Structural analysis and DNA binding of the HMG domains of the human mitochondrial transcription factor A

DNA sequence and lesion-dependent mitochondrial transcription factor A (TFAM)-DNA-binding modulates DNA repair activities and products

Mitochondrial DNA (mtDNA) is indispensable for mitochondrial function and is maintained by DNA repair, turnover, mitochondrial dynamics and mitophagy, along with the inherent redundancy of mtDNA. Base excision repair (BER) is a major DNA repair mechanism in mammalian mitochondria. Mitochondrial BER enzymes are implicated in mtDNA-mediated immune response and inflammation. mtDNA is organized into mitochondrial nucleoids by mitochondrial transcription factor A (TFAM). The regulation of DNA repair activities by TFAM-DNA interactions remains understudied. Here, we demonstrate the modulation of DNA repair enzymes by TFAM concentrations, DNA sequences and DNA modifications. Unlike previously reported inhibitory effects, we observed that human uracil-DNA glycosylase 1 (UNG1) and AP endonuclease I (APE1) have optimal activities at specific TFAM/DNA molar ratios. High TFAM/DNA ratios inhibited other enzymes, OGG1 and AAG. In addition, TFAM reduces the accumulation of certain repair intermediates. Molecular dynamics simulations and DNA-binding experiments demonstrate that the presence of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) in certain sequence motifs enhances TFAM-DNA binding, partially explaining the inhibition of OGG1 activity. Bioinformatic analysis of published 8-oxodG, dU, and TFAM-footprint maps reveals a correlation between 8-oxodG and TFAM locations in mtDNA. Collectively, these results highlight the complex regulation of mtDNA repair by DNA sequence, TFAM concentrations, lesions and repair enzymes.

Biallelic HMGXB4 loss-of-function variant causes intellectual disability, developmental delay, and dysmorphic features

Background

HMGXB4 (additionally known as HMG2L1) is a non-histone DNA-binding protein that contains a single HMG-box domain. HMGXB4 was originally described in Xenopus where it was seen to negatively regulate the Wnt/β-catenin signaling pathway.

Materials and methods

In this study, we conducted a genetic and clinical evaluation of a single family with three affected individuals suffering from intellectual disability (ID), global developmental delay (GDD) and dysmorphic facial features.Whole genome sequencing (WGS) and Sanger sequencing were performed on the affected individuals' DNA to identify genetic variations. Additionally, a reverse transcription-quantitative polymerase chain reaction (RT-qPCR) was used to assess gene expression in both the affected and unaffected individuals in the family.

Result

WGS identified a homozygous frameshift variant c.1193_1196del p. (Lys398Argfs × 25) in exon 5 of the HMGXB4 gene (OMIM 604702), which completely segregated the disease phenotype in the family. Furthermore, RT-qPCR revealed a substantial decrease in the HMGXB4 gene expression in the affected individuals as compared to the unaffected individuals of the family.

Conclusions

The current study is the first evidence linking a genetic variant in the HMGXB4 gene to ID, GDD, and dysmorphic facial features. Therefore, it is possible that HMGXB4 contributes significantly to developmental milestones and may be responsible for neurodevelopmental disorders in humans.

Mechanisms and regulation of human mitochondrial transcription

The expression of mitochondrial genes is regulated in response to the metabolic needs of different cell types, but the basic mechanisms underlying this process are still poorly understood. In this Review, we describe how different layers of regulation cooperate to fine tune initiation of both mitochondrial DNA (mtDNA) transcription and replication in human cells. We discuss our current understanding of the molecular mechanisms that drive and regulate transcription initiation from mtDNA promoters, and how the packaging of mtDNA into nucleoids can control the number of mtDNA molecules available for both transcription and replication. Indeed, a unique aspect of the mitochondrial transcription machinery is that it is coupled to mtDNA replication, such that mitochondrial RNA polymerase is additionally required for primer synthesis at mtDNA origins of replication. We discuss how the choice between replication-primer formation and genome-length RNA synthesis is controlled at the main origin of replication (OriH) and how the recent discovery of an additional mitochondrial promoter (LSP2) in humans may change this long-standing model.

