Previous screening of a single-gene knockout library consisting of 3,908 disrupted-mutant strains allowed us to identify 51 thermotolerant genes that are essential for survival at a critical high temperature (CHT) in Escherichia coli [Murata M, Fujimoto H, Nishimura K, Charoensuk K, Nagamitsu H, Raina S, Kosaka T, Oshima T, Ogasawara N, Yamada M (2011) PLoS ONE 6: e20063]. In this study, we identified another 21 thermotolerant genes. E. coli thus has 72 thermotolerant genes in total. The genes are classified into 8 groups: genes for energy metabolism, outer membrane organization, DNA double-strand break repair, tRNA modification, protein quality control, translation control, cell division and transporters. This classification and physiological analysis indicate the existence of fundamental strategies for survival at a CHT, which seems to exclude most of the heat shock responses.
Research Article; Biology and life sciences; Biochemistry; Biosynthesis; Cell biology; Cellular structures and organelles; Cell membranes; Membrane proteins; Outer membrane proteins; Nucleic acids; RNA; Non-coding RNA; Transfer RNA; Intracellular membranes; Proteins; Antiport proteins; Genetics; DNA; DNA repair; Membrane protein complexes; Protein complexes; Metabolism; Metabolic processes; Oxidative phosphorylation
Like general essential genes that are imperative for growth, there are genes, called thermotolerant genes, that are indispensable for survival at a critical high temperature (CHT), a level close to that causing cell death [[
Here, we update thermotolerant genes in E. coli and discuss the possible thermotolerant mechanisms for survival at a CHT. Notably, we obtained evidence that ATP synthesis by oxidative phosphorylation is essential at a CHT but not at temperatures below the CHT.
Oligonucleotide primers for polymerase chain reaction (PCR) were purchased from FASMAC Co, Ltd (Atsugi, Japan). Other chemicals were all of analytical grade.
The strains used in this study were derivatives of E. coli K-12. BW25113 (rrnB3, ΔlacZ4787, hsdR514, Δ(araBAD)567, Δ(rhaBAD)568, rph-1) [[
To examine the effects of supplements, glucose (0.5% (w/v)) or MgCl
Cultures were grown in LB medium at 37°C until the exponential phase, and then the temperature was up-shifted to 46°C and incubation was continued for 8 min. Total RNA was immediately prepared from the heat-stressed cells by the hot phenol method [[
Recombinant plasmids for expression of thermotolerant genes, atpA, gntK, lpxL, fimG, yccM, yraN, cydD and nhaA, were constructed, generating pUC-atpA, pUC-gntK, pUC-lpxL, pUC-fimG, pUC-yccM, pUC-yraN, pUC-cydD and pUC-nhaA, respectively. The DNA fragment of each gene, including the sequence from 40-bp upstream from the translation initiation codon to the stop codon, was amplified and inserted into pUC19 by In-Fusion cloning (Clontech, USA). Amplification of the DNA fragment was performed by PCR using a specific primer set for each gene (S2 Table) and the genomic DNA of BW25113 as a template [[
pUC-atpA, pUC-gntK, pUC-lpxL, pUC-fimG, pUC-yccM, pUC-yraN, pUC-cydD and pUC-nhaA DNAs were introduced into BW25113atpA: :kan, BW25113gntK: :kan, BW25113lpxL: :kan, BW25113fimG: :kan, BW25113yccM: :kan, BW25113yraN: :kan, BW25113cydD: :kan and BW25113nhaA: :kan cells, respectively. The empty plasmid DNA of pUC19 was also introduced into each of these mutant strains and BW25113, which were used as controls. These transformants were grown in LB medium at 37°C or 45°C for appropriate times. The experiments were performed three times, and the results were confirmed to be reproducible.
In a previous study, three successive screening steps were performed in a single-gene knockout library [[
null: Thermotolerant genes identified in this study.
