Strategies to Down Regulate Ethanol Pathway in Zymomonas

Zymomonas mobilis has been engineered for efficient conversion of lignocellulosic biomass to ethanol because of its natural tolerance to ethanol and high sugar concentrations, fast sugar utilization rate, low biomass yield, high product yield, and its ability to grow and ferment under anaerobic conditions. By applying classical recombinant DNA technologies and advanced genetic tools, we have not only augmented Z. mobilis for efficient ethanol production from cellulosic biomass hydrolysates, but also further introduced and expressed heterologous genes into Z. mobilis for the production of other biofuel molecules or precursors (e.g., hydrocarbons), taking advantage of a key intermediate in the Entner–Doudoroff pathway, pyruvate.  However, despite our efforts to overexpress pathway enzymes for the production of new molecules, ethanol remains the dominant product from fermentation, indicating the strong activity of pyruvate decarboxylase (PDC) in Zymomonas. PDC shunts pyruvate to ethanol, leaving little substrate for other pathways and this enzyme has been shown to be highly expressed in this organism. Here, we will discuss strategies for down regulating PDC with the intention of increasing the availability of pyruvate as the substrate for desirable (non-ethanol) products in Zymomonas.