used to degrade Napier grass and rice straw to generate soluble fermentable sugars. The resultant Napier grass and rice straw broths were used as growth media for the engineered K. marxianus. In the dual microbial system, H. thermocellum degraded the biomass feedstock to produce both C₅ and C₆ sugars. As the bacterium only used hexose sugars, the remaining pentose sugars could be metabolized by K. marxianus to produce ethanol. The transformant RubisCO K. marxianus strains grew well in hydrolyzed Napier grass and rice straw broths and produced bioethanol more efficiently than the wild type. Therefore, these engineered K. marxianus strains could be used with H. thermocellum in a bacterium-yeast coculture system for ethanol production directly from biomass feedstocks.

Reducing antigenicity of bovine whey proteins by Kluyveromyces marxianus fermentation combined with ultrasound treatment

This work investigated the reduction of bovine whey proteins antigenicity by ultrasonic pretreatment and microbial fermentation. Firstly, bovine whey proteins was pretreated by ultrasonic techniques, and its secondary structure was detected by circular dichroism. The pretreated whey proteins was used as the fermentation substrate by Kluyveromyces marxianus for microbial transformation. The single factor design and Box-Behnken Design (BBD) were carried out with the aim to optimize culture temperature, initial pH, inoculum volume and rotation speed. After optimization process, culture temperature, initial pH, inoculum volume and rotation speed were determined. Under culture temperature 35 °C, pH 7.25, inoculum level 10% and shaking speed 150 rpm, the α-LA and β-LG antigenicity in bovine whey proteins were reduced by 29% and 53%, respectively. The findings showed that combined with microbial fermentation for hydrolysis of whey proteins, ultrasonic pretreatment can be used in order to produce hypoallergenic bovine whey proteins.

Substrates' and products' inhibition in fructanase production by a new Kluyveromyces marxianus CF15 from Agave tequilana fructan in a batch reactor

This study focuses on fructanase production in a batch reactor by a new strain isolated from agave juice (K. marxianus var. drosophilarum) employing different Agave tequilana fructan (ATF) concentrations as substrate. The experimental data suggest that the fructanase production may be inhibited or repressed by high substrate (50 g/L) and ethanol (20.7 g/L) concentrations present in culture medium. To further analyze these phenomena an unstructured kinetic mathematical model taking into account substrate and products inhibition was proposed and fitted. The mathematical model considers six reaction kinetics and the ethanol evaporation, and predicts satisfactorily the biomass, fructan, glucose, fructose, ethanol, and fructanase behavior for different raw material initial concentrations. The proposed model is the first to satisfactorily describe the production of fructanase from branched ATF with a new strain of K. marxianus.

Fermentative capabilities of native yeast strains grown on juices from different Agave species used for tequila and mezcal production

The Asparagaceae family is endemic from America, being the Agave genus the most important. The Agave species possess economic relevance and are use as raw material to produce several distilled alcoholic beverages, as bacanora, tequila, and mezcal. The fermentation process has been carry out either spontaneously or by adding a selected yeast strain. The latter is generally responsible for the production of ethanol and volatile compounds. This study comprised five Agave species (A. angustifolia, A. cupreata, A. durangensis, A. salmiana, and A. tequilana) and eight endogenous yeast strains: five of them were non-Saccharomyces (Torulaspora delbrueckii, Zygosaccharomyces bisporus, Candida ethanolica, and two Kluyveromyces marxianus) and three Saccharomyces cerevisiae strains. The results showed that the S. cerevisiae strains were not able to grow on A. durangensis and A. salmiana juices. The Kluyveromyces marxianus strains grew and fermented all the agave juices and displayed high ethanol production (48-52 g L^-1) and volatile compounds. The ethanol production was higher on A. angustifolia juice (1.1-2.8-fold), whereas the volatile compound was dependent on both yeast strain and the Agave species. The use of endogenous non-Saccharomyces yeast strains is feasible, as they may outperform S. cerevisiae regarding the production of fermented beverages from agave plants with a high content of ethanol and aromatic compounds. Graphical abstract.

Identification and Characterization of Oleaginous Yeast Isolated from Kefir and Its Ability to Accumulate Intracellular Fats in Deproteinated Potato Wastewater with Different Carbon Sources

The search for efficient oleaginous microorganisms, which can be an alternative to fossil fuels and biofuels obtained from oilseed crops, has been going on for many years. The suitability of microorganisms in this regard is determined by their ability to biosynthesize lipids with preferred fatty acid profile along with the concurrent utilization of energy-rich industrial waste. In this study, we isolated, characterized, and identified kefir yeast strains using molecular biology techniques. The yeast isolates identified were Candida inconspicua, Debaryomyces hansenii, Kluyveromyces marxianus, Kazachstania unispora, and Zygotorulaspora florentina. We showed that deproteinated potato wastewater, a starch processing industry waste, supplemented with various carbon sources, including lactose and glycerol, is a suitable medium for the growth of yeast, which allows an accumulation of over 20% of lipid substances in its cells. Fatty acid composition primarily depended on the yeast strain and the carbon source used, and, based on our results, most of the strains met the criteria required for the production of biodiesel. In particular, this concerns a significant share of saturated fatty acids, such as C16:0 and C18:0, and unsaturated fatty acids, such as C18:1 and C18:2. The highest efficiency in lipid biosynthesis exceeded 6.3 g L-1. Kazachstania unispora was able to accumulate the high amount of palmitoleic acid.

The evolution of Lachancea thermotolerans is driven by geographical determination, anthropisation and flux between different ecosystems

The yeast Lachancea thermotolerans (formerly Kluyveromyces thermotolerans) is a species with remarkable, yet underexplored, biotechnological potential. This ubiquist occupies a range of natural and anthropic habitats covering a wide geographic span. To gain an insight into L. thermotolerans population diversity and structure, 172 isolates sourced from diverse habitats worldwide were analysed using a set of 14 microsatellite markers. The resultant clustering revealed that the evolution of L. thermotolerans has been driven by the geography and ecological niche of the isolation sources. Isolates originating from anthropic environments, in particular grapes and wine, were genetically close, thus suggesting domestication events within the species. The observed clustering was further validated by several means including, population structure analysis, F-statistics, Mantel’s test and the analysis of molecular variance (AMOVA). Phenotypic performance of isolates was tested using several growth substrates and physicochemical conditions, providing added support for the clustering. Altogether, this study sheds light on the genotypic and phenotypic diversity of L. thermotolerans, contributing to a better understanding of the population structure, ecology and evolution of this non-Saccharomyces yeast.

