Pectin is an important carbon source for the saprotrophic fungus Aspergillus niger, which is an efficient
producer of pectin-degrading enzymes. D-galacturonic acid (GA) is the main product of pectin
degradation. In A. niger, GA is transported into the cell by the sugar transporter GatA and subsequently
catabolized into pyruvate and glycerol through a pathway consisting of four enzymes: GaaA, Dgalacturonate
reductase; GaaB, L-galactonate dehydratase; GaaC, 2-keto-3-deoxy-L-galactonate
aldolase; and GaaD, L-glyceraldehyde reductase. It has been shown that GA or a derivative of GA is
required for the induction of the genes needed for pectin degradation, GA transport, and GA catabolism.
In order to identify the GA derivative that acts as an inducer, we constructed GA catabolic pathway
deletion mutants (ΔgaaA, ΔgaaB, ΔgaaC and ΔgaaD) anticipating that the mutants would accumulate the
substrate of the deleted enzyme when grown on GA. The growth of both ΔgaaB and ΔgaaC was
abolished on GA pointing out that there are no redundant enzymes replacing GaaB and
GaaC. ΔgaaB and ΔgaaC, pregrown in fructose medium and then transferred to GA medium,
accumulated pathway intermediates L-galactonate and 2-keto-3-deoxy-L-galactonate, respectively.
Northern analysis showed that the expression of the GA induced genes is drastically reduced
in ΔgaaB and highly increased in ΔgaaC compared to the wild type strain. Genome wide gene expression
analysis via RNA-seq indicated that the accumulation of 2-keto-3-deoxy-L-galactonate in ΔgaaC results in
the induction of the GA induced genes. Identification of the inducer of the pectinase genes, together with
the recent identification of the transcriptional activator-repressor module controlling pectinase expression
would facilitate the industrial use of A. niger in pectinase production.
Full conference title:
- Asperfest 14 (2017)