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Hayashi N, Lai Y, Fuerte-Stone J, Mimee M, Lu TK. Cas9-assisted biological containment of a genetically engineered human commensal bacterium and genetic elements. Nat Commun 2024; 15:2096. [PMID: 38453913 PMCID: PMC10920895 DOI: 10.1038/s41467-024-45893-w] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/07/2021] [Accepted: 02/07/2024] [Indexed: 03/09/2024] Open
Abstract
Sophisticated gene circuits built by synthetic biology can enable bacteria to sense their environment and respond predictably. Engineered biosensing bacteria outfitted with such circuits can potentially probe the human gut microbiome to prevent, diagnose, or treat disease. To provide robust biocontainment for engineered bacteria, we devised a Cas9-assisted auxotrophic biocontainment system combining thymidine auxotrophy, an Engineered Riboregulator (ER) for controlled gene expression, and a CRISPR Device (CD). The CD prevents the engineered bacteria from acquiring thyA via horizontal gene transfer, which would disrupt the biocontainment system, and inhibits the spread of genetic elements by killing bacteria harboring the gene cassette. This system tunably controlled gene expression in the human gut commensal bacterium Bacteroides thetaiotaomicron, prevented escape from thymidine auxotrophy, and blocked transgene dissemination. These capabilities were validated in vitro and in vivo. This biocontainment system exemplifies a powerful strategy for bringing genetically engineered microorganisms safely into biomedicine.
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Affiliation(s)
- Naoki Hayashi
- Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA
- JSR-Keio University Medical and Chemical Innovation Center (JKiC), JSR Corp., 35 Shinanomachi, Shinjuku, Tokyo, 160-8582, Japan
| | - Yong Lai
- Synthetic Biology Group, MIT Synthetic Biology Center, Massachusetts Institute of Technology (MIT), Cambridge, MA, 02139, USA
- Research Laboratory of Electronics, MIT, Cambridge, MA, 02139, USA
- Department of Chemical and Biological Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR
| | - Jay Fuerte-Stone
- Department of Microbiology, The University of Chicago, Chicago, IL, 60637, USA
| | - Mark Mimee
- Department of Microbiology, The University of Chicago, Chicago, IL, 60637, USA.
- Pritzker School of Molecular Engineering, The University of Chicago, Chicago, IL, 60637, USA.
| | - Timothy K Lu
- Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA.
- Synthetic Biology Group, MIT Synthetic Biology Center, Massachusetts Institute of Technology (MIT), Cambridge, MA, 02139, USA.
- Research Laboratory of Electronics, MIT, Cambridge, MA, 02139, USA.
- Broad Institute, Cambridge, MA, 02139, USA.
- Harvard-MIT Division of Health Sciences and Technology, Cambridge, MA, 02139, USA.
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2
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The past, present, and future of chemotherapy with a focus on individualization of drug dosing. J Control Release 2022; 352:840-860. [PMID: 36334860 DOI: 10.1016/j.jconrel.2022.10.043] [Citation(s) in RCA: 8] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/16/2022] [Revised: 10/14/2022] [Accepted: 10/24/2022] [Indexed: 11/16/2022]
Abstract
While there have been rapid advances in developing new and more targeted drugs to treat cancer, much less progress has been made in individualizing dosing. Even though the introduction of immunotherapies such as CAR T-cells and checkpoint inhibitors, as well as personalized therapies that target specific mutations, have transformed clinical treatment of cancers, chemotherapy remains a mainstay in oncology. Chemotherapies are typically dosed on either a body surface area (BSA) or weight basis, which fails to account for pharmacokinetic differences between patients. Drug absorption, distribution, metabolism, and excretion rates can vary between patients, resulting in considerable differences in exposure to the active drugs. These differences result in suboptimal dosing, which can reduce efficacy and increase side-effects. Therapeutic drug monitoring (TDM), genotype guided dosing, and chronomodulation have been developed to address this challenge; however, despite improving clinical outcomes, they are rarely implemented in clinical practice for chemotherapies. Thus, there is a need to develop interventions that allow for individualized drug dosing of chemotherapies, which can help maximize the number of patients that reach the most efficacious level of drug in the blood while mitigating the risks of underdosing or overdosing. In this review, we discuss the history of the development of chemotherapies, their mechanisms of action and how they are dosed. We discuss substantial intraindividual and interindividual variability in chemotherapy pharmacokinetics. We then propose potential engineering solutions that could enable individualized dosing of chemotherapies, such as closed-loop drug delivery systems and bioresponsive biomaterials.