Sequential application of waste whey as a medium component for Kluyveromyces lactis cultivation and a co-feeder for lipase immobilization by CLEA method

Currently, much attention is paid to technologies which can be drivers of the circular economy across different sectors, in particular, to develop technologies for utilization or reusability of biocompatible materials from industrial waste. One of such is the milk whey, which is a cheap biobased raw material, the disposal of which is a major problem for the dairy industry. Our proposed and investigated technology is based on a continuous exploitation of the whey combining microbiology and biotechnology. Primarily, whey was used as a nutrition source for the cultivation of Kluyveromyces lactis with the aim to produce the targeted biocatalyst-lipase. During cultivation, the whey was transformed into the hydrolyzed form, which was further successfully applied as a protein feeder (external linker) for immobilization of lipase by cross-linked enzyme aggregate (CLEA) method. The first time use of whey as a co-feeder for immobilization of enzymes by CLEA method has shown promising results and increased the stability of lipases for temperature and organic solvents. Hydrolysis of rapeseed oil catalyzed with immobilized derivatives was obtained with 45-96% efficiency at non-optimized conditions. Additionally, the determined kinetic parameters indicated that the rate of p-nitrophenyl palmitate hydrolysis was not changed drastically after immobilization.

Growth, ethanol production, and inulinase activity on various inulin substrates by mutant Kluyveromyces marxianus strains NRRL Y-50798 and NRRL Y-50799

Economically important plants contain large amounts of inulin. Disposal of waste resulting from their processing presents environmental issues. Finding microorganisms capable of converting inulin waste to biofuel and valuable co-products at the processing site would have significant economic and environmental impact. We evaluated the ability of two mutant strains of Kluyveromyces marxianus (Km7 and Km8) to utilize inulin for ethanol production. In glucose medium, both strains consumed all glucose and produced 0.40 g ethanol/g glucose at 24 h. In inulin medium, Km7 exhibited maximum colony forming units (CFU)/mL and produced 0.35 g ethanol/g inulin at 24 h, while Km8 showed maximum CFU/mL and produced 0.02 g ethanol/g inulin at 96 h. At 24 h in inulin + glucose medium, Km7 produced 0.40 g ethanol/g (inulin + glucose) and Km8 produced 0.20 g ethanol/g (inulin + glucose) with maximum CFU/mL for Km8 at 72 h, 40 % of that for Km7 at 36 h. Extracellular inulinase activity at 6 h for both Km7 and Km8 was 3.7 International Units (IU)/mL.

Easy to use and reliable technique for online dissolved oxygen tension measurement in shake flasks using infrared fluorescent oxygen-sensitive nanoparticles

BACKGROUND

Despite the progressive miniaturization of bioreactors for screening purposes, shake flasks are still widespread in biotechnological laboratories and industry as cultivation vessels. Shake flasks are often applied as the first or second step in applications such as strain screening or media optimization. Thus, there are ongoing efforts to develop online measurement techniques for shake flasks, to gain as much information as possible about the cultured microbial system. Since dissolved oxygen tension (DOT) is a key experimental parameter, its accurate determination during the course of experiment is critical. Some of the available DOT measurement techniques can lead to erroneous measurements or are very difficult to handle. A reliable and easy to use DOT measurement system, based on suspended oxygen-sensitive nanoparticles, is presented in this work.

RESULTS

In a cultivation of Kluyveromyces lactis, a new DOT measurement technique via suspended oxygen-sensitive nanoparticles was compared with the conventional DOT measurement via fixed sensor spots. These experiments revealed the main disadvantage of applying sensor spots. With further cultivations of Escherichia coli and Hansenula polymorpha, the new measurement technique was successfully validated. In combination with a RAMOS device, kLa values were determined during the presented cultivations. The determined kLa values are in good agreement with a correlation recently found in the literature.

CONCLUSIONS

The presented DOT measurement technique via suspended oxygen-sensitive nanoparticles in shake flasks turned out to be easy to use, robust and reliable under all applied combinations of shaking frequencies and filling volumes. Its applicability as an online monitoring system for cultivations was shown by means of four examples. Additionally, in combination with a RAMOS device, the possibility of experimental kLa determination was successfully demonstrated.

Bioethanol production from taro waste using thermo-tolerant yeast Kluyveromyces marxianus K21

In the present study, evaluation and optimization of taro waste (TW), which was mainly composed of taro peels that contain many starch residues, as the main carbon source in medium were studied. The flask studies showed the optimal medium was using 170g/L of TW which is about 100g/L of glucose and 9g/L of CGM as alternative nitrogen source. Simultaneous saccharification and fermentation (SSF) exhibited higher bioethanol productivity toward separation hydrolysis and fermentation (SHF). The optimal condition of SSF was 5% of Kluyveromyces marxianus K21 inoculum at 40°C resulting in the maximum ethanol concentration (48.98g/L) and productivity (2.23g/L/h) after 22h of cultivation. The scaling up experiment in a 5L bioreactor demonstrated that K21 can still maintain its capability. After 20h of cultivation, 43.78g/L of ethanol (2.19g/L/h of productivity) was achieved corresponding to a 94.2% theoretical ethanol yield.

Protein enrichment of an Opuntia ficus-indica cladode hydrolysate by cultivation of Candida utilis and Kluyveromyces marxianus

BACKGROUND

The cladodes of Opuntia ficus-indica (prickly pear cactus) have a low protein content; for use as a balanced feed, supplementation with other protein sources is therefore desirable. We investigated protein enrichment by cultivation of the yeasts Candida utilis and Kluyveromyces marxianus in an enzymatic hydrolysate of the cladode biomass.

RESULTS

Dilute acid pretreatment and enzymatic hydrolysis of sun-dried cladodes resulted in a hydrolysate containing (per litre) 45.5 g glucose, 6.3 g xylose, 9.1 g galactose, 10.8 g arabinose and 9.6 g fructose. Even though K. marxianus had a much higher growth rate and utilized l-arabinose and d-galactose more completely than C. utilis, its biomass yield coefficient was lower due to ethanol and ethyl acetate production despite aerobic cultivation. Yeast cultivation more than doubled the protein content of the hydrolysate, with an essential amino acid profile superior to sorghum and millet grains.

CONCLUSIONS

This K. marxianus strain was weakly Crabtree positive. Despite its low biomass yield, its performance compared well with C. utilis. This is the first report showing that the protein content and quality of O. ficus-indica cladode biomass could substantially be improved by yeast cultivation, including a comparative evaluation of C. utilis and K. marxianus.

