Combinatorial control of gene expression in Aspergillus niger grown on sugar beet pectin

Joanna E Kowalczyk,Ronnie J M Lubbers,Ronald P De Vries,Mao Peng,Evy Battaglia,Jaap Visser

doi:10.1038/s41598-017-12362-y

Joanna E Kowalczyk, Ronnie J M Lubbers + Show 4 more

Open Access

https://doi.org/10.1038/s41598-017-12362-y

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Abstract

Aspergillus niger produces an arsenal of extracellular enzymes that allow synergistic degradation of plant biomass found in its environment. Pectin is a heteropolymer abundantly present in the primary cell wall of plants. The complex structure of pectin requires multiple enzymes to act together. Production of pectinolytic enzymes in A. niger is highly regulated, which allows flexible and efficient capture of nutrients. So far, three transcriptional activators have been linked to regulation of pectin degradation in A. niger. The L-rhamnose-responsive regulator RhaR controls the production of enzymes that degrade rhamnogalacturonan-I. The L-arabinose-responsive regulator AraR controls the production of enzymes that decompose the arabinan and arabinogalactan side chains of rhamnogalacturonan-II. The D-galacturonic acid-responsive regulator GaaR controls the production of enzymes that act on the polygalacturonic acid backbone of pectin. This project aims to better understand how RhaR, AraR and GaaR co-regulate pectin degradation. For that reason, we constructed single, double and triple disruptant strains of these regulators and analyzed their growth phenotype and pectinolytic gene expression in A. niger grown on sugar beet pectin.

Highlights

Filamentous fungi secrete an arsenal of enzymes that decompose plant polymers such as storage and cell wall polysaccharides
The third Transcription Factors (TFs) involved in pectin degradation, AraR, activates expression of genes encoding enzymes needed for L-arabinose release from rhamnogalacturonan I (RG-I) side chains and its metabolism[16]
Growth was abolished in the triple mutant, ΔgaaRΔaraRΔrhaR, on D-galacturonic acid, L-arabinose and L-rhamnose because conversion of those monosaccharides relies on GaaR, AraR and RhaR, respectively[14,15,16]

Summary

Introduction

Filamentous fungi secrete an arsenal of enzymes that decompose plant polymers such as storage (starch, inulin) and cell wall polysaccharides (cellulose, hemicellulose, pectin). Large polysaccharides, such as pectin, have potential to release multiple inducers simultaneously This will result in a complex transcriptional response including activation, repression and de-repression mechanisms mediated by several TFs. So far, three transcriptional activators (GaaR, RhaR and AraR) and two transcriptional repressors (GaaX, CreA) have been linked to pectin degradation in A. niger[14,15,16,17,18]. A conserved GARE motif CC[ACTG]CCAA was found in the promoters of all genes up-regulated in the presence of D-galacturonic acid[21] and this regulatory element was shown to be essential for GaaR-dependent gene expression in A. niger[18]. The third TF involved in pectin degradation, AraR, activates expression of genes encoding enzymes needed for L-arabinose release from RG-I side chains (abfA, abfB) and its metabolism (larA/gaaD, ladA, lxrA, xdhA, xkiA)[16]. Several D-galacturonic acid-induced pectinolytic genes (pgaI, pgaII, pgaC, pgaE, pelB, pelC, plyA, rhgA, rgaeA) were shown to be under control of the general carbon catabolite repressor, CreA17,18

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