Enhancing <scp>l</scp>-Malic Acid Production in <i>Aspergillus niger</i> via Natural Activation of <i>sthA</i> Gene Expression

Abstract

The efficient production of l-malic acid using <i>Aspergillus niger</i> requires overcoming challenges in synthesis efficiency and excessive byproduct buildup. This study addresses these hurdles, improving the activity of NADH-dependent malate dehydrogenase (Mdh) in the early stages of the fermentation process. By employing a constitutive promoter to express the <i>Escherichia coli</i> <i>sthA</i> responsible for the transfer of reducing equivalents between NAD(H) and NADP(H) in <i>A. niger</i>, the l-malic acid production was significantly elevated. However, this resulted in conidiation defects of <i>A. niger</i>, limiting industrial viability. To mitigate this, we discovered and utilized the PmfsA promoter, enabling the specific expression of <i>sthA</i> during the fermentation stage. This conditional expression strain showed similar phenotypes to its parent strain while exhibiting exceptional performance in a 5 L fermenter. Notably, it achieved a 65.5% increase in productivity, reduced fermentation cycle by 1.5 days, and lowered succinic acid by 76.2%. This work marks a promising advancement in industrial l-malic acid synthesis via biological fermentation, showcasing the potential of synthetic biology in <i>A. niger</i> for broader applications.