Classification Sub-classification Gene Function 30°Ca 37°Cb 44°Cc 45°Cd 46°Ce Glcf Mg2+g H2O2h Energy metabolism (Group A) Oxidative phosphorylation atpA F1 sector of membrane-bound ATP synthase, alpha subunit - Oxidative phosphorylation atpD F1 sector of membrane-bound ATP synthase, beta subunit - + ++ S Oxidative phosphorylation atpG F1 sector of membrane-bound ATP synthase, gamma subunit - Oxidative phosphorylation nuoC NADH:ubiquinone oxidoreductase, chain C, D - Pentose phosphate pathway gntK Gluconate kinase 2 - + S Pentose phosphate pathway phnN Ribose 1,5-bisphosphokinase - Ubiquinone/menaquinone biosynthesis ubiE Ubiquinone/menaquinone biosynthesis C-methyltransferase UbiE - Ubiquinone biosynthesis ubiH 2-octaprenyl-6-methoxyphenol hydroxylase - - - S Ubiquinone biosynthesis ubiX Flavin prenyltransferase UbiX - Amino acid metabolism dapF Diaminopimelate epimerase - - - Amino acid metabolism trpB Tryptophan synthase, beta chain - + S Vitamin B6 metabolism pdxH Pyridoxin/pyridoxamine 5'-phosphate oxidase - - + S Outer membrane biosynthesis (Group B) Lipopolysaccharide biosynthesis lpxL Lipid A biosynthesis lauroyltransferase - - ++ S Secretion system fimG Minor component of type I fimbriae - ++ S DNA repair (Group C) Putative endonuclease yraN Predicted Mrr Cat superfamily - ++ S tRNA modification (Group D) tRNA modification yccM Sulfur relay system, 4Fe-4S membrane protein - - + Others ABC transporter cydD ATP-binding/membrane protein CydD - ++ ++ S Sodium-proton antiporter nhaA Na+:H+ antiporter, NhaA family - + ++ S Hypothetical protein yjiY Putative transporter, Carbon starvation protein CstA superfamily - S Hypothetical protein ydgH Uncharacterized deacetylase - ++ S Hypothetical protein yaiS DUF1471 family periplasmic protein - S
1 a to e“-” means very weak growth at indicated temperatures.
- 2 f According to the data in S2 Fig, ratios of growth in the presence of glucose to that in the absence of glucose at 46°C were estimated.
- 5 “++” and “+” represent more than 2.0 and 1.5–2.0, respectively.
- 3 gAccording to the data in S2 Fig, ratios of growth in the presence of MgCl2 to that in the absence of MgCl2 at 46°C were estimated.
- 6 “++” and “+” represent more than 2.0 and 1.5–2.0, respectively.
- 4 hAccording to the data in S2 Fig, ratios of growth in the presence of H2O2 to that in the absence of H2O2 at 30°C were estimated.
- 7 “S” represents less than 0.5.
Next, the 21 genes corresponding to the 21 thermosensitive mutants were classified on the basis of their functions by using public databases including the KEGG pathway database. Interestingly, most of the 21 thermotolerant genes were found to be classified into categories defined in a previous study: energy metabolism, outer membrane stabilization and tRNA modification (Table 1). It is noteworthy that this study allowed us to find lacking pieces in the following system or pathway. Six genes that are involved in the sulfur-relay system for tRNA modification are included in the previous list of thermotolerant genes [[
The functions of the newly identified thermotolerant genes are summarized in Table 1. Most of them were categorized into group A, including genes for components or their synthesis in the respiratory chain and in the complex of the F
Notably, functions of the members of group A suggest that ATP synthesis by substrate-level phosphorylation can support cell survival at temperatures up to 46°C but that ATP synthesis by oxidative phosphorylation is essential at a CHT. On the other hand, mutations of genes for subunits interacting with the membrane in the complex of the ATP synthase were found to be very sensitive even to low temperatures compared to other subunits of the complex. Presumably, mutations for the membrane-interacting subunits cause H
In total, 72 thermotolerant genes were found in E. coli (Table 2). Intriguingly, thermotolerant genes in E. coli include only a small number of heat shock genes and are mostly genes responsible for functions to stabilize the membrane or to assist fundamental metabolism [[
null: List of thermotolerant genes in E. coli.
