# Upregulation of Key Metabolic and Stress-Response Genes Enhanced Growth and High-Temperature Ethanol Fermentation of Thermotolerant Zymomonas mobilis 200M and PYK

## TL;DR

Scientists improved bacteria to produce more ethanol at high temperatures, which could make biofuel production more efficient and sustainable.

## Contribution

The study reveals how specific gene upregulation in thermotolerant Zymomonas mobilis strains enhances ethanol production and stress tolerance.

## Key findings

- Thermotolerant Zymomonas mobilis strains 200M and PYK produced up to 6.4-fold more ethanol at 40 °C compared to the wild-type.
- These strains showed superior tolerance to heat, ethanol, acetic acid, formic acid, and H2O2.
- RT-qPCR analysis showed coordinated overexpression of genes related to ethanol production and stress response.

## Abstract

Bioethanol is a renewable fuel that helps reduce our dependence on fossil fuels and lowers carbon emissions. To increase the fuel yield and reduce production costs, the bacteria used in the fermentation process must remain active even under stressful industrial conditions. In this study, two heat-tolerant strains of the bacterium Zymomonas mobilis were tested for their growth and ethanol production under heat stress. We found that these new strains could grow and produce six times more ethanol at 40 °C (104 °F) compared to the original parental strain. They also showed a better ability to survive other industrial challenges, such as high acidity and alcohol levels. By studying their internal activity, we identified specific protective systems that these bacteria activate to withstand high temperatures. These findings are valuable to society because these newly developed bacteria can make biofuel production more efficient and sustainable. This leads to the development of cheaper green energy and a cleaner environment.

Among various ethanologenic microorganisms, thermotolerant Zymomonas mobilis has emerged as a promising candidate for industrial ethanol production at elevated temperatures. However, the comparative fermentation efficiency and the underlying molecular mechanisms driving thermotolerance in newly developed strains remain largely unexplored, hindering their industrial application. In this study, the recently developed thermotolerant strains Z. mobilis 200M and Z. mobilis PYK exhibited critical high temperatures for growth approximately 2.0 and 2.5 °C higher than the wild-type, respectively. While 40 °C represents severe heat stress that completely inhibits the growth of the wild-type, the thermotolerant strains remained viable, exhibiting significantly shorter cell lengths under these conditions. This study provides the first evidence of their superior multi-stress tolerance toward heat, ethanol, acetic acid, formic acid, and H2O2. Furthermore, the thermotolerant strains exhibited significantly higher ethanol fermentation efficiencies than the wild-type. At 40 °C, Z. mobilis 200M produced approximately 5.8-fold and 3.0-fold more ethanol than the wild-type after 24 and 48 h, respectively, while Z. mobilis PYK yielded 6.4-fold and 3.1-fold increases. Novel transcriptional insights via RT-qPCR revealed the simultaneous overexpression of genes involved in ethanol production, protein quality control, and signal transduction, particularly during the exponential phase under heat stress. Collectively, these findings bridge the gap between strain development and molecular understanding, suggesting that the coordinated upregulation of these genetic pathways enhances the adaptive capacity and fermentation efficiency of these thermotolerant strains during sustained growth at 40 °C.