Kinetics of soluble glucan production by<Emphasis Type="Italic">Claviceps viridis</Emphasis>

Among 18 tested strains of Claviceps spp., 7 produced significant amounts of exocellular polysaccharide (EPS). The maximum production of EPS was found in fermentation broth of Claviceps viridis. The kinetics of growth, substrate consumption, and EPS production in the batch, aerobic, submerged culture of this fungus were investigated in detail. The experimental data were processed by a simple mathematical model describing mass balance of growth, substrate consumption, formation of intermediates, and production of EPS. The parameters of the model were estimated from data obtained in cultivation performed in flasks and two laboratory fermentors of different size. Physiological similarity was obtained during process scale-up in volumetric ratio 1:100. The sugar consumption efficiency (52%) and observed EPS productivity (1.9 kg/m3 per d) were comparable with literature data.     

Overproduction of lipase by<Emphasis Type="Italic"> Yarrowia lipolytica</Emphasis> mutants

Non-genetically modified mutants with increased capacities of extracellular lipase production were obtained from Yarrowia lipolytica strain CBS6303 by chemical mutagenesis. Of the 400 mutants isolated, LgX64.81 had the highest potential for the development of an industrial lipase production process. This mutant exhibits lipase production uncoupled from catabolite repression by glucose, and a 10-fold increased productivity upon addition of oleic acid. Using a LIP2- LacZ reporter gene, we demonstrate that the mutant phenotype originates from a trans-acting mutation. The glucose uptake capacity of LgX64.81 is reduced 2.5-fold compared to the wild-type-strain, and it exhibits high lipase production on glucose medium. A trans-acting mutation in a gene involved in glucose transport could thus explain this mutant phenotype.     

Viable nonsense mutants for the essential gene <Emphasis Type="Italic">SUP45</Emphasis> of <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis>

Background  

Termination of protein synthesis in eukaryotes involves at least two polypeptide release factors (eRFs) – eRF1 and eRF3. The highly conserved translation termination factor eRF1 in Saccharomyces cerevisiae is encoded by the essential gene SUP45.     

Biosorption of manganese by <Emphasis Type="Italic">Aspergillus niger</Emphasis> and <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis>

Summary Biosorption of manganese from its aqueous solution using yeast biomass Saccharomyces cerevisiae and fungal biomass Aspergillus niger was carried out. Manganese biosorption equilibration time for A. niger and S. cerevisiae were found to be 60 and 20 min, with uptakes of 19.34 and 18.95 mg/g, respectively. Biosorption increased with rise in pH, biomass, and manganese concentration. The biosorption equilibrium data fitted with the Freundlich isotherm model revealed that A. niger was a better biosorbent of manganese than S. cerevisiae.     

Effect of <Emphasis Type="Italic">HXT</Emphasis> 1 and <Emphasis Type="Italic">HXT</Emphasis> 7 hexose transporter overexpression on wild-type and lactic acid producing <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> cells

Background  

Since about three decades, Saccharomyces cerevisiae can be engineered to efficiently produce proteins and metabolites. Even recognizing that in baker's yeast one determining step for the glucose consumption rate is the sugar uptake, this fact has never been conceived to improve the metabolite(s) productivity.     

Methods designed for the identification and characterization of<Emphasis Type="Italic">in vitro</Emphasis> and<Emphasis Type="Italic">in vivo</Emphasis> chromatin assembly mutants in<Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis>

Assembly of DNA into chromatin allows for the formation of a barrier that protects naked DNA from protein and chemical agents geared to degrade or metabolize DNA. Chromatin assembly occurs whenever a length of DNA becomes exposed to the cellular elements, whether during DNA synthesis or repair. This report describes tools to study chromatin assembly in the model systemSaccharomyces cerevisiae. Modifications to anin vitro chromatin assembly assay are described that allowed a brute force screen of temperature sensitive (ts) yeast strains in order to identify chromatin assembly defective extracts. This screen yielded mutations in genes encoding two ubiquitin protein ligases (E3s):RSP5, and a subunit of the Anaphase Promoting Complex (APC),APC5. Additional modifications are described that allow for a rapid analysis and anin vivo characterization of yeast chromatin assembly mutants, as well as any other mutant of interest. Our analysis suggests that thein vitro andin vivo chromatin assembly assays are responsive to different cellular signals, including cell cycle cues that involve different molecular networks. Published: July 3, 2003     