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3
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Patel V, Matange N. Adaptation and compensation in a bacterial gene regulatory network evolving under antibiotic selection. eLife 2021; 10:70931. [PMID: 34591012 PMCID: PMC8483737 DOI: 10.7554/elife.70931] [Citation(s) in RCA: 10] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/02/2021] [Accepted: 09/22/2021] [Indexed: 12/28/2022] Open
Abstract
Gene regulatory networks allow organisms to generate coordinated responses to environmental challenges. In bacteria, regulatory networks are re-wired and re-purposed during evolution, though the relationship between selection pressures and evolutionary change is poorly understood. In this study, we discover that the early evolutionary response of Escherichia coli to the antibiotic trimethoprim involves derepression of PhoPQ signaling, an Mg2+-sensitive two-component system, by inactivation of the MgrB feedback-regulatory protein. We report that derepression of PhoPQ confers trimethoprim-tolerance to E. coli by hitherto unrecognized transcriptional upregulation of dihydrofolate reductase (DHFR), target of trimethoprim. As a result, mutations in mgrB precede and facilitate the evolution of drug resistance. Using laboratory evolution, genome sequencing, and mutation re-construction, we show that populations of E. coli challenged with trimethoprim are faced with the evolutionary ‘choice’ of transitioning from tolerant to resistant by mutations in DHFR, or compensating for the fitness costs of PhoPQ derepression by inactivating the RpoS sigma factor, itself a PhoPQ-target. Outcomes at this evolutionary branch-point are determined by the strength of antibiotic selection, such that high pressures favor resistance, while low pressures favor cost compensation. Our results relate evolutionary changes in bacterial gene regulatory networks to strength of selection and provide mechanistic evidence to substantiate this link.
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Affiliation(s)
- Vishwa Patel
- Dr. Vikram Sarabhai Institute of Cell and Molecular Biology, The Maharaja Sayajirao University of Baroda, Vadodara, India.,Department of Microbiology and Cell Biology, Indian Institute of Science, Bangalore, India
| | - Nishad Matange
- Indian Institute of Science Education and Research (IISER), Pune, India
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4
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de Moraes MH, Hsu F, Huang D, Bosch DE, Zeng J, Radey MC, Simon N, Ledvina HE, Frick JP, Wiggins PA, Peterson SB, Mougous JD. An interbacterial DNA deaminase toxin directly mutagenizes surviving target populations. eLife 2021; 10:62967. [PMID: 33448264 PMCID: PMC7901873 DOI: 10.7554/elife.62967] [Citation(s) in RCA: 10] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/11/2020] [Accepted: 01/14/2021] [Indexed: 12/12/2022] Open
Abstract
When bacterial cells come in contact, antagonism mediated by the delivery of toxins frequently ensues. The potential for such encounters to have long-term beneficial consequences in recipient cells has not been investigated. Here, we examined the effects of intoxication by DddA, a cytosine deaminase delivered via the type VI secretion system (T6SS) of Burkholderia cenocepacia. Despite its killing potential, we observed that several bacterial species resist DddA and instead accumulate mutations. These mutations can lead to the acquisition of antibiotic resistance, indicating that even in the absence of killing, interbacterial antagonism can have profound consequences on target populations. Investigation of additional toxins from the deaminase superfamily revealed that mutagenic activity is a common feature of these proteins, including a representative we show targets single-stranded DNA and displays a markedly divergent structure. Our findings suggest that a surprising consequence of antagonistic interactions between bacteria could be the promotion of adaptation via the action of directly mutagenic toxins.
