Inducer-independent production of pectinases in Aspergillus niger by overexpression of the D-galacturonic acid-responsive transcription factor gaaR

Ebru Alazi,Adrian Tsang,Jaap Visser,Marcos Di Falco,Ian D. Reid,Arthur F. J. Ram,Tim Knetsch,Mark Arentshorst

doi:10.1007/s00253-018-8753-7

Ebru Alazi, Adrian Tsang + Show 6 more

Open Access

https://doi.org/10.1007/s00253-018-8753-7

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Abstract

The transcription factor GaaR is needed for the expression of genes required for pectin degradation and transport and catabolism of the main degradation product, D-galacturonic acid (GA) in Aspergillus niger. In this study, we used the strong constitutive gpdA promoter of Aspergillus nidulans to overexpress gaaR in A. niger. Overexpression of gaaR resulted in an increased transcription of the genes encoding pectinases, (putative) GA transporters, and catabolic pathway enzymes even under non-inducing conditions, i.e., in the absence of GA. Exoproteome analysis of a strain overexpressing gaaR showed that this strain secretes highly elevated levels of pectinases when grown in fructose. The genes encoding exo-polygalacturonases were found to be subjected to CreA-mediated carbon catabolite repression, even in the presence of fructose. Deletion of creA in the strain overexpressing gaaR resulted in a further increase in pectinase production in fructose. We showed that GaaR localizes mainly in the nucleus regardless of the presence of an inducer, and that overexpression of gaaR leads to an increased concentration of GaaR in the nucleus.

Highlights

Aspergillus niger is an important filamentous fungus for the industrial production of pectinases (Pedrolli et al 2009)
We further show that the effect of gaaR overexpression on pectinase production is sensitive to CreA-mediated carbon catabolite repression even when fructose, a less repressing carbon source compared to glucose, was used
To create strains that overexpress the galacturonic acid (GA)-responsive transcription factor gene gaaR (OEgaaR), the A. niger gaaR gene was fused with the strong constitutive A. nidulans gpdA promoter and transformed into a ΔgaaR strain

Summary

Introduction

Aspergillus niger is an important filamentous fungus for the industrial production of pectinases (Pedrolli et al 2009). Pectinases are widely used in the food industry (Kashyap et al 2001; Toushik et al 2017; Khan et al 2013) and are important enzymes in the utilization of pectin-rich feedstock in biofuel production (Edwards and Doran-Peterson 2012). A. niger contains a large number of enzymes potentially acting on pectin substructures (Martens-Uzunova and Schaap 2009; Coutinho et al 2009; De Vries et al 2017). Loss of function of GaaX leads to constitutive and inducer-independent expression of pectinases

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