Classificationa Gene Definition Function Energy Metabolism (Group A) aceE Pyruvate metabolism Pyruvate dehydrogenase compenent E1 decarboxylase component E1 aceF Pyruvate metabolism Pyruvate dehydrogenase, dihydrolipoyltransacetylase compenent E2 lpd Pyruvate metabolism Lipoamide dehydrogenase, E3 component, subunit of three comlexes lipA Pyruvate mechanism Liponate synthase ackA Pyruvate metabolism Acetate kinase A and propionate kinase 2 rpe Pentose phosphate pathway D-ribulose-5-phosphate 3-epimerase cydB Respiratory chain Cytochrome d ubiquinol oxidase subunit II yhcB Respiratory chain Cytochrome d ubiquinol oxidase subunit III atpAb Oxidative phosphorylation F1 sector of membrane-bound ATP synthase, alpha subunit atpD Oxidative phosphorylation F1 sector of membrane-bound ATP synthase, beta subunit atpG Oxidative phosphorylation F1 sector of membrane-bound ATP synthase, gamma subunit nuoC Oxidative phosphorylation NADH:ubiquinone oxidoreductase, chain C, D gntK Pentose phosphate pathway Gluconate kinase 2 phnN Pentose phosphate pathway Ribose 1,5-bisphosphokinase ubiE Ubiquinone/menaquinone biosynthesis Ubiquinone/menaquinone biosynthesis C-methyltransferase UbiE ubiH Ubiquinone biosynthesis 2-octaprenyl-6-methoxyphenol hydroxylase ubiX Ubiquinone biosynthesis Flavin prenyltransferase UbiX dapF Amino acid metabolism Diaminopimelate epimerase trpB Amino acid metabolism Tryptophan synthase, beta subunit pdxH Vitamin B6 metabolism Pyridoxine/pyridoxamine 5'-phosphate oxidase ybhH Hypothetical protein Putative isomerase Outer membrane stabilization (Group B) gmhB Lipopolysaccharide biosynthesis D,D-heptose 1,7-bisphosphate phosphatase lpcA (gmhA) Lipopolysaccharide biosynthesis D-sedoheptulose 7-phosphate isomerase rfaC (waaC) Lipopolysaccharide biosynthesis ADP-heptose:LPS heptosyl transferase I rfaD (waaD) Lipopolysaccharide biosynthesis ADP-L-glycero-D-mannoheptose-6-epimerase, NAD(P)-binding rfaE (gmhC) Lipopolysaccharide biosynthesis Fused heptose7-phosphate kinase and heptose 1-phosphate adenyltransferase rfaF (waaF) Lipopolysaccharide biosynthesis ADP-heptose:LPS heptosyltransferase II rfaG (waaG) Lipopolysaccharide biosynthesis Glucosyltransferase I lpxL Lipopolysaccharide biosynthesis Lipid A biosynthesis lauroyltransferase ydcL Peptidoglycan-associated lipoprotein Predicted lipoprotein yfgL Peptidoglycan-associated lipoprotein Protein assembly complex, lipoprotein component ynbE Peptidoglycan-associated lipoprotein Predicted lipoprotein nlpI Peptidoglycan-associated lipoprotein Conserved protein ycdO Peptidoglycan-associated lipoprotein Conserved protein pal Outer membrane integrity Tol/Pal system, peptidoglycan-associated outer membrane lipoprotein tolQ Outer membrane integrity Tol/Pal system, membrane-spanning protein tolR Outer membrane integrity Tol/Pal system, membrane-spanning protein yciM Outer membrane integrity Conserved hypothetical protein fimG Secretion system Minor component of type 1 fimbriae DNA repair (Group C) dnaQ DNA replication & repair, DSBR DNA polymerase III subunit, epsilon holC DNA replication & repair, DSBR DNA polymerase III subunit, chi priA DNA replication & repair, DSBR Primosome factor n' ruvA DNA replication & repair, DSBR Component of RuvABC resolvasome, endonuclease ruvC DNA replication & repair, DSBR Conserved protein required for cell growth yraN Hypothetical protein Predicted Mrr Cat superfamily tRNA modification (Group D) iscS tRNA modification Sulfer relay system, cysteine desulfurase yheL (tusB) tRNA modification Sulfer relay system, predicted