Regulation of pentose catabolic pathway genes of Aspergillus niger

The aim of this study was to obtain a better understanding of the pentose catabolism in Aspergillus niger and the regulatory systems that affect it. To this end, we have cloned and characterised the genes encoding A. niger L-arabitol dehydrogenase (ladA) and xylitol dehydrogenase (xdhA), and compared the regulation of these genes to other genes of the pentose catabolic pathway. This demonstrated that activation of the pathway depends on two transcriptional regulators, the xylanolytic activator (XlnR) and an unidentified L-arabinose specific regulator (AraR). These two regulators affect those genes of the pentose catabolic pathway that are related to catabolic conversion of their corresponding inducers (D-xylose and L-arabinose, respectively)

Microbial cell surfaces and secretion systems

Microbial cell surfaces, surface-exposed organelles, and secreted proteins are important for the interaction with the environment, including adhesion to hosts, protection against host defense mechanisms, nutrient acquisition, and intermicrobial competition. Here, we describe the structures of the cell envelopes of bacteria , fungi, and oomycetes , and the mechanisms they have evolved for the transport of proteins across these envelopes to the cell surface and into the extracellular milieu

Microbial cell surfaces and secretion systems

Microbial cell surfaces, surface-exposed organelles, and secreted proteins are important for the interaction with the environment, including adhesion to hosts, protection against host defense mechanisms, nutrient acquisition, and intermicrobial competition. Here, we describe the structures of the cell envelopes of bacteria , fungi, and oomycetes , and the mechanisms they have evolved for the transport of proteins across these envelopes to the cell surface and into the extracellular milieu

A surface active protein involved in aerial hyphae formation in the filamentous fungus Schizophillum commune restores the capacity of a bald mutant of the filamentous bacterium Streptomyces coelicolor to erect aerial structures

The filamentous bacterium Streptomyces coelicolor undergoes a complex process of morphological differentiation involving the formation of a dense lawn of aerial hyphae that grow away from the colony surface into the air to form an aerial mycelium, Bald mutants of S, coelicolor, which are blocked in aerial mycelium formation, regain the capacity to erect aerial structures when exposed to a small hydrophobic protein called SapB, whose synthesis is temporally and spatially correlated with morphological differentiation. We now report that SapB is a surfactant that is capable of reducing the surface tension of water from 72 mJ m(-2) to 30 mJ m(-2) at a concentration of 50 mu g ml(-1) We also report that SapB, like the surface-active peptide streptofactin produced by the species S. tendae, was capable of restoring the capacity of bald mutants of S. tendae to erect aerial structures. Strikingly, a member (SC3) of the hydrophobin family of fungal proteins involved in the erection of aerial hyphae in the filamentous fungus Schizophyllum commune was also capable of restoring the capacity of S, coelicolor and S, tendae bald mutants to erect aerial structures. SC3 is unrelated in structure to SapB and streptofactin but, like the streptomycetes proteins, the fungal protein is a surface active agent. Scanning electron microscopy revealed that aerial structures produced in response to both the bacterial or the fungal proteins were undifferentiated vegetative hyphae that had grown away from the colony surface but had not commenced the process of spore formation, We conclude that the production of SapB and streptofactin at the start of morphological differentiation contributes to the erection of aerial hyphae by decreasing the surface tension at the colony surface but that subsequent morphogenesis requires additional developmentally regulated events under the control of bald genes

Fruiting Body Formation in Basidiomycetes

Establishment of the dikaryotic mycelium and formation of fruiting bodies are highly complex developmental programmes that are activated by a combination of environmental cues. A wide variety of proteins are expected to regulate and coordinate these programmes or to fulfil enzymatic conversions or structural roles. With the identification of the first genes involved in mushroom development, we are only at the beginning of understanding fruiting body formation. The process of identification of genes will be accelerated by whole genome expression studies and increased availability of molecular tools to assign functions to genes. Establishment of the dikaryon and emergence of fruiting bodies in basidiomycetes are regulated by the mating-type genes. These genes encode DNA-binding proteins and pheromones and their receptors. Regulation of fruiting by the mating-type genes is mediated by downstream transcription factors. Several genes encoding such regulatory proteins have now been identified. Regulatory circuits ultimately activate genes encoding structural proteins or enzymes that are involved in fruiting body formation. The role of hydrophobins is well established. They enable hyphae to escape the aqueous environment to allow fruiting body development. Moreover, they coat aerial structures and line air channels in mushrooms. The hydrophobic coating irreversibly directs growth of hyphae into the air, allows dispersal of spores and ensures gas exchange in fruiting bodies under humid conditions. Apart from hydrophobins, phenolics polymerised by the action of laccases may contribute to surface hydrophobicity of fruiting bodies. These enzymes have also been proposed to cross-link cell walls of hyphae in the fruiting bodies but this still has to be established. Experimental evidence indicates that cytochrome P450 enzymes, lectins, haemolysins and expansins also function in mushroom development. Lectins may be involved in aggregation of hyphae, haemolysins in signalling particularly to induce apoptosis of selected hyphae in the fruiting body, while expansins may be involved in cell wall modification and extension