Domain engineering of <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> exoglucanases

To illustrate the effect of a cellulose-binding domain (CBD) on the enzymatic characteristics of non-cellulolytic exoglucanases, 10 different recombinant enzymes were constructed combining the Saccharomyces cerevisiae exoglucanases, EXG1 and SSG1, with the CBD2 from the Trichoderma reesei cellobiohydrolase, CBH2, and a linker peptide. The enzymatic activity of the recombinant enzymes increased with the CBD copy number. The recombinant enzymes, CBD2-CBD2-L-EXG1 and CBD2-CBD2-SSG1, exhibited the highest cellobiohydrolase activity (17.5 and 16.3 U mg ^–1 respectively) on Avicel cellulose, which is approximately 1.5- to 2-fold higher than the native enzymes. The molecular organisation of CBD in these recombinant enzymes enhanced substrate affinity, molecular flexibility and synergistic activity, contributing to their elevated action on the recalcitrant substrates as characterised by adsorption, kinetics, thermostability and scanning electron microscopic analysis.     

Inactivation of NMD increases viability of <Emphasis Type="Italic">sup45</Emphasis> nonsense mutants in <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis>

Background  

The nonsense-mediated mRNA decay (NMD) pathway promotes the rapid degradation of mRNAs containing premature termination codons (PTCs). In yeast Saccharomyces cerevisiae, the activity of the NMD pathway depends on the recognition of the PTC by the translational machinery. Translation termination factors eRF1 (Sup45) and eRF3 (Sup35) participate not only in the last step of protein synthesis but also in mRNA degradation and translation initiation via interaction with such proteins as Pab1, Upf1, Upf2 and Upf3.     

Three new DNA helicases from<Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis>

At least six DNA helicases have been identified during fractionation of extracts from the yeastSaccharomyces cerevisiae. Three of those, DNA helicases B, C, and D, have been further purified and characterized. DNA helicases B and C co-purified with DNA polymerse δ through several chromatographic steps, but were separated from the polymerase by hydrophobic chromatography. DNA helicase D co-purified with Replication Factor C over seven chromatographic steps, and was only separated from it by glycerol gradient centrifugation in the presence of 0.2 M NaCl. All three helicases are DNA dependent ATPases with K_m values for ATP of 190 μM, 325 μM, and 60 μM for DNA helicases B, C, and D, respectively. Their DNA helicase activities are comparable. They are 5′–3′ helicases and have pH optima of 6.5–7 and Mg²⁺ optima of 1–2 mM. However, they differ in the nucleotide requirement for helicase action. Whereas all three helicases preferred ATP, dATP, UTP, CTP, and dCTP as cofactors, DNA helicase C also used GTP, but not dTTP. On the other hand, DNA helicase D used dTTP, but not GTP, and DNA helicase B used neither nucleotide as cofactor. These studies allowed us to conclude that DNA helicases B, C, and D are not only distinct enzymes, but also different from two previously identified yeast DNA helicases, the RAD3 protein and ATPase III.     

Patterns of Gene Duplication in <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> and <Emphasis Type="Italic">Caenorhabditis elegans</Emphasis>

In this paper we present a new method for detecting block duplications in a genome. It is more stringent than previous ones in that it requires a more rigorous definition of paralogous genes and that it requires the paralogous proteins on the two blocks to be contiguous. In addition, it provides three criterion choices: (1) the same composition (i.e., having the same paralogues in the two windows), (2) the same composition and gene order, and (3) the same composition, gene order, and gene orientation. The method is completely automated, requiring no visual inspection as in previous methods. We applied it to analyze the complete genomes of S. cerevisiae and C. elegans. In yeast we detected fewer duplicated blocks than previously reported. In C. elegans, however, we detected more block duplications than previously reported, indicating that although our method has a more stringent definition of block duplication than previous ones, it may be more sensitive in detection because it considers every possible window rather than only fixed nonoverlapping windows. Our results show that block duplication is a common phenomenon in both organisms. The patterns of block duplication in the two species are, however, markedly different. The yeast shows much more extensive block duplication than the nematode, with some chromosomes having more than 40% of the duplications derived from block duplications. Moreover, in the yeast the majority of block duplications occurred between chromosomes, while in the nematode most block duplications occurred within chromosomes.     