intracellular sulfur oxidation protein yheM (tusC) tRNA modification Sulfer relay system, predicted intracellular sulfur oxidation protein yheN (tusD) tRNA modification Sulfer relay system, predicted intracellular sulfur oxidation protein yhhP (tusA) tRNA modification Sulfer relay system, conserved protein required for cell growth yccM tRNA modification Sulfur relay system, 4Fe-4S membrane protein miaA tRNA modification Delta(2)-isopentenylpyrophosphate tRNA-adenosine transferase trmU tRNA modification tRNA (5-methylaminomethyl-2-thiouridylate)-methyltransferase truA tRNA modification Pseudouridine synthase A Chaperone /protease (Group E) dnaJ Chaperone system Chaperone Hsp40, co-chaperone with DnaK dnaK Chaperone system Chaperone Hsp70, co-chaperone with DnaJ degP Chaperone system Chaperone/serine endoprotease rseA Chaperone regulator Anti-sigma factor Translational control (Group F) rpmJ Translational control 50S ribosomal subunit L36, related to secY expression rpsF Translational control 30S ribosomal subunit S6, modified with glutamic acid or phosphate dksA Translational control DNA-binding transcriptional regulator or rRNA transcription smpB Translational control Component of trans-translation process Cell division (Group G) xerC Related to cell division Site-specific tyrosine recombinase for chromosome dimmer resolution dedD Related to cell division Membrane-anchored periplasmic protein involved in separation envC Related to cell division Regulator of cell wall hydrolases responsible for cell separation Transporter (Group H) zntA Membrane transport Zinc/cadmium/mercury/lead-exporting ATPase ybgH Membrane transport Predicted proton-dependent oligopeptide transporter, POT family cydD Membrane transport ATP-binding/permease protein CydD nhaA Membrane transport Na+/H+antiporter, NhaA family yjiY Hypothetical protein Putative transporter, carbon starvation protein CstA superfamily Others yaiS Hypothetical protein Uncharacterized deacetylase ydgH Hypothetical protein DUF1471 family periplasmic protein
- 8 aA new group of transporter (Group H) is added. Transporters were classified in others in previous paper [
1 ]. - 9 bThermotolerant genes identified in this study are shown by bold letters.
Taken together, we propose a model of intracellular problems at a CHT and their protection mechanisms as shown in Fig 1. Under conditions at high temperatures close to a CHT, membrane fluidity may dramatically increase, causing leakage of electrons from the respiratory chain to generate ROS that give rise to oxidative damages to macromolecules. DNA double-strand breaks and protein denaturation by oxidative modifications seem to occur in the cytoplasm or periplasm because the corresponding genes become indispensable at a CHT. As protection mechanisms against these problems as thermotolerant mechanisms, LPS and membrane proteins may strengthen the membrane structure to prevent membrane fluidity. However, ROS scavenging genes were not found as thermotolerant genes. This may be due to the existence of homologues that can perform the same function in E. coli. Notably, overexpression of sodA for superoxide dismutase or katE for catalase significantly increased the number of viable and culturable cells at a CHT [[
Since the screening in this study was performed with a single-gene knockout library covering all non-essential genes in E. coli, knowledge of physiological functions of the 72 thermotolerant genes will be very beneficial for the genetic conversion of non-thermotolerant to thermotolerant bacteria.
