Thermotolerant genes essential for survival at a critical high temperature in thermotolerant ethanologenic Zymomonas mobilis TISTR 548

Kannikar Charoensuk,Mamoru Yamada,Pornthap Thanonkeo,Tomoyuki Kosaka,Tomoko Sakurada,Amina Tokiyama,Masayuki Murata

doi:10.1186/s13068-017-0891-0

Abstract

BackgroundHigh-temperature fermentation (HTF) technology is expected to reduce the cost of bioconversion of biomass to fuels or chemicals. For stable HTF, the development of a thermotolerant microbe is indispensable. Elucidation of the molecular mechanism of thermotolerance would enable the thermal stability of microbes to be improved.ResultsThermotolerant genes that are essential for survival at a critical high temperature (CHT) were identified via transposon mutagenesis in ethanologenic, thermotolerant Zymomonas mobilis TISTR 548. Surprisingly, no genes for general heat shock proteins except for degP were included. Cells with transposon insertion in these genes showed a defect in growth at around 39 °C but grew normally at 30 °C. Of those, more than 60% were found to be sensitive to ethanol at 30 °C, indicating that the mechanism of thermotolerance partially overlaps with that of ethanol tolerance in the organism. Products of these genes were classified into nine categories of metabolism, membrane stabilization, transporter, DNA repair, tRNA modification, protein quality control, translation control, cell division, and transcriptional regulation.ConclusionsThe thermotolerant genes of Escherichia coli and Acetobacter tropicalis that had been identified can be functionally classified into 9 categories according to the classification of those of Z. mobilis, and the ratio of thermotolerant genes to total genomic genes in Z. mobilis is nearly the same as that in E. coli, though the ratio in A. tropicalis is relatively low. There are 7 conserved thermotolerant genes that are shared by these three or two microbes. These findings suggest that Z. mobilis possesses molecular mechanisms for its survival at a CHT that are similar to those in E. coli and A. tropicalis. The mechanisms may mainly contribute to membrane stabilization, protection and repair of damage of macromolecules and maintenance of cellular metabolism at a CHT. Notably, the contribution of heat shock proteins to such survival seems to be very low.

Highlights

High-temperature fermentation (HTF) technology is expected to reduce the cost of bioconversion of biomass to fuels or chemicals
The thermotolerant genes of Escherichia coli and Acetobacter tropicalis that had been identified can be functionally classified into 9 categories according to the classification of those of Z. mobilis, and the ratio of thermotolerant genes to total genomic genes in Z. mobilis is nearly the same as that in E. coli, though the ratio in A. tropicalis is relatively low
There are 7 conserved thermotolerant genes that are shared by these three or two microbes. These findings suggest that Z. mobilis possesses molecular mechanisms for its survival at a critical high temperature (CHT) that are similar to those in E. coli and A. tropicalis

Summary

Introduction

High-temperature fermentation (HTF) technology is expected to reduce the cost of bioconversion of biomass to fuels or chemicals. Elucidation of the molecular mechanism of thermotolerance would enable the thermal stability of microbes to be improved. Since the ethanol fermentation process is exothermic [7, 8], ethanologenic microorganisms are exposed to heat stress in addition to other stresses including ethanol [9, 10]. Z. mobilis TISTR 548 is a thermotolerant strain that can grow even at 39 °C [19,20,21], which is 5–10 °C higher than the optimum temperature for the same genus [22] and the same species [1, 23], and it can efficiently produce ethanol to an extent similar to that of ZM4 [3]. Information on the molecular mechanism of the thermotolerance of thermotolerant Z. mobilis is limited, though some heat shock proteins have been analyzed [24, 25]

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Results

Discussion

Conclusion