Sustainable conversion of coffee and other crop wastes to biofuels and bioproducts using coupled biochemical and thermochemical processes in a multi-stage biorefinery concept

The environmental impact of agricultural waste from the processing of food and feed crops is an increasing concern worldwide. Concerted efforts are underway to develop sustainable practices for the disposal of residues from the processing of such crops as coffee, sugarcane, or corn. Coffee is crucial to the economies of many countries because its cultivation, processing, trading, and marketing provide employment for millions of people. In coffee-producing countries, improved technology for treatment of the significant amounts of coffee waste is critical to prevent ecological damage. This mini-review discusses a multi-stage biorefinery concept with the potential to convert waste produced at crop processing operations, such as coffee pulping stations, to valuable biofuels and bioproducts using biochemical and thermochemical conversion technologies. The initial bioconversion stage uses a mutant Kluyveromyces marxianus yeast strain to produce bioethanol from sugars. The resulting sugar-depleted solids (mostly protein) can be used in a second stage by the oleaginous yeast Yarrowia lipolytica to produce bio-based ammonia for fertilizer and are further degraded by Y. lipolytica proteases to peptides and free amino acids for animal feed. The lignocellulosic fraction can be ground and treated to release sugars for fermentation in a third stage by a recombinant cellulosic Saccharomyces cerevisiae, which can also be engineered to express valuable peptide products. The residual protein and lignin solids can be jet cooked and passed to a fourth-stage fermenter where Rhodotorula glutinis converts methane into isoprenoid intermediates. The residues can be combined and transferred into pyrocracking and hydroformylation reactions to convert ammonia, protein, isoprenes, lignins, and oils into renewable gas. Any remaining waste can be thermoconverted to biochar as a humus soil enhancer. The integration of multiple technologies for treatment of coffee waste has the potential to contribute to economic and environmental sustainability.

Mixed culture of Kluyveromyces marxianus and Candida krusei for single-cell protein production and organic load removal from whey

The study was conducted to evaluate the potential of mixed culture of Kluyveromyces marxianus and Candida krusei to enhance COD removal efficiency, minimize contamination at extreme conditions (high temperature 40°C and low pH 3.5) during batch and continuous aerobic fermentation and to obtain improved quality single-cell protein (SCP) using whey as substrate. The batch fermentation of mono-culture and mixed culture result showed that the mixed culture resulted in 8.8% higher COD removal efficacy with 19% higher biomass yield and 33% increased productivity. The maximum COD removal 80.2% (including residual protein) was obtained at 24h HRT with biomass productivity of 0.17 g/L/h; however, maximum biomass productivity of 0.38 g/L/h and 34% COD removal were obtained at 6h HRT. The results showed that the mixed culture of acid resistance and thermo-tolerant yeasts was a potential way to produce SCP (animal feed) and simultaneous COD removal under extreme operating conditions.

Production of human lysozyme in biofilm reactor and optimization of growth parameters of Kluyveromyces lactis K7

Lysozyme (1,4-β-N-acetylmuramidase) is a lytic enzyme, which degrades the bacterial cell wall. Lysozyme has been of interest in medicine, cosmetics, and food industries because of its anti-bactericidal effect. Kluyveromyces lactis K7 is a genetically modified organism that expresses human lysozyme. There is a need to improve the human lysozyme production by K. lactis K7 to make the human lysozyme more affordable. Biofilm reactor provides high biomass by including a solid support, which microorganisms grow around and within. Therefore, the aim of this study was to produce the human lysozyme in biofilm reactor and optimize the growth conditions of K. lactis K7 for the human lysozyme production in biofilm reactor with plastic composite support (PCS). The PCS, which includes polypropylene, soybean hull, soybean flour, bovine albumin, and salts, was selected based on biofilm formation on PCS (CFU/g), human lysozyme production (U/ml), and absorption of lysozyme inside the support. To find the optimum combination of growth parameters, a three-factor Box-Behnken design of response surface method was used. The results suggested that the optimum conditions for biomass and lysozyme productions were different (27 °C, pH 6, 1.33 vvm for biomass production; 25 °C, pH 4, no aeration for lysozyme production). Then, different pH and aeration shift strategies were tested to increase the biomass at the first step and then secrete the lysozyme after the shift. As a result, the lysozyme production amount (141 U/ml) at 25 °C without pH and aeration control was significantly higher than the lysozyme amount at evaluated pH and aeration shift conditions (p < 0.05).

Low pH D-xylonate production with Pichia kudriavzevii

D-xylonic acid is one of the top 30 most desirable chemicals to be derived from biomass sugars identified by the US Department of Energy, being applicable as a non-food substitute for D-gluconic acid and as a platform chemical. We engineered the non-conventional yeast Pichia kudriavzevii VTT C-79090T to express a D-xylose dehydrogenase coding gene from Caulobacter crescentus. With this single modification the recombinant P. kudriavzevii strain produced up to 171 g L(-1) of D-xylonate from 171 g L(-1) D-xylose at a rate of 1.4 g L(-1) h(-1) and yield of 1.0 g [g substrate consumed](-1), which was comparable with D-xylonate production by Gluconobacter oxydans or Pseudomonas sp. The productivity of the strain was also remarkable at low pH, producing 146 g L(-1) D-xylonate at 1.2 g L(-1) h(-1) at pH 3.0. This is the best low pH production reported for D-xylonate. These results encourage further development towards industrial scale production.

[Fungicidal activity of yeast isolated from chal (camel cultured milk product)]

A Kluyveromyces strain secreting a fungicidal proteinaceous toxin has been isolated. Its maximal activity is observed at pH 5.0 and an increased osmotic pressure. This agent has been identified as a mycocin; it is active towards species belonging to the genus Kluyveromyces and some representatives of taxonomically related groups.

Growth of the yeast Kluyveromyces marxianus CBS 6556 on different sugar combinations as sole carbon and energy source

The yeast Kluyveromyces marxianus has been pointed out as a promising microorganism for a variety of industrial bioprocesses. Although genetic tools have been developed for this yeast and different potential applications have been investigated, quantitative physiological studies have rarely been reported. Here, we report and discuss the growth, substrate consumption, metabolite formation, and respiratory parameters of K. marxianus CBS 6556 during aerobic batch bioreactor cultivations, using a defined medium with different sugars as sole carbon and energy source, at 30 and 37 °C. Cultivations were carried out both on single sugars and on binary sugar mixtures. Carbon balances closed within 95 to 101 % in all experiments. Biomass and CO2 were the main products of cell metabolism, whereas by-products were always present in very low proportion (<3 % of the carbon consumed), as long as full aerobiosis was guaranteed. On all sugars tested as sole carbon and energy source (glucose, fructose, sucrose, lactose, and galactose), the maximum specific growth rate remained between 0.39 and 0.49 h(-1), except for galactose at 37 °C, which only supported growth at 0.31 h(-1). Different growth behaviors were observed on the binary sugar mixtures investigated (glucose and lactose, glucose and galactose, lactose and galactose, glucose and fructose, galactose and fructose, fructose and lactose), and the observations were in agreement with previously published data on the sugar transport systems in K. marxianus. We conclude that K. marxianus CBS 6556 does not present any special nutritional requirements; grows well in the range of 30 to 37 °C on different sugars; is capable of growing on sugar mixtures in a shorter period of time than Saccharomyces cerevisiae, which is interesting from an industrial point of view; and deviates tiny amounts of carbon towards metabolite formation, as long as full aerobiosis is maintained.

