Stoichiometry and compartmentation of NADH metabolism in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, reduction of NAD(+) to NADH occurs in dissimilatory as well as in assimilatory reactions. This review discusses mechanisms for reoxidation of NADH in this yeast, with special emphasis on the metabolic compartmentation that occurs as a consequence of the impermeability of the mitochondrial inner membrane for NADH and NAD(+). At least five mechanisms of NADH reoxidation exist in S. cerevisiae. These are: (1) alcoholic fermentation; (2) glycerol production; (3) respiration of cytosolic NADH via external mitochondrial NADH dehydrogenases; (4) respiration of cytosolic NADH via the glycerol-3-phosphate shuttle; and (5) oxidation of intramitochondrial NADH via a mitochondrial 'internal' NADH dehydrogenase. Furthermore, in vivo evidence indicates that NADH redox equivalents can be shuttled across the mitochondrial inner membrane by an ethanol-acetaldehyde shuttle. Several other redox-shuttle mechanisms might occur in S. cerevisiae, including a malate-oxaloacetate shuttle, a malate-aspartate shuttle and a malate-pyruvate shuttle. Although key enzymes and transporters for these shuttles are present, there is as yet no consistent evidence for their in vivo activity. Activity of several other shuttles, including the malate-citrate and fatty acid shuttles, can be ruled out based on the absence of key enzymes or transporters. Quantitative physiological analysis of defined mutants has been important in identifying several parallel pathways for reoxidation of cytosolic and intramitochondrial NADH. The major challenge that lies ahead is to elucidate the physiological function of parallel pathways for NADH oxidation in wild-type cells, both under steady-state and transient-state conditions. This requires the development of techniques for accurate measurement of intracellular metabolite concentrations in separate metabolic compartments.     

Kinetic study and modelling of the xylitol production using Candida parapsilosis in oxygen-limited culture conditions

Kinetic studies are presented for xylitol production and growth of the yeast Candida parapsilosis ATCC 28474. The oxygen supply influence on xylitol production from xylose was investigated. No metabolic activity was detected in anaerobic conditions. In contrast, it was found that under low aeration rates (0.1-0.2 vvm), xylitol is produced. Xylitol production decreases when air flow to reactor is augmented. An unstructured model is proposed for the kinetic behaviour analysis of yeast growing in batch culture. A simplex method was used for the estimation of model parameters. The parameter confidence intervals were also calculated.     

Effect of yeast extract concentration on growth ofSchizoccharomyces pombe

Summary The effect of yeast extract on the growth ofSchizosaccharomyces pombe was investigated using a complex-synthetic medium. Batch cultures at low-glucose concentration show that a too low concentration of yeast extract may limit the biomass formation. On this medium kinetics and yields were found to be similar to those obtained on a synthetic-defined medium under both aerobic and anaerobic conditions.     

Effects of cutting and rainfall pattern on resprouting vigour and growth of Erica arborea L.

Abstract. This study analyses the effects of recurrent cutting and rainfall pattern on sprout and genet growth in the resprouting shrub Erica arborea. Three different intervals between consecutive clear-cutting events were applied: 30, 5 and 2 yr. Above-ground genet biomass after 2 yr of treatment application was correlated with burl size and decreased with increasing recurrence of cutting, indicating that more than 5 yr were needed to re-establish the resprouting potential of genets. The effect of rainfall on growth was evaluated through the correlations between sprout RGR and several variables related to the rainfall pattern at different regeneration stages. The effect of the rainfall pattern on sprout RGR was dependent on the regeneration age, suggesting that water availability becomes more limiting after two years of regrowth.     

Estimating the capacity of Chamaecrista fasciculata for adaptation to change in precipitation

Adaptation through natural selection may be the only means by which small and fragmented plant populations will persist through present day environmental change. A population's additive genetic variance for fitness (V_A(W)) represents its immediate capacity to adapt to the environment in which it exists. We evaluated this property for a population of the annual legume Chamaecrista fasciculata through a quantitative genetic experiment in the tallgrass prairie region of the Midwestern United States, where changing climate is predicted to include more variability in rainfall. To reduce incident rainfall, relative to controls receiving ambient rain, we deployed rain exclusion shelters. We found significant V_A(W) in both treatments. We also detected a significant genotype‐by‐treatment interaction for fitness, which suggests that the genetic basis of the response to natural selection will differ depending on precipitation. For the trait‐specific leaf area, we detected maladaptive phenotypic plasticity and an interaction between genotype and environment. Selection for thicker leaves was detected with increased precipitation. These results indicate capacity of this population of C. fasciculata to adapt in situ to environmental change.     

Vital role for Plasmodium berghei Kinesin8B in axoneme assembly during male gamete formation and mosquito transmission

Sexual development is an essential phase in the Plasmodium life cycle, where male gametogenesis is an unusual and extraordinarily rapid process. It produces 8 haploid motile microgametes, from a microgametocyte within 15 minutes. Its unique achievement lies in linking the assembly of 8 axonemes in the cytoplasm to the three rounds of intranuclear genome replication, forming motile microgametes, which are expelled in a process called exflagellation. Surprisingly little is known about the actors involved in these processes. We are interested in kinesins, molecular motors that could play potential roles in male gametogenesis. We have undertaken a functional characterization in Plasmodium berghei of kinesin‐8B (PbKIN8B) expressed specifically in male gametocytes and gametes. By generating Pbkin8B‐gfp parasites, we show that PbKIN8B is specifically expressed during male gametogenesis and is associated with the axoneme. We created a ΔPbkin8B knockout cell line and analysed the consequences of the absence of PbKIN8B on male gametogenesis. We show that the ability to produce sexually differentiated gametocytes is not affected in ΔPbkin8B parasites and that the 3 rounds of genome replication occur normally. Nevertheless, the development to free motile microgametes is halted and the life cycle is interrupted in vivo. Ultrastructural analysis revealed that intranuclear mitoses are unaffected whereas cytoplasmic microtubules, although assembled in doublets and elongated, fail to assemble in the normal axonemal ‘9+2' structure and become motile. Absence of a functional axoneme prevented microgamete assembly and release from the microgametocyte, severely reducing infection of the mosquito vector. This is the first functional study of a kinesin involved in male gametogenesis. These results reveal a previously unknown role for PbKIN8B in male gametogenesis, providing new insights into Plasmodium flagellar formation.     

Phylogenetic diversity of stress signalling pathways in fungi

Background  

Microbes must sense environmental stresses, transduce these signals and mount protective responses to survive in hostile environments. In this study we have tested the hypothesis that fungal stress signalling pathways have evolved rapidly in a niche-specific fashion that is independent of phylogeny. To test this hypothesis we have compared the conservation of stress signalling molecules in diverse fungal species with their stress resistance. These fungi, which include ascomycetes, basidiomycetes and microsporidia, occupy highly divergent niches from saline environments to plant or mammalian hosts.