Deletion of the Aspergillus niger pro-protein processing protease gene kexB results in a pH-dependent morphological transition during submerged cultivations and increases cell wall chitin content
There is a growing interest for the use of post-fermentation mycelial waste to obtain cell wall chitin as an added-value product. In the pursuit to identify suitable production strains that can be used for post-fermentation cell wall harvesting, we turned to an Aspergillus niger strain in which the kexB (also named pclA in literature) gene was deleted. Previous work has shown that deletion of kexB causes hyper-branching and thicker cell walls, which is beneficial as these properties reduce fermentation viscosity and lysis. The deletion of kexB has been shown to exhibit a pH-dependent hyper-branching on solid agar plates at pH 6.0, but not at pH 5.0, whereas this phenotype was reported to be less pronounced during submerged growth. Here, we show a series of controlled batch cultivations performed at a pH range of 5, 5.5, and 6 to examine the pellet phenotype in liquid medium of the ?kexB strain. Morphological analysis showed that the ?kexB formed wild type-like pellets at pH 5.0 whereas the characteristic hyper-branching ?kexB phenotype was found at pH 6.0. Cultivations at pH 5.5 showed that the ?kexB strain grows as an intermediate phenotype of pH 5.0 and pH 6.0. Analyzing the cell walls of the ?kexB strain from these controlled pH-conditions showed an increase in chitin content compared to wild type at all three pH values tested. Surprisingly, the increase in chitin content was found to be irrespective of the hyper-branching morphology. Evidence for alterations in cell wall make-up are corroborated by transcriptional analysis that showed a significant cell wall stress response in addition to upregulation of genes encoding other unrelated cell well biosynthetic genes. Overall design: Expression profiles A. niger strains; wild type versus kexB deletion strain from bioreactor cultures, using RNA-sequencing, duplicate biological samples,  90% of maximum attained biomass per biological samples. 4 samples total