Essentiality of respiratory activity for pentose utilization in thermotolerant yeast Kluyveromyces marxianus DMKU 3-1042

By random integrative mutagenesis with a kanMX4 cassette in Kluyveromyces marxianus DMKU 3-1042, we obtained three mutants of COX15, ATP25 and CYC3 encoding a cytochrome oxidase assembly factor (singleton), a transcription factor required for assembly of the Atp9p subunit of mitochondrial ATP synthase and cytochrome c heme lyase, respectively, as mutants lacking growth capability on xylose and/or arabinose. They exhibited incapability of growth on non-fermentable carbon sources, such as acetate or glycerol, and thermosensitiveness. Their biomass formation in glucose medium was reduced, but ethanol yields were increased with a high ethanol level in the medium, compared to those of the parental strain. Experiments with respiratory inhibitors showed that cox15 and cyc3, but not atp25, were able to grow in glucose medium containing antimycin A and that the atp25 mutant was KCN-resistant. Activities of NADH and ubiquinol oxidases in membrane fractions of each mutant became a half of that of the parent and negligible, respectively, and their remaining NADH oxidase activities were found to be resistant to KCN. Absolute absorption spectral analysis revealed that the peak corresponding to a + a 3 was very small in atp25 and negligible in cox15 and cyc3. These findings suggest that the K. marxianus strain possesses an alternative KCN-resistant oxidase that is located between primary dehydrogenases and the ubiquinone pool and that the respiratory activity is essential for utilization of pentoses.

[Mycocin sensitivity patterns of the species Kluyveromyces: Kluyveromyces sensu lato vs. Kluyveromyces sensu stricto]

The Kluyveromyces species reassigned to the genera Lachancea and Vanderwaltozyma are insensitive to five mycocins secreted by Pichiamembranifaciens. The remaining Kluyveromyces species including species transferred to the genera Kazachstania, Nakaseomyces, and Tetrapisispora are sensitive to them. Only the neotype strain is insensitive to mycocins among Kluyveromyceslactis cultures.

The influence of presaccharification, fermentation temperature and yeast strain on ethanol production from sugarcane bagasse

Ethanol can be produced from cellulosic biomass in a process known as simultaneous saccharification and fermentation (SSF). The presence of yeast together with the cellulolytic enzyme complex reduces the accumulation of sugars within the reactor, increasing the ethanol yield and saccharification rate. This paper reports the isolation of Saccharomyces cerevisiae LBM-1, a strain capable of growth at 42 °C. In addition, S. cerevisiae LBM-1 and Kluyveromyces marxianus UFV-3 were able to ferment sugar cane bagasse in SSF processes at 37 and 42 °C. Higher ethanol yields were observed when fermentation was initiated after presaccharification at 50°C than at 37 or 42° C. Furthermore, the volumetric productivity of fermentation increased with presaccharification time, from 0.43 g/L/h at 0 h to 1.79 g/L/h after 72 h of presaccharification. The results suggest that the use of thermotolerant yeasts and a presaccharification stage are key to increasing yields in this process.

Trehalose overproduction affects the stress tolerance of Kluyveromyces marxianus ambiguously

A thermotolerant Kluyveromyces marxianus mutant was developed by exposing yeast cultures repeatedly to 48°C incubation temperature, and the strain was characterized with a significantly increased trehalose content. Unexpectedly, the strain was sensitive to alcohol, osmotic and oxidative stress, which correlated with the increases in the trehalose concentrations. Intracellular glutathione levels declined in both wild-type and mutant cells when exposed to elevating incubation temperatures. Finally, we reached the surprising conclusion that neither trehalose nor glutathione metabolisms should be aimed at in future strain development programs with K. marxianus.

Disruption of PMR1 in Kluyveromyces lactis improves secretion of calf prochymosin

BACKGROUND

Chymosin is an important industrial enzyme widely used in cheese manufacturing. Kluyveromyces lactis is a promising host strain for expression of the chymosin gene. However, only low yields of chymosin (80 U mL(-1) in shake flask culture) have been obtained using K. lactis GG799. The aim of this study was to increase the amount of recombinant calf chymosin secreted by K. lactis GG799 by disrupting the PMR1 gene.

RESULTS

Kluyveromyces lactis GG799 harbouring the disrupted PMR1 gene showed reduced growth in ethylene glycol tetraacetic acid-containing and Ca(2+) -deficient medium, but Ca(2+) supplementation eliminated the growth problem. The calf chymosin gene was ligated into the K. lactis GG799 expression vector, generating the plasmid pKLAC1-N-prochymosin. The linearised plasmid was homologously integrated into the genome of K. lactis GG799. In shake flask culture, chymosin activity was 496 U mL(-1) in the K. lactis PMR1-deficient mutant, sixfold higher than that in wild-type K. lactis GG799.

CONCLUSION

Disrupting the PMR1 gene improved chymosin production in K. lactis GG799 sixfold. This knowledge could be applied to industrial chymosin production.

Evolutionary aspects of a genetic network: studying the lactose/galactose regulon of Kluyveromyces lactis

The budding yeast Kluyveromyces lactis has diverged from the Saccharomyces lineage before the whole-genome duplication and its genome sequence reveals lower redundancy of many genes. Moreover, it shows lower preference for fermentative carbon metabolism and a broader substrate spectrum making it a particularly rewarding system for comparative and evolutionary studies of carbon-regulated genetic networks. The lactose/galactose regulon of K. lactis, which is regulated by the prototypic transcription activator Gal4 exemplifies important aspects of network evolution when compared with the model GAL regulon of Saccharomyces cerevisiae. Differences in physiology relate to different subcellular compartmentation of regulatory components and, importantly, to quantitative differences in protein-protein interactions rather than major differences in network architecture. Here, we introduce genetic and biochemical tools to study K. lactis in general and the lactose/galactose regulon in particular. We present methods to quantify relevant protein-protein interactions in that network and to visualize such differences in simple plate assays allowing for genetic approaches in further studies.