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Affiliation(s)
- Marcos H de Moraes
- Department of Microbiology, University of Washington School of Medicine, Seattle, United States
| | - FoSheng Hsu
- Department of Microbiology, University of Washington School of Medicine, Seattle, United States
| | - Dean Huang
- Department of Physics, University of Washington, Seattle, United States
| | - Dustin E Bosch
- Department of Laboratory Medicine and Pathology, University of Washington School of Medicine, Seattle, United States
| | - Jun Zeng
- Department of Microbiology, University of Washington School of Medicine, Seattle, United States
| | - Matthew C Radey
- Department of Microbiology, University of Washington School of Medicine, Seattle, United States
| | - Noah Simon
- Department of Biostatistics, University of Washington School of Public Health, Seattle, United States
| | - Hannah E Ledvina
- Department of Microbiology, University of Washington School of Medicine, Seattle, United States
| | - Jacob P Frick
- Department of Microbiology, University of Washington School of Medicine, Seattle, United States
| | - Paul A Wiggins
- Department of Physics, University of Washington, Seattle, United States
| | - S Brook Peterson
- Department of Microbiology, University of Washington School of Medicine, Seattle, United States
| | - Joseph D Mougous
- Department of Microbiology, University of Washington School of Medicine, Seattle, United States.,Department of Biochemistry, University of Washington School of Medicine, Seattle, United States.,Howard Hughes Medical Institute, University of Washington, Seattle, United States
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5
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Martín CM, Zaritsky A, Fishov I, Guzmán EC. Transient enhanced cell division by blocking DNA synthesis in Escherichia coli. MICROBIOLOGY (READING, ENGLAND) 2020; 166:516-521. [PMID: 32118529 PMCID: PMC7376268 DOI: 10.1099/mic.0.000888] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 11/03/2019] [Accepted: 01/09/2020] [Indexed: 11/22/2022]
Abstract
Duplication of the bacterial nucleoid is necessary for cell division hence specific arrest of DNA replication inhibits divisions culminating in filamentation, nucleoid dispersion and appearance of a-nucleated cells. It is demonstrated here that during the first 10 min however, Escherichia coli enhanced residual divisions: the proportion of constricted cells doubled (to 40%), nucleoids contracted and cells remodelled dimensions: length decreased and width increased. The preliminary data provides further support to the existence of temporal and spatial couplings between the nucleoid/replisome and the sacculus/divisome, and is consistent with the idea that bacillary bacteria modulate width during the division process exclusively.
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Affiliation(s)
- Carmen Mata Martín
- Departamento de Bioquímica Biología Molecular y Genética, Universidad de Extremadura, Badajoz 06071, Spain
- Present address: CICAB Clinical Research Centre, Extremadura University Hospital and Medical School, Badajoz, Spain
| | - Arieh Zaritsky
- Faculty of Natural Sciences, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
| | - Itzhak Fishov
- Department of Life Sciences, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
| | - Elena C. Guzmán
- Departamento de Bioquímica Biología Molecular y Genética, Universidad de Extremadura, Badajoz 06071, Spain
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6
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Schmidt TT, Sharma S, Reyes GX, Kolodziejczak A, Wagner T, Luke B, Hofer A, Chabes A, Hombauer H. Inactivation of folylpolyglutamate synthetase Met7 results in genome instability driven by an increased dUTP/dTTP ratio. Nucleic Acids Res 2020; 48:264-277. [PMID: 31647103 PMCID: PMC7145683 DOI: 10.1093/nar/gkz1006] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/08/2019] [Revised: 10/11/2019] [Accepted: 10/16/2019] [Indexed: 12/14/2022] Open
Abstract
The accumulation of mutations is frequently associated with alterations in gene function leading to the onset of diseases, including cancer. Aiming to find novel genes that contribute to the stability of the genome, we screened the Saccharomyces cerevisiae deletion collection for increased mutator phenotypes. Among the identified genes, we discovered MET7, which encodes folylpolyglutamate synthetase (FPGS), an enzyme that facilitates several folate-dependent reactions including the synthesis of purines, thymidylate (dTMP) and DNA methylation. Here, we found that Met7-deficient strains show elevated mutation rates, but also increased levels of endogenous DNA damage resulting in gross chromosomal rearrangements (GCRs). Quantification of deoxyribonucleotide (dNTP) pools in cell extracts from met7Δ mutant revealed reductions in dTTP and dGTP that cause a constitutively active DNA damage checkpoint. In addition, we found that the absence of Met7 leads to dUTP accumulation, at levels that allowed its detection in yeast extracts for the first time. Consequently, a high dUTP/dTTP ratio promotes uracil incorporation into DNA, followed by futile repair cycles that compromise both mitochondrial and nuclear DNA integrity. In summary, this work highlights the importance of folate polyglutamylation in the maintenance of nucleotide homeostasis and genome stability.
