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Improving Citric Acid Production of an Industrial Aspergillus Niger CGMCC

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Improving Citric Acid Production of an Industrial Aspergillus Niger CGMCC Xue et al. Microb Cell Fact (2021) 20:168 https://doi.org/10.1186/s12934-021-01659-3 Microbial Cell Factories RESEARCH Open Access Improving citric acid production of an industrial Aspergillus niger CGMCC 10142: identifcation and overexpression of a high-afnity glucose transporter with diferent promoters Xianli Xue1†, Futi Bi1,2†, Boya Liu1,2, Jie Li1, Lan Zhang1, Jian Zhang1, Qiang Gao1 and Depei Wang1,2,3* Abstract Background: Glucose transporters play an important role in the fermentation of citric acid. In this study, a high- afnity glucose transporter (HGT1) was identifed and overexpressed in the industrial strain A. niger CGMCC 10142. HGT1-overexpressing strains using the PglaA and Paox1 promoters were constructed to verify the glucose transporter functions. Result: As hypothesized, the HGT1-overexpressing strains showed higher citric acid production and lower residual sugar contents. The best-performing strain A. niger 20-15 exhibited a reduction of the total sugar content and residual reducing sugars by 16.5 and 44.7%, while the fnal citric acid production was signifcantly increased to 174.1 g/L, representing a 7.3% increase compared to A. niger CGMCC 10142. Measurement of the mRNA expression levels of relevant genes at diferent time-points during the fermentation indicated that in addition to HGT1, citrate synthase and glucokinase were also expressed at higher levels in the overexpression strains. Conclusion: The results indicate that HGT1 overexpression resolved the metabolic bottleneck caused by insufcient sugar transport and thereby improved the sugar utilization rate. This study demonstrates the usefulness of the high- afnity glucose transporter HGT1 for improving the citric acid fermentation process of Aspergillus niger CGMCC 10142. Keywords: Aspergillus niger, Glucose transporter, HGT1, Citric acid fermentation Background most commonly used organic acids in the world [1, 2]. Citric acid (CA) is a naturally occurring tricarboxylic Moreover, the demand for microbially fermented CA has acid that is widely used in food, chemistry, medicine, been increasing at an annual rate of 3.7% in recent years. and other felds, whose high demand places it among the Nowadays, CA is mostly produced using Aspergillus niger fermentation on the industrial scale. As a saprophytic fungus, A. niger secretes abundant hydrolytic enzymes *Correspondence: [email protected] which are benefcial for CA production on complex sub- † Xianli Xue and Futi Bi contributed equally to this article strates [3, 4]. 1 Key Laboratory of Industrial Microbiology & Engineering Research Center of Food Biotechnology of Ministry of Education, College A. niger is one of the most important industrial micro- of Biotechnology, Tianjin University of Science and Technology, organisms, with tremendous commercial value. With the Tianjin 300457, People’s Republic of China development of sequencing and genomic technologies, Full list of author information is available at the end of the article © The Author(s) 2021. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. The Creative Commons Public Domain Dedication waiver (http:// creat iveco mmons. org/ publi cdoma in/ zero/1. 0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Xue et al. Microb Cell Fact (2021) 20:168 Page 2 of 13 rational molecular evolution and modifcation instead did not [20]. Tis native state of A. niger with a low-afn- of random mutagenesis are increasingly being applied ity glucose transporter with high Km and high-afnity to increase the CA fermentation efciency of A. niger glucose transporters with extremely low-expressed levels, [5]. To date, the principal metabolic engineering strate- can explain the limited citrate yields and large amounts of gies in the CA industry focus on the improvement of residual glucose in the fermentation broth [21, 22]. Con- the central metabolic fuxes and the efciency of energy trolling and coordinating these proteins can change the generation in the respiratory chain of A. niger [6–8], nutrient uptake ability of the strains in a targeted manner, mostly via knockout and overexpression of key genes making them more suitable for industrial fermentation. involved in the EMP (glycolysis) pathway, the TCA cycle, Terefore, the glucose transport efciency of A. niger is and electron transport [9–11]. Recent studies focused crucial for increasing CA production and may be one of on energy metabolism and electron transport [12, 13]. the limiting factors for obtaining higher CA yields [21, Te metabolism of A. niger