Aspergillus niger is an industrially important source for gluconic acid and glucose
oxidase (GOx), a secreted commercially important flavoprotein which catalyses the
oxidation of b-D-glucose by molecular oxygen to D-glucolactone and hydrogen
peroxide. Expression of goxC, the GOx encoding gene and the concomitant two step
conversion of glucose to gluconic acid requires oxygen and the presence of significant
amounts of glucose in the medium and is optimally induced at pH 5.5. The molecular
mechanisms underlying regulation of goxC expression are, however, still enigmatic.
Genetic studies aimed at understanding GOx induction have indicated the involvement
of at least seven complementation groups, for none of which the molecular basis has
been resolved. In this study, a mapping-by-sequencing forward genetics approach was
used to uncover the molecular role of the goxB locus in goxC expression. Using the
Illumina and PacBio sequencing platforms a hybrid high quality draft genome assembly
of laboratory strain N402 was obtained and used as a reference for mapping of genomic
reads obtained from the derivative NW103:goxB mutant strain. The goxB locus encodes
a thioredoxin reductase. A deletion of the encoding gene in the N402 parent strain led
to a high constitutive expression level of the GOx and the lactonase encoding genes
required for the two-step conversion of glucose in gluconic acid and of the catR gene
encoding catalase R. This high constitutive level of expression was observed to be
irrespective of the carbon source and oxidative stress applied. A model clarifying the
role of GoxB in the regulation of the expression of goxC involving hydrogen peroxide as
second messenger is presented.
