Abstract
Thermotolerant genes, which are essential for survival at a high temperature, have been identified in three mesophilic microbes, including Zymomonas mobilis. Contrary to expectation, they include only a few genes for reactive oxygen species (ROS)-scavenging enzymes and heat shock proteins, which are assumed to play key roles at a critical high temperature (CHT) as an upper limit of survival. We thus examined the effects of increased expression of these genes on the cell growth of Z. mobilis strains at its CHT. When overexpressed, most of the genes increased the CHT by about one degree, and some of them enhanced tolerance against acetic acid. These findings suggest that ROS-damaged molecules or unfolded proteins that prevent cell growth are accumulated in cells at the CHT.
Highlights
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 tends to be more temperatureresistant
To examine the contribution of RSEs and heat shock proteins (HSPs) to the survival of Z. mobilis at the CHT and to the improvement of the CHT, the genes coding for these enzymes and proteins (Table 1) were individually cloned into pZA22 under the control of the pdc promoter, which is a relatively strong promoter derived from Z. mobilis, and the effects of expression of these genes were evaluated by the two-step cultivation assay, which enables determination of the CHT of mesophiles (Kosaka et al, 2019)
Summary
Microorganisms intrinsically have an upper temperature limit for survival called a critical high temperature (CHT) (Matsushita et al, 2015; Kosaka et al, 2019). Improvement of Thermotolerance of Zymomonas mobilis strains have adapted to the environmental temperature. The thermotolerance of mesophiles, E. coli W3110, Z. mobilis CP4, and Z. mobilis TISTR 548, has been improved by in vivo thermal adaptation, suggesting that they have a genomic capacity for adaptation to higher temperature environments (Kosaka et al, 2019). The change of only a few degrees is physiologically important, which was determined by an accurate method, called a two-step cultivation assay that eliminates the effects of start temperature and can distinguish CHT differences between two different strains of the same species (Kosaka et al, 2019)
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