includes a complex network 22]. of replenishment pathways, which can overcome the Many putative sugar transporters have been identifed efects of individual gene overexpression, leading to lim- in fungi. Interestingly, several newly discovered xylose ited improvement of citric acid production. For example, transporters from A. nidulans, A. niger, and T. reesei can the overexpression or deletion of citrate synthase did also transport glucose [23, 24]. Additionally, the low- not improve citric acid production [14, 15]. Tus, this afnity glucose transporter HxtB, which is involved in enzyme does not catalyze a rate-limiting step. glucose signaling was also characterized [25]. However, Accordingly, Torres et al. calculated that the key lim- there are a few reports on the specifc glucose transport- iting points of citric acid production are the intake and ers of A. niger. Based on a conserved protein domain phosphorylation of hexoses [16]. If a high glucose con- search, 86 putative sugar transporter genes were identi- centration is maintained during citric acid fermentation, fed and annotated in the genome of A. niger [26]. Two the CA production peak period of the fermentation pro- new putative high-afnity glucose transporters, named cess will be advanced, but the fnal yield will not increase MstG and MstH, were identifed in A. niger by mem- [17]. In general terms, the type of carbon source and its brane-associated proteome analysis and biochemically concentration are both critical factors determining the characterized [27]. Additionally, it was found that MstA efciency of citric acid fermentation. Xu et al. found that is a high-afnity glucose transporter in A. niger, and its adding 10% w/v of sugars, such as maltose, sucrose, man- disruption resulted in a two- to fvefold reduction of nose or fructose, led to the highest yields of citric acid, cellular glucose uptake [28]. Tis protein is expressed while 75 g/L glucose concentration gave the best results. by the enzyme-producing strain A. niger CBS 513.88 as Furthermore, there was no citric acid production in well as the original industrial acid-producing strain A. the media with less than 2.5% sugar [18]. Mischak et al. niger ATCC 1015 [29], but it is not expressed by A. niger clearly showed that the uptake of glucose was inhibited CGMCC 10142. Additionally, two glucose transporters by small quantities of extracellular citric acid, at concen- were identifed in A. niger H915-1, a low-afnity glucose trations over 0.5 mM [19]. Tus, citric acid production transporter with high expression at high glucose con- and its rate are strongly related to glucose concentration centration, and a high-afnity glucose transporter with and its uptake rate. Furthermore, the absorption rate of extremely low-expression level [20]. Liu et al. increased glucose exhibits a simple linear relationship with the glu- the glucose transport rate and shortened the fermen- cose concentration, and as a highly hydrophilic molecule, tation cycle by overexpressing a low-afnity glucose glucose cannot passively pass through the cell membrane. transporter [30]. Notably, the glucose transporter HGT1 Accordingly, facilitated difusion and active transporter (high-afnity glucose transporter 1), frst identifed in are the main glucose uptake modes [20]. Kluyveromyces lactis, was also found in the A. niger CBS Tere are two types of glucose transporters in A. 513.88 and A. niger CGMCC 10142 [31, 32]. niger, a low-afnity glucose transporter with Km of Te primary substrate used in the A. niger CA indus- 3.67 mmol/L, and a high-afnity glucose transporter with try is corn steep liquor with high amounts of total sugars, Km of 260 µmol/L. Torres et al. found that a high glucose including glucose, disaccharides, and polysaccharides. A. concentration (> 50 g/L) is needed for the low-afnity niger secretes abundant glucoamylases that can hydrolyze glucose transporter to become active, which is capable polysaccharides into readily bioavailable glucose. Tus, of providing the high fux of glucose required for cit- the main carbon source at the beginning of the fermenta- rate production [16]. Transcriptomic analysis of A. niger tion is glucose, and an adequate supply of carbon sources H915-1 during the fermentation process revealed that a can greatly increase the CA production. Undoubtedly, low-afnity glucose transporter maintained high tran- the transport rate of glucose understandably becomes script levels, while 5 high-afnity glucose transporters a limiting factor preventing the further increase of CA Xue et al. Microb Cell Fact (2021) 20:168 Page 3 of 13 yields. Terefore, it is desirable to accelerate the trans- relatively higher glucose concentrations (> 50 g/L) [20].
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