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Affiliation(s)
- Tobias T Schmidt
- DNA Repair Mechanisms and Cancer, German Cancer Research Center (DKFZ), Heidelberg D-69120, Germany.,Faculty of Bioscience, Heidelberg University, Heidelberg D-69120, Germany
| | - Sushma Sharma
- Department of Medical Biochemistry and Biophysics, Umeå University, Umeå SE-901 87 Sweden
| | - Gloria X Reyes
- DNA Repair Mechanisms and Cancer, German Cancer Research Center (DKFZ), Heidelberg D-69120, Germany
| | - Anna Kolodziejczak
- DNA Repair Mechanisms and Cancer, German Cancer Research Center (DKFZ), Heidelberg D-69120, Germany.,Faculty of Bioscience, Heidelberg University, Heidelberg D-69120, Germany
| | - Tina Wagner
- Institute of Developmental Biology and Neurobiology, Johannes Gutenberg Universität, 55128 Mainz, Germany
| | - Brian Luke
- Institute of Developmental Biology and Neurobiology, Johannes Gutenberg Universität, 55128 Mainz, Germany.,Institute of Molecular Biology (IMB), 55128 Mainz, Germany
| | - Anders Hofer
- Department of Medical Biochemistry and Biophysics, Umeå University, Umeå SE-901 87 Sweden
| | - Andrei Chabes
- Department of Medical Biochemistry and Biophysics, Umeå University, Umeå SE-901 87 Sweden.,Laboratory for Molecular Infection Medicine Sweden (MIMS), Umeå University, SE-901 87 Umeå, Sweden
| | - Hans Hombauer
- DNA Repair Mechanisms and Cancer, German Cancer Research Center (DKFZ), Heidelberg D-69120, Germany
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7
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In Vivo Rate of Formaldehyde Condensation with Tetrahydrofolate. Metabolites 2020; 10:metabo10020065. [PMID: 32059429 PMCID: PMC7073904 DOI: 10.3390/metabo10020065] [Citation(s) in RCA: 12] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/29/2019] [Revised: 02/10/2020] [Accepted: 02/11/2020] [Indexed: 12/28/2022] Open
Abstract
Formaldehyde is a highly reactive compound that participates in multiple spontaneous reactions, but these are mostly deleterious and damage cellular components. In contrast, the spontaneous condensation of formaldehyde with tetrahydrofolate (THF) has been proposed to contribute to the assimilation of this intermediate during growth on C1 carbon sources such as methanol. However, the in vivo rate of this condensation reaction is unknown and its possible contribution to growth remains elusive. Here, we used microbial platforms to assess the rate of this condensation in the cellular environment. We constructed Escherichia coli strains lacking the enzymes that naturally produce 5,10-methylene-THF. These strains were able to grow on minimal medium only when equipped with a sarcosine (N-methyl-glycine) oxidation pathway that sustained a high cellular concentration of formaldehyde, which spontaneously reacts with THF to produce 5,10-methylene-THF. We used flux balance analysis to derive the rate of the spontaneous condensation from the observed growth rate. According to this, we calculated that a microorganism obtaining its entire biomass via the spontaneous condensation of formaldehyde with THF would have a doubling time of more than three weeks. Hence, this spontaneous reaction is unlikely to serve as an effective route for formaldehyde assimilation.
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8
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Roe K. Dual-peptide ligand masks: a proposed treatment approach to stop prion disease dementias. Drug Discov Today 2019; 25:15-21. [PMID: 31560948 DOI: 10.1016/j.drudis.2019.09.012] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/02/2019] [Revised: 08/15/2019] [Accepted: 09/17/2019] [Indexed: 12/19/2022]
Abstract
Prion disease dementias are currently not practically treatable. However, a proposed treatment approach using specifically targeted dual-peptide ligand masks can mask prion surface proteins and treat specific prion diseases. Different approaches might be used to treat these prion diseases. One treatment introduces genetically modified cells into the gastrointestinal tract or other locations to produce dual-peptide ligand masks; and another treatment introduces only the dual-peptide ligand masks into the center of prion infections to mask prion surface proteins. An independent group introduced genetically modified therapeutic bacteria into large numbers of mammals, including several human volunteers, with safe and effective experimental results, without long-term colonization by the bacteria, which experimentally supports the feasibility of the first treatment. These approaches offer several advantages compared with other potential treatments against prion diseases in humans.
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9
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Akiyama MT, Oshima T, Chumsakul O, Ishikawa S, Maki H. Replication fork progression is paused in two large chromosomal zones flanking the DNA replication origin in Escherichia coli. Genes Cells 2016; 21:907-14. [PMID: 27353572 DOI: 10.1111/gtc.12388] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/26/2016] [Accepted: 05/16/2016] [Indexed: 11/27/2022]
Abstract
Although the speed of nascent DNA synthesis at individual replication forks is relatively uniform in bacterial cells, the dynamics of replication fork progression on the chromosome are hampered by a variety of natural impediments. Genome replication dynamics can be directly measured from an exponentially growing cell population by sequencing newly synthesized DNA strands that were specifically pulse-labeled with the thymidine analogue 5-bromo-2'-deoxyuridine (BrdU). However, a short pulse labeling with BrdU is impracticable for bacteria because of poor incorporation of BrdU into the cells, and thus, the genomewide dynamics of bacterial DNA replication remain undetermined. Using a new thymidine-requiring Escherichia coli strain, eCOMB, and high-throughput sequencing, we succeeded in determining the genomewide replication profile in bacterial cells. We also found that fork progression is paused in two ~200-kb chromosomal zones that flank the replication origin in the growing cells. This origin-proximal obstruction to fork progression was overcome by an increased thymidine concentration in the culture medium and enhanced by inhibition of transcription. These indicate that DNA replication near the origin is sensitive to the impediments to fork progression, namely a scarcity of the DNA precursor deoxythymidine triphosphate and probable conflicts between replication and transcription machineries.
