Catabolism of l-rhamnose in A. nidulans proceeds via the non-phosphorylated pathway and is glucose repressed by a CreA-independent mechanism

• Published in: Microbial cell factories Vol. 19; no. 1; pp. 1 - 15
• Main Authors: MacCabe, Andrew P., Ninou, Elpinickie I., Pardo, Ester, Orejas, Margarita
• Format: Journal Article
• Language: English
• Published: London BioMed Central Ltd 02.10.2020; BioMed Central; BMC
• Subjects: Aspergillus nidulans; Bioactive compounds; Carbon; Catabolism; CCR; CreA-independent; Dehydrogenases; E coli; Enzymes; Experiments; Extracellular enzymes; Fungi; Gene expression; Gene silencing; Genes; Genetic research; Glucose; Kinases; L-Rhamnose; l-rhamnose catabolism; Lactose; Mannose; Metabolism; Metabolites; Microorganisms; Phenotypes; Polysaccharides; Regulatory mechanisms (biology); Rhamnose; RhaR; Saccharides; Substrates; Sugar; Transcription (Genetics); Transcriptional regulation; Yeast
• DOI: 10.1186/s12934-020-01443-9
• ISSN: 1475-2859

l -rhamnose (6-deoxy-mannose) occurs in nature mainly as a component of certain plant structural polysaccharides and bioactive metabolites but has also been found in some microorganisms and animals. The release of l -rhamnose from these substrates is catalysed by extracellular enzymes including α- l -rhamnosidases, the production of which is induced in its presence. The free sugar enters cells via specific uptake systems where it can be metabolized. Of two l -rhamnose catabolic pathways currently known in microorganisms a non-phosphorylated pathway has been identified in fungi and some bacteria but little is known of the regulatory mechanisms governing it in fungi. In this study two genes ( lraA and lraB ) are predicted to be involved in the catabolism of l -rhamnose, along with lraC , in the filamentous fungus Aspergillus nidulans . Transcription of all three is co-regulated with that of the genes encoding α- l -rhamnosidases, i.e. induction mediated by the l -rhamnose-responsive transcription factor RhaR and repression of induction in the presence of glucose via a CreA-independent mechanism. The participation of lraA /AN4186 (encoding l -rhamnose dehydrogenase) in l -rhamnose catabolism was revealed by the phenotypes of knock-out mutants and their complemented strains. lraA deletion negatively affects both growth on l -rhamnose and the synthesis of α- l -rhamnosidases, indicating not only the indispensability of this pathway for l -rhamnose utilization but also that a metabolite derived from this sugar is the true physiological inducer.