Minimal models of growth and decline of microbial populations

Dynamics of growth and decline of microbial populations were analysed and respective models were developed in this investigation. Analysis of the dynamics was based on general considerations concerning the main properties of microorganisms and their interactions with the environment which was supposed to be affected by the activity of the population. Those considerations were expressed mathematically by differential equations or systems of the equations containing minimal sets of parameters characterizing those properties. It has been found that: (1) the factors leading to the decline of the population have to be considered separately, namely, accumulation of metabolites (toxins) in the medium and the exhaustion of resources; the latter have to be separated again into renewable ('building materials') and non-renewable (sources of energy); (2) decline of the population is caused by the exhaustion of sources of energy but no decline is predicted by the model because of the exhaustion of renewable resources; (3) the model determined by the accumulation of metabolites (toxins) in the medium does not suggest the existence of a separate 'stationary phase'; (4) in the model determined by the exhaustion of energy resources the 'stationary' and 'decline' phases are quite discernible; and (5) there is no symmetry in microbial population dynamics, the decline being slower than the rise. Mathematical models are expected to be useful in getting insight into the process of control of the dynamics of microbial populations. The models are in agreement with the experimental data.

Soybean toxin (SBTX), a protein from soybeans that inhibits the life cycle of plant and human pathogenic fungi

Soybean toxin (SBTX) is a 44 kDa glycoprotein that is lethal to mice (LD(50) = 5.6 mg/kg). This study reports the toxicity of SBTX on pathogenic fungi and yeasts and the mechanism of its action. SBTX inhibited spore germination of Aspergillus niger and Penicillium herguei and was toxic to Candida albicans, Candida parapsilosis, Kluyveromyces marxiannus , Pichia membranifaciens, and Saccharomyces cerevisiae. In addition, SBTX hampered the growth of C. albicans and K. marxiannus and inhibited the glucose-stimulated acidification of the incubation medium by S. cerevisiae, suggesting that SBTX interferes with intracellular proton transport to the external medium. Moreover, SBTX caused cell-wall disruption, condensation/shrinkage of cytosol, pseudohyphae formation, and P. membranifaciens and C. parapsilosis cell death. SBTX is toxic to fungi at concentrations far below the dose lethal to mice and has potential in the design of new antifungal drugs or in the development of transgenic crops resistant to pathogens.

[Gene synthesis of the bovine prochymosin gene and high-level expression in Kluyvermyces lactis]

Chymosin is an important industrial enzyme widely used in cheese manufacture. To improve expression efficiency of recombinant bovine chymosin in Kluyveromyces lactis strain GG799, we designed and synthesized a DNA sequence encoding bovine prochymosin gene (GenBank Accession No. AA30448) by using optimized codons. The synthesized prochymosin gene was amplified by two-step PCR method, and then cloned into the expression vector pKLAC1, resulting in pKLAC1-Prochy. pKLAC1-Prochy was linearized and transformed into K. lactis GG799 by electrotransformation. Positive clones were screened by YEPD plates containing 1% casein. A recombinant strain chyl with highest activities and multi-copy integration which was detected by using specifical integration primers was chosen and fermented in flask. Prochymosin was expressed in K. lactis successfully. SDS-PAGE analysis revealed that the purified recombinant bovine prochymosin had a molecular mass of 41 kDa. After acid treatment, molecular weight of chymosin is about 36 kDa, the same as native bovine chymosin. Activity tests showed that the chymosin activity of the culture supernatant was 99.67 SU/mL after 96 h cultivation. The activities of chymosin were not prominent increased when galactose was used as carbon source instead of glucose, which proved that the fermentation of recombinant strain does not need galactose inducing. The recombinant K. lactis strain obtained in this study could be further used to produce recombinant chymosin for cheese making.

Mathematical modeling of Kluyveromyces marxianus growth in solid-state fermentation using a packed-bed bioreactor

This work investigated the growth of Kluyveromyces marxianus NRRL Y-7571 in solid-state fermentation in a medium composed of sugarcane bagasse, molasses, corn steep liquor and soybean meal within a packed-bed bioreactor. Seven experimental runs were carried out to evaluate the effects of flow rate and inlet air temperature on the following microbial rates: cell mass production, total reducing sugar and oxygen consumption, carbon dioxide and ethanol production, metabolic heat and water generation. A mathematical model based on an artificial neural network was developed to predict the above-mentioned microbial rates as a function of the fermentation time, initial total reducing sugar concentration, inlet and outlet air temperatures. The results showed that the microbial rates were temperature dependent for the range 27-50 degrees C. The proposed model efficiently predicted the microbial rates, indicating that the neural network approach could be used to simulate the microbial growth in SSF.

High-temperature ethanol fermentation and transformation with linear DNA in the thermotolerant yeast Kluyveromyces marxianus DMKU3-1042

We demonstrate herein the ability of Kluyveromyces marxianus to be an efficient ethanol producer and host for expressing heterologous proteins as an alternative to Saccharomyces cerevisiae. Growth and ethanol production by strains of K. marxianus and S. cerevisiae were compared under the same conditions. K. marxianus DMKU3-1042 was found to be the most suitable strain for high-temperature growth and ethanol production at 45 degrees C. This strain, but not S. cerevisiae, utilized cellobiose, xylose, xylitol, arabinose, glycerol, and lactose. To develop a K. marxianus DMKU3-1042 derivative strain suitable for genetic engineering, a uracil auxotroph was isolated and transformed with a linear DNA of the S. cerevisiae ScURA3 gene. Surprisingly, Ura(+) transformants were easily obtained. By Southern blot hybridization, the linear ScURA3 DNA was found to have inserted randomly into the K. marxianus genome. Sequencing of one Lys(-) transformant confirmed the disruption of the KmLYS1 gene by the ScURA3 insertion. A PCR-amplified linear DNA lacking K. marxianus sequences but containing an Aspergillus alpha-amylase gene under the control of the ScTDH3 promoter together with an ScURA3 marker was subsequently used to transform K. marxianus DMKU3-1042 in order to obtain transformants expressing Aspergillus alpha-amylase. Our results demonstrate that K. marxianus DMKU3-1042 can be an alternative cost-effective bioethanol producer and a host for transformation with linear DNA by use of S. cerevisiae-based molecular genetic tools.

Effect of the yeast and bacteria biomass on the microbiota in the rumen

This study aims at obtaining a probiotic product based on viable biomass from 6 yeast strains and 2 strains of lactic bacteria used for nutrition of animals. The strains are subjected to some resistance tests, at temperature, pH, pepsin, pancreatin and biliary salts so as to make obvious their viability. Tests were done by comparison to the witness strain and respectively a protective solution based on mucin and casein. Based on the resulted viabilities 2 products are formulated. Their effect is tested by inoculating fresh rumen content and supervising the microbic balance for a period of 12 days. After the final tests, it resulted that the product Fpl (20% Saccharomyces cerevisiae 1-29, 10% Kluyveromyces marxianus R-CS, 20% Issatchenkia orientalis R-BC, 30% Lactobacillus paracasei CMGB16, 20% Enterococcus faecium GM8) was chosen because anaerobic strains were preponderant as a consequence of the tests performed with rumen.

