Improving Citric acid Production of an Industrial Aspergillus Niger CGMCC 10142: Identification and Overexpression of a High-affinity Glucose Transporter with Different Promoters

Background: The transporter of glucose plays an important role in the fermentation of citric acid. In this research, a high-affinity glucose transporter (HGT1) was identified and overexpressed in industrial strain A. niger CGMCC 10142. The overexpressed-HGT1 strains were obtained to verify the glucose transporter functions with different promoters PglaA and Paox1.Result: The sugar contents and citric acid production during fermentation were tracked. As speculated, the HGT1-overexpressed strains showed higher citric acid production and lower residual sugar contents. The best-performing strain on the fermentation is A. niger 20-15 transformant of which the total sugar content and residual reducing sugar both were reduced by 31% and 44.9% comparing to A. niger CGMCC 10142, and the final citric acid production was correspondingly increased to 174.1 g/L comparing to A. niger CGMCC 10142 with 162.3 g/L. Besides HGT1, the transcription levels of related genes as citrate synthase and glucokinase in transformants were also significantly higher than A. niger CGMCC 10142.Conclusion: The results indicate that HGT1 understandably alleviates the metabolic restriction caused by insufficient sugar transporters and finally improves the utilization of sugar. This study confirms the important role of glucose transporter HGT1 in the citric acid fermentation process of Aspergillus niger.