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Affiliation(s)
- Masahiro Tatsumi Akiyama
- Division of Systems Biology, Graduate School of Biological Sciences, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara, 630-0192, Japan
| | - Taku Oshima
- Division of Systems Biology, Graduate School of Biological Sciences, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara, 630-0192, Japan
| | - Onuma Chumsakul
- Division of Systems Biology, Graduate School of Biological Sciences, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara, 630-0192, Japan
| | - Shu Ishikawa
- Division of Systems Biology, Graduate School of Biological Sciences, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara, 630-0192, Japan
| | - Hisaji Maki
- Division of Systems Biology, Graduate School of Biological Sciences, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara, 630-0192, Japan
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10
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A DinB Ortholog Enables Mycobacterial Growth under dTTP-Limiting Conditions Induced by the Expression of a Mycobacteriophage-Derived Ribonucleotide Reductase Gene. J Bacteriol 2015; 198:352-62. [PMID: 26527643 DOI: 10.1128/jb.00669-15] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/13/2015] [Accepted: 10/24/2015] [Indexed: 01/30/2023] Open
Abstract
UNLABELLED Mycobacterium species such as M. smegmatis and M. tuberculosis encode at least two translesion synthesis (TLS) polymerases, DinB1 and DinB2, respectively. Although predicted to be linked to DNA repair, their role in vivo remains enigmatic. M. smegmatis mc(2)155, a strain commonly used to investigate mycobacterial genetics, has two copies of dinB2, the gene that codes for DinB2, by virtue of a 56-kb chromosomal duplication. Expression of a mycobacteriophage D29 gene (gene 50) encoding a class II ribonucleotide reductase in M. smegmatis ΔDRKIN, a strain derived from mc(2)155 in which one copy of the duplication is lost, resulted in DNA replication defects and growth inhibition. The inhibitory effect could be linked to the deficiency of dTTP that resulted under these circumstances. The selective inhibition observed in the ΔDRKIN strain was found to be due solely to a reduced dosage of dinB2 in this strain. Mycobacterium bovis, which is closely related to M. tuberculosis, the tuberculosis pathogen, was found to be highly susceptible to gene 50 overexpression. Incidentally, these slow-growing pathogens harbor one copy of dinB2. The results indicate that the induction of a dTTP-limiting state can lead to growth inhibition in mycobacteria, with the effect being maximum in cells deficient in DinB2. IMPORTANCE Mycobacterium species, such as M. tuberculosis, the tuberculosis pathogen, are known to encode several Y family DNA polymerases, one of which is DinB2, an ortholog of the DNA repair-related protein DinP of Escherichia coli. Although this protein has been biochemically characterized previously and found to be capable of translesion synthesis in vitro, its in vivo function remains unknown. Using a novel method to induce dTTP deficiency in mycobacteria, we demonstrate that DinB2 can aid mycobacterial survival under such conditions. Apart from unraveling a specific role for the mycobacterial Y family DNA polymerase DinB2 for the first time, this study also paves the way for the development of drugs that can kill mycobacteria by inducing a dTTP-deficient state.
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Hanawalt PC. A balanced perspective on unbalanced growth and thymineless death. Front Microbiol 2015; 6:504. [PMID: 26097468 PMCID: PMC4456962 DOI: 10.3389/fmicb.2015.00504] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/08/2015] [Accepted: 05/07/2015] [Indexed: 11/16/2022] Open
Abstract
The early history of the esoteric phenomenon of thymineless death (TLD) is recounted, from the pioneering discovery by Seymour Cohen and Hazel Barner, through my graduate studies at Yale and postdoctoral research in Copenhagen. My principal contribution was the discovery that restricted synthesis of protein and RNA permits cultures of Escherichia coli to complete their DNA replication cycles without initiating new ones, and that cells held in this physiological state are immune to the lethality of thymine deprivation; unbalanced growth is not the fundamental cause of TLD. The successful synchronization of the DNA replication cycle contributed to formulation of the replicon concept. Studies at Stanford revealed a specific requirement for transcription and led to the discovery of a TLD-resistant mutant in a new gene, termed recQ, with important homologs in humans and most other organisms. The lessons learned from research on TLD underscore the value of basic research in bacterial systems that can have profound implications for human health.
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