Effect of antioxidants on<Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> mutants deficient in superoxide dismutases

S. cerevisiae strain delta sodl lacking Cu,Zn-superoxide dismutase and delta sodl delta sod2 mutant lacking both Cu,Zn-SOD and Mn-superoxide dismutase displayed strongly reduced aerobic growth on glucose, glycerol and lactate; delta sod2 deletion had no effect on aerobic growth on glucose and largely precluded growth on glycerol and lactate. The oxygen-induced growth defects and their alleviation by antioxidants depended on growth conditions, in particular on oxygen supply to cells. Under strong aeration, vitamins A and E had a low effect, 100 mumol/L quercetin alleviated the growth defects of all three mutants while beta-carotene had no growth-restoring effect. The superoxide producer paraquat inhibited the aerobic growth of all three mutants in a concentration-dependent manner. Low concentrations of antioxidants had no effect on paraquat toxicity while higher concentrations supported the toxic effect of the agent.     

Quantitative study of interactions between <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> and <Emphasis Type="Italic">Oenococcus </Emphasis><Emphasis Type="Italic">oeni</Emphasis> strains

This study examines the interactions that occur between Saccharomyces cerevisiae and Oenococcus oeni strains during the process of winemaking. Various yeast/bacteria pairs were studied by applying a sequential fermentation strategy which simulated the natural winemaking process. First, four yeast strains were tested in the presence of one bacterial strain leading to the inhibition of the bacterial component. The extent of inhibition varied widely from one pair to another and closely depended on the specific yeast strain chosen. Inhibition was correlated to weak bacterial growth rather than a reduction in the bacterial malolactic activity. Three of the four yeast strains were then grown with another bacteria strain. Contrary to the first results, this led to the bacterial stimulation, thus highlighting the importance of the bacteria strain. The biochemical profile of the four yeast fermented media exhibited slight variations in ethanol, SO(2) and fatty acids produced as well as assimilable consumed nitrogen. These parameters were not the only factors responsible for the malolactic fermentation inhibition observed with the first bacteria strain. The stimulation of the second has not been reported before in such conditions and remains unexplained.     

Electrically enhanced ethanol fermentation by <Emphasis Type="Italic">Clostridium thermocellum</Emphasis> and <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis>

Ethanol production by Clostridium thermocellum ATCC 35609 and Saccharomyces cerevisiae ATCC 26603 was improved in an electrochemical bioreactor system. It was increased by 61% with Cl. thermocellum and 12% with S. cerevisiae in the presence of -1.5 V of electric potential. These increases were attributed to high production rates due to regeneration and availability of increased reduced equivalents in the presence of electric potential. The electric current caused considerable shift in the metabolite concentrations on a molar basis in Cl. thermocellum fermentation but less in S. cerevisiae fermentation. Increasing electric potential in Cl. thermocellum fermentation resulted in less acetate and more lactate production. Acetate production was also reduced with increased electric potential in S. cerevisiae fermentation. The high electric potential of -5 V adversely affected the Cl. thermocellum fermentation, but not the S. cerevisiae fermentation even at a high electric potential of -10 V.     

Comparative characterization of endo-polygalacturonase (<Emphasis Type="Italic">Pgu1</Emphasis>) from <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> and <Emphasis Type="Italic">Saccharomyces paradoxus</Emphasis> under winemaking conditions

A peptide antibiotic, gramicidin A, was covalently bound to cystamine self-assembled monolayers on gold surfaces. Each step of the surface functionalization was characterized by polarization modulation infrared reflection absorption spectroscopy and X-ray photoelectron spectroscopy. The antimicrobial activity of the anchored gramicidin was tested against three Gram-positive bacteria (Listeria ivanovii, Enterococcus faecalis, and Staphylococcus aureus), the Gram-negative bacterium Escherichia coli and the yeast Candida albicans. The results revealed that the adsorbed gramicidin reduced, from 60% for E. coli to 90% for C. albicans, the number of culturable microorganisms attached to the surface. The activity was proven to be persistent overtime, up to 6 months after the first use. The bacteria attached to the functionalized surfaces were permeabilized as shown by confocal microscopy. Taken together, these results indicate a bacteriostatic mode of action of the immobilized peptide. Finally, using green fluorescent protein-expressing bacteria, it was shown that the development of a bacterial biofilm was delayed on peptide-grafted surfaces for at least 24 h.     