null. Growth of thermosensitive mutants in LB liquid culture at different temperatures.Each of the 26 thermosensitive mutant strains (open circles) and the parental strain, BW25113 (closed circles), were grown in 30 ml LB medium at 30°C, 37°C, 39°C, 44°C, 45°C and 46°C. At the times indicated, turbidity at OD
null. Effects of addition of glucose and MgCl2 and sensitivity to H2O2.Thermosensitive mutant strains are shown by gene names. Growth conditions are described in Materials and Methods. Black and white columns represent turbidity under the conditions with and without supplements (0.5% glucose (A) or 20 mM MgCl
null. Gene organizations around genes having either an essential gene or a thermotolerant gene as a just downstream gene.Gene organizations around 19 thermotolerant genes that may have either an essential gene or a thermotolerant gene as a just downstream gene are depicted. Black boxes represent 19 identified thermotolerant genes. Grey boxes represent possible essential or thermotolerant genes. The direction of boxes shows the direction of transcription.(PDF)
null. Testing of possible polar effects by aph insertion.Total RNA was prepared from cells cultured at 37°C (a) and 46°C (b) and subjected to RT-PCR as described in Materials and Methods. RT-PCR was performed with primers specific for a just downstream gene of each thermotolerant gene to amplify about 500-bp DNA fragments. After RT reaction, PCR was performed for 15, 20, 25 and 30 cycles and each PCR product was electrophoresed on 1.2% agarose gel, followed by staining with ethidium bromide. Arrowheads indicate amplified products by RT-PCR.(PDF)
null. Complementation experiments with plasmid clones of representative thermotolerant genes.Transformants with plasmid clones (open circles), BW25113atpA: :kan (pUC-atpA), BW25113gntK: :kan (pUC-gntK), BW25113lpxL: :kan (pUC-lpxL), BW25113fimG: :kan (pUC-fimG), BW25113yccM: :kan (pUC-yccM), BW25113yraN: :kan (pUC-yraN), BW25113cydD: :kan (pUC-cydD) and BW25113nhaA: :kan (pUCNHAA), and transformants with an empty vector (closed circles), BW25113atpA: :kan (pUC19), BW25113gntK: :kan (pUC19), BW25113lpxL: :kan (pUC19), BW25113fimG: :kan (pUC19), BW25113yccM: :kan (pUC19), BW25113yraN: :kan (pUC19), BW25113cydD: :kan (pUC19) and BW25113nhaA: :kan (pUC19), and BW25113 (pUC19) (open triangles) were grown in 30 ml LB medium at 37°C (at left side) and 45°C (at right side), except that cells in the BW25113lpxL: :kan background and its control cells were examined at 37°C (at left side) and 43°C (at right side) because of the negative effect of pUC19 (see related description in the text). At the times indicated, turbidity at OD
null. RT-PCR primers used in this study.(DOC)
null. Primers for gene cloning for complementation experiments.(DOC)
DIAGRAM: Fig 1: A model of intracellular problems at a CHT and their protection mechanisms. At a CHT, the level of ROS is increased as described in the text, resulting in damage of macromolecules. There are various possible protection mechanisms as thermotolerant mechanisms, such as stabilization of the membrane to protect electron leakage from the respiratory chain, scavenging ROS, and stabilization of tRNA by modification and repair of DNA double-strand breaks or denatured proteins. Abbreviations used are: OM, outer membrane; IM, inner membrane; O2•-, superoxide radical anion; H2O2, hydrogen peroxide; •OH, hydroxyl radical.
By Masayuki Murata, Writing – review & editing; Ayana Ishii, Investigation; Hiroko Fujimoto, Investigation; Kaori Nishimura, Investigation; Tomoyuki Kosaka, Formal analysis; Hirotada Mori, Formal analysis and Mamoru Yamada, Writing – review & editing