Spatial induction of genes encoding secreted proteins in micro-colonies of Aspergillus niger

Aspergillus niger is used by the industry to produce enzymes and metabolites such as citric acid. In liquid cultures, it can grow as a dispersed mycelium or as micro-colonies with a width in the micrometer to millimeter range. Here, it was assessed whether expression of genes encoding secreted enzymes depends on mycelium morphology. To this end, expression of the reporter gene gfp from the promoters of the glucoamylase gene glaA, the feruloyl esterase gene faeA and the α-glucuronidase gene aguA was causally related to micro-colony size within a liquid shaken culture. Data could be fitted by hyperbolic functions, implying that the genes encoding these secreted proteins are expressed in a shell at the periphery of the micro-colony. The presence of such a shell was confirmed by confocal microscopy. Modelling predicted that the width of these zones was 13 to 156 µm depending on growth medium and micro-colony diameter. Together, data indicate that the highest productive micro-colonies are those colonies that have a radius ≤ the width of the peripheral expression zone.