Morphological regulation of Aspergillus niger to improve citric acid production by chsC gene silencing

Abstract

The mycelial morphology of Aspergillus niger, a major filamentous fungus used for citric acid production, is important for citric acid synthesis during submerged fermentation. To investigate the involvement of the chitin synthase gene, chsC, in morphogenesis and citric acid production in A. niger, an RNAi system was constructed to silence chsC and the morphological mutants were screened after transformation. The compactness of the mycelial pellets was obviously reduced in the morphological mutants, with lower proportion of dispersed mycelia. These morphological changes have caused a decrease in viscosity and subsequent improvement in oxygen and mass transfer efficiency, which may be conducive for citric acid accumulation. All the transformants exhibited improvements in citric acid production; in particular, chsC-3 showed 42.6% higher production than the original strain in the shake flask. Moreover, the high-yield strain chsC-3 exhibited excellent citric acid production potential in the scale-up process.The citric acid yield and the conversion rate of glucose of chsC-3 were both improved by 3.6%, when compared with that of the original strain in the stirred tank bioreactor.

Moulding the mould: understanding and reprogramming filamentous fungal growth and morphogenesis for next generation cell factories

“…As just one example, the A. niger chitin synthase encoding gene An12g10380 ( chsE ) is transcriptionally upregulated during citrate fermentation [63] and following over-expression of a glucoamylase encoding gene [85], strongly suggesting that (i) chitin synthesis at the cell wall is a critical component of morphological development during industrial applications, and (ii) genetic targeting of this process could be used to modify and possibly optimise morphology. This hypothesis has been validated by RNAi knockdown of chitin synthase encoding genes in A. niger ( chsC ) and P. chrysogenum ( chs4 ), which result in compact pellets and highly branched morphology, and eventually in elevated citric acid (40%) and penicillin product titres (27–41%), respectively [89, 90].…”

Citric acid and itaconic acid accumulation: variations of the same story?

“…Consistent with this, Yin et al ( 2017 ) found that expression of chitin synthase C ( chsC ) and pstA , encoding a cell surface glycosylphosphatidylinositol-anchored protein important for cell wall integrity and resistance against low pH (Pardo et al 2004 ; Gil-Bona et al 2015 ), were highly upregulated during citric acid fermentation. Sun et al ( 2018 ) silenced chsC in an industrial mutant strain which resulted in strains with altered morphology and with lower proportion of dispersed mycelia, which caused the desired decrease in viscosity, improved oxygen, and mass transfer efficiency and improved citric acid production. Dai et al ( 2004 ) applied a different strategy (suppression subtractive hybridization) to identify genes that are differentially expressed under manganese sufficient and deficient conditions, and identified a gene in A .…”

Disruption or reduced expression of the orotidine-5′-decarboxylase gene pyrG increases citric acid production: a new discovery during recyclable genome editing in Aspergillus niger

“…Currently, this toolkit remains a key objective for biotechnologists as genetic leads for generating A. niger citric acid hyper-producing isolates are limited [5,16,17]. For example, several studies have used gene knock-down of the chitin synthase chsC [18] or amino acid transporter Brsa-25 [19] to elevate citric acid titers about 42.6% [18] and 10% [19], respectively. Alternatively, the over-expression of the organic acid transporter cexA has been used for hyperproduction [20].…”