4-05: Transcriptome of Zymomonas mobilis 8b to acetate in mixed sugars

The efficient utilization of mixed sugars of glucose and xylose in the biomass hydrolysate is crucial for economic biofuel production. Acetate is one of the major inhibitors in pretreated biomass hydrolysate.  We have previously engineered Zymomonas mobilis 8b to utilize both glucose and xylose and studied the acetate stress response when grown in either of these sugars, observing significant differences. However, the molecular response of 8b in mixed sugars of glucose and xylose hasn’t been investigated. In this study, microarray technology was used to investigate the acetate stress response of 8b in mixed glucose/xylose fermentations. In the presence of acetate, 8b still can utilize all the glucose (though xylose utilization was inhibited) with similar ethanol yield although the growth, final biomass, and ethanol production rate were reduced. Transcriptomic studies of 8b in different growth phases (log, mid-log and stationary) indicated that the response to acetate is dynamic and complex. The presence of acetate caused genes related to biosynthesis, flagellar system, and glycolysis to be downregulated, and genes related to stress responses and energy metabolism to be upregulated. In addition, we reviewed the microarray data from these experiments in light of previous work, and compared the different acetate effects on Z. mobilis when grown under different sugar regimes. Our study has provided insights into the molecular responses of the model ethanologenic bacterium Z. mobilis to pretreatment inhibitor acetate in different sugar sources, which will help identify the gene candidates for better xylose utilization and acetate tolerance.