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

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.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-017-0891-0) contains supplementary material, which is available to authorized users.

Improvement of Thermotolerance of Zymomonas mobilis by Genes for Reactive Oxygen Species-Scavenging Enzymes and Heat Shock Proteins

“…This study was motivated by the surprising fact that only a few genes for RSEs and HSPs have been identified as thermotolerant genes (Charoensuk et al, 2017;Murata et al, 2018), which are essential for survival at a CHT. The finding presented suggests that many genes for RSEs and HSPs have the potential to improve the CHT, although the range of improvement is within 1 • C. It is thus likely that there are several genes that have overlapping functions in cells, and thus, the disruption of one gene is complemented by another gene.…”

“…Microorganisms intrinsically have an upper temperature limit for survival called a critical high temperature (CHT) (Matsushita et al, 2015;Kosaka et al, 2019). Genome-wide analysis of three mesophiles, Escherichia coli, Acetobacter tropicalis, and Zymomonas mobilis, by screening thermosensitive mutants either with a single-knockout mutant library or with a transposoninserted mutant library has revealed that about 1.5% of genomic genes, called thermotolerant genes, are responsible for cell survival at a CHT (Charoensuk et al, 2017;Murata et al, 2018), but there is no sufficient information to conclude that a mesophile with a larger number of genomic genes has a larger number of thermotolerant genes and thus tends to be more temperatureresistant. Other factors including gene expression of key proteins may also contribute to the degree of thermotolerance.…”

“…Other factors including gene expression of key proteins may also contribute to the degree of thermotolerance. Thermotolerant genes are categorized into nine groups, including genes for metabolism, membrane stabilization, transporter, DNA repair, tRNA modification, protein quality control, translation control, cell division, and transcriptional regulation (Murata et al, 2011(Murata et al, , 2018Soemphol et al, 2011;Charoensuk et al, 2017), which are mostly related to fundamental activities of cells. The CHT differs to some extent from strain to strain even in the same species such that relatively thermotolerant strains can be isolated in tropical areas, assuming that thermotolerant strains have adapted to the environmental temperature.…”

“…It is thus assumed that the impact of the CHT causes intracellular oxidative stress to elicit harmful effects on cells as a secondary stress. However, only one gene and no gene for reactive oxygen species (ROS)scavenging enzymes (RSEs) was found as a thermotolerant gene in A. tropicalis and Z. mobilis, respectively (Soemphol et al, 2011;Charoensuk et al, 2017). The CHT would also cause damage of proteins to be unfolded or denatured.…”

“…The CHT would also cause damage of proteins to be unfolded or denatured. Surprisingly, no genes for general heat shock proteins (HSPs), except for degP, dnaK, and dnaJ in E. coli (Murata et al, 2011(Murata et al, , 2018, degP in A. tropicalis (Soemphol et al, 2011), and degP in Z. mobilis (Charoensuk et al, 2017), have been identified as thermotolerant genes.…”