Switching from a unicellular to multicellular organization in an Aspergillus niger hypha

UNLABELLED: Pores in fungal septa enable cytoplasmic streaming between hyphae and their compartments. Consequently, the mycelium can be considered unicellular. However, we show here that Woronin bodies close ~50% of the three most apical septa of growing hyphae of Aspergillus niger. The incidence of closure of the 9th and 10th septa was even ≥94%. Intercompartmental streaming of photoactivatable green fluorescent protein (PA-GFP) was not observed when the septa were closed, but open septa acted as a barrier, reducing the mobility rate of PA-GFP ~500 times. This mobility rate decreased with increasing septal age and under stress conditions, likely reflecting a regulatory mechanism affecting septal pore diameter. Modeling revealed that such regulation offers effective control of compound concentration between compartments. Modeling also showed that the incidence of septal closure in A. niger had an even stronger impact on cytoplasmic continuity. Cytoplasm of hyphal compartments was shown not to be in physical contact when separated by more than 4 septa. Together, data show that apical compartments of growing hyphae behave unicellularly, while older compartments have a multicellular organization. IMPORTANCE: The hyphae of higher fungi are compartmentalized by porous septa that enable cytosolic streaming. Therefore, it is believed that the mycelium shares cytoplasm. However, it is shown here that the septa of Aspergillus niger are always closed in the oldest part of the hyphae, and therefore, these compartments are physically isolated from each other. In contrast, only part of the septa is closed in the youngest part of the hyphae. Still, compartments in this hyphal part are physically isolated when separated by more than 4 septa. Even open septa act as a barrier for cytoplasmic mixing. The mobility rate through such septa reduces with increasing septal age and under stress conditions. Modeling shows that the septal pore width is set such that its regulation offers maximal control of compound concentration levels within the compartments. Together, we show for the first time that Aspergillus hyphae switch from a unicellular to multicellular organization

Increasing carbohydrate degradation in compost during commercial mushroom production of Agaricus bisporus?

After 2-3 flushes of Agaricus bisporus mushroom production, champost still contains about50% of the carbohydrates and 45% of the lignin originally present in compost. Increaseduptake of the unused pool of polysaccharides may increase mushroom yield. Lignin isremoved during the vegetative growth phase of A. bisporus (phase III) but less efficientlyduring mushroom formation (phase IV). Here, we overexpressed the manganese peroxidasegene mnp1 in A. bisporus by placing it under control of the actin promoter to improve theremoval of lignin, thereby promoting accessibility of hemicellulose and cellulose.Transformants produced MnP activity in liquid malt extract while the wild type strain did not.MnP activity was 3-4 fold increased in wheat bran medium. During a semi-commercialproduction cycle, MnP activity per gram wet compost was increased significantly at the endof phase III (30%) while the activity was similar to the wild type strain at the initiation ofmushroom formation in phase IV. This indicates that other factors than mRNA accumulationmay be limiting in MnP1 production at this stage. After the 1st and 2nd flush, MnP activitywas increased 3-4 fold. There was no difference in mushroom yield or biomass formation incompost as measured by chitin release. In addition, carbohydrates released after enzymatictreatment of milled compost and carbohydrate content was not affected. Finally, lignin wasnot affected differently by the mnp1 overexpressor as determined by pyrolysis. Efficient MnPactivity in compost may have been impaired by cofactor limitation

Regulation of pentose catabolic pathway genes of Aspergillus niger

The aim of this study was to obtain a better understanding of the pentose catabolism in Aspergillus niger and the regulatory systems that affect it. To this end, we have cloned and characterised the genes encoding A. niger L-arabitol dehydrogenase (ladA) and xylitol dehydrogenase (xdhA), and compared the regulation of these genes to other genes of the pentose catabolic pathway. This demonstrated that activation of the pathway depends on two transcriptional regulators, the xylanolytic activator (XlnR) and an unidentified L-arabinose specific regulator (AraR). These two regulators affect those genes of the pentose catabolic pathway that are related to catabolic conversion of their corresponding inducers (D-xylose and L-arabinose, respectively)