Lactic acid production by mixed cultures of Kluyveromyces marxianus, Lactobacillus delbrueckii ssp. bulgaricus and Lactobacillus helveticus

Lactic acid production using Kluyveromyces marxianus (IFO 288), Lactobacillus delbrueckii ssp. bulgaricus (ATCC 11842) and Lactobacillus helveticus (ATCC 15009) individually or as mixed culture on cheese whey in stirred or static fermentation conditions was evaluated. Lactic acid production, residual sugar and cell biomass were the main features examined. Increased lactic acid production was observed, when mixed cultures were used in comparison to individual ones. The highest lactic acid concentrations were achieved when K. marxianus yeast was combined with L. delbrueckii ssp. bulgaricus, and when all the strains were used revealing possible synergistic effects between the yeast and the two lactic acid bacteria. The same synergistic effects were further observed and verified when the mixed cultures were applied in sourdough fermentations, proving that the above microbiological system could be applied in the food fermentations where high lactic acid production is sought.

Transition of the ability to generate petites in theSaccharomyces/Kluyveromycescomplex

Petite-positivity - the ability to tolerate the loss of mtDNA - was examined after the treatment with ethidium bromide (EB) in over hundred isolates from the Saccharomyces/Kluyveromyces complex. The identity of petite mutants was confirmed by the loss of specific mtDNA DAPI staining patterns. Besides unequivocal petite-positive and petite-negative phenotypes, a few species exhibited temperature sensitive petite positive phenotype and petiteness of a few other species could be observed only at the elevated EB concentrations. Several yeast species displayed a mixed 'moot' phenotype, where a major part of the population did not tolerate the loss of mtDNA but several cells did. The genera from postwhole-genome duplication lineages (Saccharomyces, Kazachstania, Naumovia, Nakaseomyces) were invariably petite-positive. However, petite-positive traits could also be observed among the prewhole-genome duplication species.

Mixed cultures ofSerratia marcescensandKluyveromyces fragilisfor simultaneous protease production and COD removal of whey

AIMS

The aim of this study was to investigate the behaviour of a Serratia marcescens-Kluyveromyces fragilis mixed culture in whey, with the objective of proteases production and organic waste reduction.

METHODS AND RESULTS

Discontinuous aerobic fermentations in whey were carried out using individual pure cultures and mixed cultures of S. marcescens and K. fragilis. Cell growth, protease production, lactose and proteins consumption and COD/TOC reduction were monitored. Lactose and protein content of the fermenting medium was almost depleted in the mixed cultures, achieving a reduction in the organic content much higher than in both pure cultures. Interestingly, proteolytic activity in the mixed cultures was similar to that obtained for S. marcescens in pure culture. In addition, protease stability was increased in the mixed cultures. Kinetic models were developed fitting well with the experimental results.

CONCLUSIONS

Mixed cultures were found to maintain the achievements of each individual fermentation, yielding a high and stable production of proteases and a significant reduction of COD/TOC.

SIGNIFICANCE AND IMPACT OF THE STUDY

Mixed cultures tested in this work have shown a synergistic effect with possible industrial applications. These results lead to a gain in the chain value for enzyme production with an environmentally friendly operation.

Comparison of yeast strains for batch ethanol fermentation of cheese-whey powder (CWP) solution

AIMS

To test the suitability of cheese whey powder (CWP) solution for ethanol fermentation and to compare performances of different Kluyveromyces marxianus strains for ethanol fermentation from CWP solution.

METHODS AND RESULTS

Batch ethanol fermentation of cheese whey (CW), CWP and lactose solutions with the same initial sugar contents were compared by using two different K. marxianus strains and the CWP solution was found to be the most suitable substrate. CWP solution was fermented to ethanol using three different yeast strains and DSMZ-7239 was found to be the most suitable one yielding the highest rate and extent (3.3%, v/v) of ethanol formation.

CONCLUSIONS

CWP solution and K. marxianus strain of DSMZ-7239 were found to be more suitable for ethanol fermentation with the highest ethanol yield when compared with the other substrates and the yeast strains tested.

SIGNIFICANCE AND IMPACT OF THE STUDY

CWP can be used as a concentrated form of CW for ethanol fermentations with considerable advantages.

Identification and characterization of a novel glucose-phosphorylating enzyme inKluyveromyces lactis

Recent data suggest that hexokinase KlHxk1 (Rag5) represents the only glucose-phosphorylating enzyme of Kluyveromyces lactis, which also is required for glucose signalling. Long-term growth studies of a K. lactis rag5 mutant, however, reveal slow growth on glucose, but no growth on fructose. Isolation of the permissive glucose-phosphorylating enzyme, mass spectrometric tryptic peptide analysis and determination of basic kinetic data identify a novel glucokinase (KlGlk1) encoded by ORF KLLA0C01,155g. In accordance with the growth characteristics of the rag5 mutant, KlGlk1 phosphorylates glucose, but fails to act on fructose as a sugar substrate. Multiple sequence alignment indicates the presence of at least one glucokinase gene in all sequenced yeast genomes.

An approach to the hypoxic and oxidative stress responses inKluyveromyces lactisby analysis of mRNA levels

Genome duplication, after the divergence of Saccharomyces cerevisiae from Kluyveromyces lactis along evolution, has been proposed as a mechanism of yeast evolution from strict aerobics, such as Candida albicans, to facultatives/fermentatives, such as S. cerevisiae. This feature, together with the preponderance of respiration and the use of the pentose phosphate pathway in glucose utilization, makes K. lactis a model yeast for studies related to carbon and oxygen metabolism. In this work, and based on the knowledge of the sequence of the genome of K. lactis, obtained by the Génolevures project, we have constructed DNA arrays from K. lactis including a limited amount of selected probes. They are related to the aerobiosis-hypoxia adaptation and to the oxidative stress response, and have been used to test changes in mRNA levels in response to hypoxia and oxidative stress generated by H(2)O(2). The study was carried out in both wild-type and rag2 mutant K. lactis strains in which glycolysis is blocked at the phosphoglucose isomerase step. This approach is the first analysis carried out in K. lactis for the majority of the genes selected.