Raw starch conversion by <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> expressing <Emphasis Type="Italic">Aspergillus tubingensis</Emphasis> amylases

Background

Starch is one of the most abundant organic polysaccharides available for the production of bio-ethanol as an alternative transport fuel. Cost-effective utilisation of starch requires consolidated bioprocessing (CBP) where a single microorganism can produce the enzymes required for hydrolysis of starch, and also convert the glucose monomers to ethanol.

Results

The Aspergillus tubingensis T8.4 α-amylase (amyA) and glucoamylase (glaA) genes were cloned and expressed in the laboratory strain Saccharomyces cerevisiae Y294 and the semi-industrial strain, S. cerevisiae Mnuα1. The recombinant AmyA and GlaA displayed protein sizes of 110–150 kDa and 90 kDa, respectively, suggesting significant glycosylation in S. cerevisiae. The Mnuα1[AmyA-GlaA] and Y294[AmyA-GlaA] strains were able to utilise 20 g l^-1 raw corn starch as sole carbohydrate source, with ethanol titers of 9.03 and 6.67 g l^-1 (0.038 and 0.028 g l^-1 h^-1), respectively, after 10 days. With a substrate load of 200 g l^-1 raw corn starch, Mnuα1[AmyA-GlaA] yielded 70.07 g l^-1 ethanol (0.58 g l^-1 h^-1) after 120 h of fermentation, whereas Y294[AmyA-GlaA] was less efficient at 43.33 g l^-1 ethanol (0.36 g l^-1 h^-1).

Conclusions

In a semi-industrial amylolytic S. cerevisiae strain expressing the A. tubingensis α-amylase and glucoamylase genes, 200 g l^-1 raw starch was completely hydrolysed (saccharified) in 120 hours with 74% converted to released sugars plus fermentation products and the remainder presumably to biomass. The single-step conversion of raw starch represents significant progress towards the realisation of CBP without the need for any heat pretreatment. Furthermore, the amylases were produced and secreted by the host strain, thus circumventing the need for exogenous amylases.

     

Cross-resistance to strobilurin fungicides in mitochondrial and nuclear mutants of<Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis>

In yeast the resistance to kresoxim-methyl and azoxystrobin, like the resistance to strobilurin A (mucidin) is under the control of both mitochondrial cob gene and the PDR network of nuclear genes involved in multidrug resistance. The mucidin-resistant mucl (G137R) and muc2 (L275S) mutants of Saccharomyces cerevisiae containing point mutations in mtDNA were found to be cross-resistant to kresoxim-methyl and azoxystrobin. Cross-resistance to all three strobilurin fungicides was also observed in yeast transformants containing gain-of-function mutations in the nuclear PDR3 gene. On the other hand, nuclear mutants containing disrupted chromosomal copies of the PDR1 and PDR3 genes or the PDR5 gene alone were hypersensitive to kresoxim-methyl, azoxystrobin and strobilurin A. The frequencies of spontaneous mutants selected for resistance either to kresoxim-methyl, azoxystrobin or strobilurin A were similar and resulted from mutations both in mitochondrial and nuclear genes. The results indicate that resistance to strobilurin fungicides, differing in chemical structure and specific activity, can be caused by the same molecular mechanism involving changes in the structure of apocytochrome b and/or increased efflux of strobilurins from fungal cells.     

Genetic polymorphism of sherry <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> yeasts

The review deals with natural diversity of sherry (“flor”) Saccharomyces cerevisiae yeasts. Various properties of these yeasts are analyzed: life cycles, fermentation of sugars, sensitivity to methyl violet and killer toxins, and resistance to ethanol and sulfite. Special attention is paid to molecular identification and differentiation of sherry yeasts. The history of their nomenclature is considered, including the names of possible subpopulations: “aceti,” “beticus” (“cheresienses”), “cordubensis,” “gaditensis,” “hispanica” (“prostoserdovii”), and “oxidans.”