CreA influences the metabolic fluxes of Aspergillus nidulans during growth on glucose and xylose

Abstract

The physiological phenotype of Aspergillus nidulans was investigated for different genetic and environmental conditions of glucose repression through the quantification of in vivo fluxes in the central carbon metabolism using 13 C-metabolic-flux analysis. The particular focus was the role of the carbon repressor CreA, which is the major regulatory protein mediating carbon repression in many fungal species, in the primary metabolism of A. nidulans. Batch cultivations were performed with a reference strain and a deletion mutant strain (creAD4) using [1-13 C]glucose as carbon source. The mutant strain was also grown on a mixture of [1-13 C]glucose and unlabelled xylose. Fractional enrichment data were measured by gas chromatography-mass spectrometry. A model describing the central metabolism of A. nidulans was used in combination with fractional enrichment data, and measurements of extracellular rates and biomass composition for the estimation of the in vivo metabolic fluxes. The creA-mutant strain showed a lower maximum specific growth rate than the reference strain when grown on glucose (0?11 and 0?25 h "1 , respectively), whereas the specific growth rate of the mutant strain grown on the glucose/xylose mixture was identical to that on glucose (0?11 h "1 ). Different patterns and increased levels of extracellular polyols were observed both upon deletion of the creA gene and upon addition of xylose to the growth medium of the mutant strain. Concerning metabolic fluxes, the major change observed in the flux distribution of A. nidulans upon deletion of the creA gene was a 20 % decrease in the flux through the oxidative part of the pentose-phosphate pathway. Addition of xylose to the growth medium of the mutant resulted in an increase of about 40 % in the activity of the oxidative part of the pentose-phosphate pathway, as well as decreases in the fluxes through the Embden-Meyerhof-Parnas pathway and the tricarboxylic acid cycle (in the range of 20-30 %). The derepression of key pathways leads to alterations in the demands for cofactors, thereby imposing changes in the central metabolism due to the coupling of the many different reactions via the redox and energy metabolism of the cells.

Aspergillus fumigatus Catalytic Glucokinase and Hexokinase: Expression Analysis and Importance for Germination, Growth, and Conidiation

“…Therefore, we conclude that the deletion of either enzyme unbalances the equilibrium of glucose uptake and utilization. Carbon catabolite repression may therefore depend not only on the presence of free glucose but also on the accumulation of intermediates from its degradation, as previously assumed (8,33).…”

d -Xylose as a Repressor or Inducer of Xylanase Expression in Hypocrea jecorina ( Trichoderma reesei )

“…This effect of reduced induction due to the addition of glucose must be seen as independent. Possible explanations might include the impact of glucose on D-xylose uptake, as postulated for A. niger (6), or the impact of glucose and/or cre1 deletions on D-xylose metabolism, as demonstrated for A. nidulans (4).…”

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“…Similarly mass isotopomer distributions of other metabolites give access to various other flux parameters, such as dual pathways in amino acid biosynthesis [64], bidirectional fluxes around the pyruvate node [64] or parallel pathways in different cellular compartments [18] and different flux ratios [24]. In other studies, summed fractional labelling data have been utilized for the analysis of fluxes [65,66]. Hereby, the lower degree of information for each single compound was compensated by the consideration of different ion fragments containing different parts of the carbon skeleton for the analytes [67].…”

“…Among yeasts studied on the level of intracellular fluxes by GC-MS are Saccharomyces cerevisiae with studies on the influence of growth rate [18,27], or the comparison of different mutants [66], several members of the genus Pichia [84,88,108], Phaffia rhodozyma [28] or different other species [109]. Flux studies with fungi comprise e. g. biotechnologically relevant species such as Penicillium chrysogenum [29], Aspergillus nidulans [65] or Aspergillus niger [95]. …”