The F1-ATPase inhibitor Inh1 (IF1) affects suppression of mtDNA loss-lethality in Kluyveromyces lactis

Loss of mtDNA by the petite-negative yeast Kluyveromyces lactis is lethal (rho(o)-lethality). However, mutations in the alpha, beta and gamma subunits of F(1)-ATPase can suppress lethality by increasing intramitochondrial hydrolysis of ATP. Increased hydrolysis of ATP can also occur on inactivation of Inh1, the natural inhibitor of F(1)-ATPase. However, not all strains of K. lactis show suppression of rho(o)-lethality on inactivation of INH1. Genetic analysis indicates that one or more alleles of modifying factors are required for suppression. Papillae showing enhanced resistance to ethidium bromide (EB) in INH1 disruptants have mutations in the alpha, beta and gamma subunits of F(1)-ATPase. Increased growth of double mutants on EB has been investigated by disruption of INH1 in previously characterized atp suppressor mutants. Inactivation of Inh1, with one exception, results in better growth on EB and increased F(1)-ATPase activity, indicating that suppression of rho(o)-lethality is not due to atp mutations preventing Inh1 from interacting with the F(1)-complex. By contrast, in suppressor mutants altered in Arg435 of the beta subunit, disruption of INH1 did not change the kinetic properties of F(1)-ATPase or alter growth on EB. Consequently, Arg435 appears to be required for interaction of Inh1 with the beta subunit. In a previous study, a mex1-1 allele was found to enhance mgi(atp) expression. In accord with results from double mutants, it has been found that mex1-1 is a frameshift mutation in INH1 causing inactivation of Inh1p.

Mutations of theRAG3gene encoding a regulator of fermentation inKluyveromyces lactisare suppressed by a mutation of the transcription factor geneKlGCR1

In Kluyveromyces lactis, Rag3 regulates both fermentative metabolism and thiamine biosynthesis. Regulation of fermentation is exerted at the level of transcription of KlPDC1. We have isolated and identified a mutation of the transcription factor KlGCR1, Klgcr1-1, which suppressed the fermentative-deficient phenotype associated with the RAG3 deletion. In the mutant, the transcription of KlPDC1 was restored. However, we found that the suppression was not specific to the RAG3 mutation, as the Klgcr1-1 mutation could also suppress the fermentative defect associated with mutation of Sck1, another regulator of glycolysis.

Application of the Cre-loxP system for multiple gene disruption in the yeast Kluyveromyces marxianus

The yeast Kluyveromyces marxianus presents several interesting features that make this species a promising industrial yeast for the production of several compounds. In order to take full advantage of this yeast and its particular properties, proper tools for gene disruption and metabolic engineering are needed. The Cre-loxP system is a very versatile tool that allows for gene marker rescue, resulting in mutant strains free of exogenous selective markers, which is a very important aspect for industrial application. As the Cre-loxP system works in some non-conventional yeasts, namely Kluyveromyces lactis, we wished to know whether it also works in K. marxianus. Here, we report the validation of this system in K. marxianus CBS 6556, by disrupting two copies of the LAC4 gene, which encodes a beta-galactosidase activity.

Identification and characterization of yeast isolated from the elaboration of seasoned green table olives

The purpose of this study was to investigate the yeast population during the processing of green table olives. In the fresh olives, yeast were found at concentrations of around 3.0 log cfu/g, with Cryptococcus spp. being predominant. In the brine, the yeast concentrations were greater than 4.9 log cfu/ml, with Pichia anomala, Kluyveromyces marxianus, and Saccharomyces cerevisiae being the predominant species. Unlike the yeast isolated from the fresh olives, the strains obtained from the olive brine mostly showed low pectolytic but high catalase activities. Some of these strains also exhibited other biochemical desirable properties for the fermentation of green table olives, including their lipolytic activities and their assimilation or production of organic acids in the brine. Seven strains in particular of P. anomala, K. marxianus, S. cerevisiae, and Candida maris showed the best properties for use in trials as starter culture in pilot fermenters.

A new regulatory element mediates ethanol repression of KlADH3, a Kluyveromyces lactis gene coding for a mitochondrial alcohol dehydrogenase

KlADH3 is a Kluyveromyces lactis alcohol dehydrogenase gene induced in the presence of all respiratory carbon sources except ethanol, which specifically represses this gene. Deletion analysis of the KlADH3 promoter revealed the presence of both positive and negative elements. However, by site-directed mutagenesis and gel retardation experiments, we identified a 15-bp element responsible for the transcriptional repression of this gene by ethanol. In particular, this element showed putative sites required for the sequential binding of ethanol-induced factors responsible for the repressed conditions, and the binding of additional factors relieved repression. In addition, we showed that the ethanol element was required for in vivo repression of KlAdh3 activity.

Physiology of the yeastKluyveromyces marxianusduring batch and chemostat cultures with glucose as the sole carbon source

Growth, substrate consumption, metabolite formation, biomass composition and respiratory parameters of Kluyveromyces marxianus ATCC 26548 were determined during aerobic batch and chemostat cultivations, using mineral medium with glucose as the sole carbon source, at 30 degrees C and pH 5.0. Carbon balances closed within 95-101% in all experiments. A maximum specific growth rate of 0.56 h(-1), a biomass yield on glucose of 0.51 g g(-1), and a maximum specific consumption of oxygen of 11.1 mmol g(-1) h(-1) were obtained during batch cultures. The concentration of excreted metabolites was very low at the culture conditions applied, representing 6% of the consumed carbon at most. Acetate and pyruvate were excreted to a larger extent than ethanol under the batch conditions, and the protein content accounted for 54.6% of the biomass dry weight. Steady states were obtained during chemostats at dilution rates of 0.1, 0.25 and 0.5 h(-1). At the two former dilution rates, cells grew at carbon limitation and the biomass yield on glucose was similar to that obtained under the batch conditions. Metabolite formation was rather low, accounting for a total of 0.005 C-mol C-mol(-1) substrate. At 0.5 h(-1), although the biomass yield on glucose was similar to the value obtained under the above-mentioned conditions, the cultivation was not under carbon limitation. Under this condition, 2-oxoglutarate, acetate, pyruvate and ethanol were the prevalent metabolites excreted. Total metabolite formation only accounted to 0.056 C-mol C-mol(-1) of substrate. A very high protein and a low carbohydrate content (71.9% and 9.6% of biomass dry weight, respectively) were measured in cells under this condition. It is concluded that K. marxianus aligns with the so-called aerobic-respiring or Crabtree-negative yeasts. Furthermore, it has one of the highest growth rates among yeasts, and a high capacity of converting sugar into biomass, even when carbon is not the limiting nutrient. These results provide useful data regarding the future application of K. marxianus in processes aimed at the production of biomass-linked compounds, with high yields and productivities.

The apyrase KlYnd1p of Kluyveromyces lactis affects glycosylation, secretion, and cell wall properties

The Kluyveromyces lactis ORF r_klactIV3,463 on chromosome IV, hereafter named KlYND1, encodes an endoapyrase that has nucleoside phosphatase activity with a lumenal orientation. The enzyme showed equally high activity towards GDP/UDP and ADP, and also showed activity, although to a lesser extent, towards GTP. No activity was detected with the other triphosphates and all monophosphates. The overexpression of KlYND1 in Klgda1Delta cells of K. lactis, devoid of the encoded GDPase/UDPase activity, suppressed the loss of O-glycosylation and cell wall-related defects described in such mutants, and suggests a partial overlap of function between the two genes, and therefore some redundancy. The overexpression of KlYND1 in wild-type cells enhanced the secretion of the recombinant human serum albumin and glucoamylase employed as reporters.

