Improved high-temperature ethanol production from sweet sorghum juice using Zymomonas mobilis overexpressing groESL genes.

Zymomonas mobilis may encounter various types of stress during ethanol fermentation, which reduces ethanol production efficiency. This situation may be mitigated by molecular chaperones, including the chaperonin GroESL, which confers enhanced protection against various stresses. In this study, we successfully developed a Z. mobilis strain R301 that harbors groESL genes and can be used for high-temperature ethanol production from sweet sorghum juice. Sequence analyses of GroES and GroEL from Z. mobilis TISTR548 demonstrated conserved residues at specific positions within GroES and conserved glycine-glycine-methionine (GGM) repeats at the C-terminus of GroEL. The Z. mobilis wild-type and R301 strains were then evaluated for their tolerance to stresses, including high temperatures, high sugar concentrations, and high ethanol concentrations up to 40°C, 300 g/L, and 13% (v/v), respectively. Z. mobilis R301 exhibited better growth performance than the wild-type strain under all stress conditions. This is the first report on ethanol production at 40°C by recombinant Z. mobilis using sweet sorghum juice; this strain produced an ethanol concentration of 41.66 g/L, with a productivity of 0.87 g/L/h and a theoretical ethanol yield of 88.9%. Overexpression of groESL resulted in increased ethanol production, with values approximately 11% higher than those of the wild type at 40°C. Additionally, at 37°C, Z. mobilis R301 gave a higher theoretical ethanol yield (92.6%) than that shown in previous research. This work illustrates the potential for future enhancement of industrial-scale ethanol production at high temperatures utilizing Z. mobilis R301 in the bioconversion of sweet sorghum juice, a promising energy crop. KEY POINTS: • The groESL-overexpressing Z. mobilis strain was successfully constructed. • The recombinant Z. mobilis exhibited higher stress tolerance than the wild-type strain. • Overexpression of groESL genes improved ethanol production efficiency at high temperatures.