Publisher Summary This chapter provides a brief description of nitrogen catabolite repression in Saccharomyces cerevisiae. The ancestral use of yeasts in industry has led to the selection of many wild-type strains of Saccharomyces cerevisiae. Saccharomyces spp. are very different from bacteria, although in vegetative growth they can be handled in a similar way. This is probably the origin of the choice of these organisms when prokaryotic cellular physiologists became interested in eukaryotic organisms. Anabolism, catabolism, and regulation of arginine metabolism have also been studied in parallel in Saccharomyces cerevisiae. The chapter also discusses the nitrogen metabolite repression in filamentous fungi. A number of filamentous fungi are able to utilize a wide range of nitrogen sources. This metabolic versatility implies a strong selective pressure for a mechanism to ensure preferential utilization of the favorable nitrogen sources. This mechanism, nitrogen metabolite repression of the syntheses of many enzymes and permeases involved in nitrogen nutrition, has been extensively studied in two filamentous fungi, the ascomycetes Aspergillus nidulans and Neurospora crassa.

Nitrogen catabolite repression in yeasts and filamentous fungi.

https://mmbr.asm.org/content/mmbr/45/3/437.full.pdf

Regulation of nitrogen metabolism and gene expression in fungi.

Filamentous fungi constitute a large group of eukaryotic microorganisms that grow by forming simple tube-like hyphae that are capable of differentiating into more-complex morphological structures and distinct cell types. Hyphae form filamentous networks by extending at their tips while branching in subapical regions. Rapid tip elongation requires massive membrane insertion and extension of the rigid chitin-containing cell wall. This process is sustained by a continuous flow of secretory vesicles that depends on the coordinated action of the microtubule and actin cytoskeletons and the corresponding motors and associated proteins. Vesicles transport cell wall-synthesizing enzymes and accumulate in a special structure, the Spitzenkorper, before traveling further and fusing with the tip membrane. The place of vesicle fusion and growth direction are enabled and defined by the position of the Spitzenkorper, the so-called cell end markers, and other proteins involved in the exocytic process. Also important for tip extension is membrane recycling by endocytosis via early endosomes, which function as multipurpose transport vehicles for mRNA, septins, ribosomes, and peroxisomes. Cell integrity, hyphal branching, and morphogenesis are all processes that are largely dependent on vesicle and cytoskeleton dynamics. When hyphae differentiate structures for asexual or sexual reproduction or to mediate interspecies interactions, the hyphal basic cellular machinery may be reprogrammed through the synthesis of new proteins and/or the modification of protein activity. Although some transcriptional networks involved in such reprogramming of hyphae are well studied in several model filamentous fungi, clear connections between these networks and known determinants of hyphal morphogenesis are yet to be established.

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Fungal Morphogenesis, from the Polarized Growth of Hyphae to Complex Reproduction and Infection Structures

Hyperoxidant states cause microbial cell differentiation by cell isolation from dioxygen.

Experiments were designed to clone and identify genes of the fungal phytopathogen Colletotrichum gloeosporioides expressed at high levels during growth on the compatible host Stylosanthes guianensis when compared with expression in axenic culture. A cDNA clone (pCgGS) that hybridised preferentially to a cDNA probe prepared from infected leaves was isolated by the differential screening of a cDNA library from a nitrogen-starved axenic culture of C. gloeosporioides. The DNA sequence of pCgGS is highly homologous to genes for glutamine synthetase (GS) in other organisms. pCgGS contained all of the conserved regions assigned as catalytic domains in GS enzymes. Comparison with genomic sequences indicated that in C. gloeosporioides the GS gene is present as a single copy with three introns. To our knowledge this is the first report of the cloning of a GS from a filamentous fungus. A second clone (pCgRL1) was also isolated and represented a partial cDNA of the 25s rRNA of C. gloeosporioides. Because pCgRL1 did not hybridise to plant rRNA under high-stringency hybridisation conditions, it was used as a reference to quantify the expression of fungal GS mRNA during pathogenesis in S. guianensis compared to fungal growth in axenic culture. The results indicated that elevated expression of GS occurred during pathogenesis of C. gloeosporioides on S. guianensis, particularly at early stages of infection where expression was about six-times higher than during growth in rich culture media. This work also demonstrates that fungal-specific 25s rRNA fragments, such as pCgRL1, have considerable utility as a reference for quantifying pathogen gene expression in infected plants.

Cloning and characterisation of glutamine synthetase from Colletotrichum gloeosporioides and demonstration of elevated expression during pathogenesis on Stylosanthes guianensis

Neurospora crassa glutamine synthetase mRNA was measured by its capacity to direct the synthesis of the specific protein in a cell-free system derived from rabbit reticulocytes. N. crassa cultures grown on glutamate as the sole nitrogen source had higher mRNA activities than did those grown on glutamine. The differences were about 10-fold when polysomal RNA was used for translation and about 5-fold when either total cellular RNA or polyadenylic acid-enriched cellular RNA was used. These data indicate that in exponentially growing N. crassa, the nitrogen source regulates glutamine synthetase by adjusting specific mRNA levels.

Wilhelm Hansberg

Papers

Nitrogen source regulates glutamine synthetase mRNA levels in Neurospora crassa.

Regulation of glutamine synthetase messenger ribonucleic acid in conidia ofNeurospora crassa