Cdk1 coordinates cell-surface growth with the cell cycle

The mechanisms that control cell growth during the cell cycle are poorly understood. In budding yeast, cyclin dependent kinase 1 (Cdk1) triggers polarization of the actin cytoskeleton and bud emergence in late G1 through activation of the Cdc42 GTPase. However, Cdk1 is not thought to be required for subsequent growth of the bud. Here, we show that Cdk1 has an unexpected role in controlling bud growth after bud emergence. Moreover, we show that G1 cyclin-Cdk1 complexes specifically phosphorylate multiple proteins associated with Cdc24, the guanine nucleotide-exchange factor (GEF) that activates the Cdc42 GTPase. A mutant form of a Cdc24-associated protein that fails to undergo Cdk1-dependent phosphorylation causes defects in bud growth. These results provide a direct link between Cdk1 activity and the control of polarized cell growth.

Potential relationship between glutathione metabolism and flocculation in the yeast Kluyveromyces lactis

Reduced glutathione (GSH) is involved in biochemical and physiological processes in cells. Flocculation is an important mechanism in microorganisms. The present study concerned the potential relationship between GSH metabolism and flocculation. Two yeast strains, a flocculent (Kluyveromyces lactis 5c) and a nonflocculent (Kluyveromyces lactis 5a) strain, were used. The level of intracellular GSH measured during the growth period was significantly higher in the nonflocculent than in the flocculent strain; in contrast, the flocculent strain exhibited brighter staining of vacuoles than the nonflocculent strain when observed using epifluorescence microscopy. Compounds acting either on flocculation (EDTA, galactose) or on GSH metabolism (buthionine sulfoximine, and N-acetylcysteine) were tested on the flocculent strain during the growth period. Both EDTA and galactose fully inhibited flocculation and induced GSH overproduction of 58% and 153%, respectively. Buthionine sulfoximine decreased GSH level by 76% but had no effect on flocculation; N-acetylcysteine increased the GSH level and flocculation by 106% and 41%, respectively. Combination of EDTA and N-acetylcysteine produced similar effects than with each of them. Combination of galactose and N-acetylcysteine increased the GSH level but decreased flocculation. These results demonstrated that GSH homeostasis is linked to the flocculation mechanism. A hypothesis related to stress is given.

The influence of a pulsating electric field on selenium accumulation inKluyveromyces marxianus cells

K. marxianus was cultivated under conditions of PEF (pulse electric field) action and a selenium source. The culture duration after which cells were treated with PEF, the field exposure time and the selenium concentration in the medium were all optimized. Optimization of culture duration caused a 33% increase in selenium accumulation in cells as compared to the control with no PEF treatment. The highest selenium accumulation--about 167 microg/g dry mass (DM)--was recorded after 3-minute PEF treatment of 16-hour culture. A roughly two-fold increase in selenium content was achieved after optimization of culture duration and PEF treatment time. Finding the optimum selenium concentration in the medium brought about a 13-fold increase of selenium accumulation in the cells of K. marxianus.

Induction by hypoxia of heterologous-protein production with the KlPDC1 promoter in yeasts

The control of promoter activity by oxygen availability appears to be an intriguing system for heterologous protein production. In fact, during cell growth in a bioreactor, an oxygen shortage is easily obtained simply by interrupting the air supply. The purpose of our work was to explore the possible use of hypoxic induction of the KlPDC1 promoter to direct heterologous gene expression in yeast. In the present study, an expression system based on the KlPDC1 promoter was developed and characterized. Several heterologous proteins, differing in size, origin, localization, and posttranslational modification, were successfully expressed in Kluyveromyces lactis under the control of the wild type or a modified promoter sequence, with a production ratio between 4 and more than 100. Yields were further optimized by a more accurate control of hypoxic physiological conditions. Production of as high as 180 mg/liter of human interleukin-1beta was obtained, representing the highest value obtained with yeasts in a lab-scale bioreactor to date. Moreover, the transferability of our system to related yeasts was assessed. The lacZ gene from Escherichia coli was cloned downstream of the KlPDC1 promoter in order to get beta-galactosidase activity in response to induction of the promoter. A centromeric vector harboring this expression cassette was introduced in Saccharomyces cerevisiae and in Zygosaccharomyces bailii, and effects of hypoxic induction were measured and compared to those already observed in K. lactis cells. Interestingly, we found that the induction still worked in Z. bailii; thus, this promotor constitutes a possible inducible system for this new nonconventional host.

Deletion of the glucose-6-phosphate dehydrogenase gene KlZWF1 affects both fermentative and respiratory metabolism in Kluyveromyces lactis

In Kluyveromyces lactis, the pentose phosphate pathway is an alternative route for the dissimilation of glucose. The first enzyme of the pathway is the glucose-6-phosphate dehydrogenase (G6PDH), encoded by KlZWF1. We isolated this gene and examined its role. Like ZWF1 of Saccharomyces cerevisiae, KlZWF1 was constitutively expressed, and its deletion led to increased sensitivity to hydrogen peroxide on glucose, but unlike the case for S. cerevisiae, the Klzwf1Delta strain had a reduced biomass yield on fermentative carbon sources as well as on lactate and glycerol. In addition, the reduced yield on glucose was associated with low ethanol production and decreased oxygen consumption, indicating that this gene is required for both fermentation and respiration. On ethanol, however, the mutant showed an increased biomass yield. Moreover, on this substrate, wild-type cells showed an additional band of activity that might correspond to a dimeric form of G6PDH. The partial dimerization of the G6PDH tetramer on ethanol suggested the production of an NADPH excess that was negative for biomass yield.

A reverse transcriptase PCR technique for the detection and viability assessment of Kluyveromyces marxianus in yoghurt

A fast and sensitive reverse transcriptase PCR (RT-PCR) method was developed for the detection of viable Kluyveromyces marxianus in yoghurt. Yeast-specific primers were used with the RT-PCR to evaluate the suitability of 18S rRNA as a target for the detection of viable yeasts in pure culture and yoghurt. The RT-PCR assay was able to detect down to 10(2) CFU ml(-1) in yoghurt samples contaminated with viable yeast cells. Application of the RT-PCR method to commercial yoghurt samples demonstrated the utility of this technique for detection of low concentrations of viable yeast cells in naturally contaminated dairy products. The 18S rRNA molecule is an appropriate target for cell viability assessment because of its limited persistence after cell death and the resultant high level of sensitivity of the assay.