Structure and dynamics of the mitochondrial DNA-compaction factor Abf2 from S. cerevisiae

Mitochondria are essential organelles that produce most of the energy via the oxidative phosphorylation (OXPHOS) system in all eukaryotic cells. Several essential subunits of the OXPHOS system are encoded by the mitochondrial genome (mtDNA) despite its small size. Defects in mtDNA maintenance and expression can lead to severe OXPHOS deficiencies and are amongst the most common causes of mitochondrial disease. The mtDNA is packaged as nucleoprotein structures, referred to as nucleoids. The conserved mitochondrial proteins, ARS-binding factor 2 (Abf2) in the budding yeast Saccharomyces cerevisiae (S. cerevisiae) and mitochondrial transcription factor A (TFAM) in mammals, are nucleoid associated proteins (NAPs) acting as condensing factors needed for packaging and maintenance of the mtDNA. Interestingly, gene knockout of Abf2 leads, in yeast, to the loss of mtDNA and respiratory growth, providing evidence for its crucial role. On a structural level, the condensing factors usually contain two DNA binding domains called high-mobility group boxes (HMG boxes). The co-operating mechanical activities of these domains are crucial in establishing the nucleoid architecture by bending the DNA strands. Here we used advanced solution NMR spectroscopy methods to characterize the dynamical properties of Abf2 together with its interaction with DNA. We find that the two HMG-domains react notably different to external cues like temperature and salt, indicating distinct functional properties. Biophysical characterizations show the pronounced difference of these domains upon DNA-binding, suggesting a refined picture of the Abf2 functional cycle.

Adenovirus protein VII binds the A-box of HMGB1 to repress interferon responses

Viruses hijack host proteins to promote infection and dampen host defenses. Adenovirus encodes the multifunctional protein VII that serves both to compact viral genomes inside the virion and disrupt host chromatin. Protein VII binds the abundant nuclear protein high mobility group box 1 (HMGB1) and sequesters HMGB1 in chromatin. HMGB1 is an abundant host nuclear protein that can also be released from infected cells as an alarmin to amplify inflammatory responses. By sequestering HMGB1, protein VII prevents its release, thus inhibiting downstream inflammatory signaling. However, the consequences of this chromatin sequestration on host transcription are unknown. Here, we employ bacterial two-hybrid interaction assays and human cell culture to interrogate the mechanism of the protein VII-HMGB1 interaction. HMGB1 contains two DNA binding domains, the A- and B-boxes, that bend DNA to promote transcription factor binding while the C-terminal tail regulates this interaction. We demonstrate that protein VII interacts directly with the A-box of HMGB1, an interaction that is inhibited by the HMGB1 C-terminal tail. By cellular fractionation, we show that protein VII renders A-box containing constructs insoluble, thereby acting to prevent their release from cells. This sequestration is not dependent on HMGB1's ability to bind DNA but does require post-translational modifications on protein VII. Importantly, we demonstrate that protein VII inhibits expression of interferon β, in an HMGB1-dependent manner, but does not affect transcription of downstream interferon-stimulated genes. Together, our results demonstrate that protein VII specifically harnesses HMGB1 through its A-box domain to depress the innate immune response and promote infection.

A novel single alpha-helix DNA-binding domain in CAF-1 promotes gene silencing and DNA damage survival through tetrasome-length DNA selectivity and spacer function

The histone chaperone chromatin assembly factor 1 (CAF-1) deposits two nascent histone H3/H4 dimers onto newly replicated DNA forming the central core of the nucleosome known as the tetrasome. How CAF-1 ensures there is sufficient space for the assembly of tetrasomes remains unknown. Structural and biophysical characterization of the lysine/glutamic acid/arginine-rich (KER) region of CAF-1 revealed a 128-Å single alpha-helix (SAH) motif with unprecedented DNA-binding properties. Distinct KER sequence features and length of the SAH drive the selectivity of CAF-1 for tetrasome-length DNA and facilitate function in budding yeast. In vivo, the KER cooperates with the DNA-binding winged helix domain in CAF-1 to overcome DNA damage sensitivity and maintain silencing of gene expression. We propose that the KER SAH links functional domains within CAF-1 with structural precision, acting as a DNA-binding spacer element during chromatin assembly.

Structural analysis of the Candida albicans mitochondrial DNA maintenance factor Gcf1p reveals a dynamic DNA-bridging mechanism

The compaction of mitochondrial DNA (mtDNA) is regulated by architectural HMG-box proteins whose limited cross-species similarity suggests diverse underlying mechanisms. Viability of Candida albicans, a human antibiotic-resistant mucosal pathogen, is compromised by altering mtDNA regulators. Among them, there is the mtDNA maintenance factor Gcf1p, which differs in sequence and structure from its human and Saccharomyces cerevisiae counterparts, TFAM and Abf2p. Our crystallographic, biophysical, biochemical and computational analysis showed that Gcf1p forms dynamic protein/DNA multimers by a combined action of an N-terminal unstructured tail and a long helix. Furthermore, an HMG-box domain canonically binds the minor groove and dramatically bends the DNA while, unprecedentedly, a second HMG-box binds the major groove without imposing distortions. This architectural protein thus uses its multiple domains to bridge co-aligned DNA segments without altering the DNA topology, revealing a new mechanism of mtDNA condensation.

35 Years of TFAM Research: Old Protein, New Puzzles

Transcription Factor A Mitochondrial (TFAM), through its contributions to mtDNA maintenance and expression, is essential for cellular bioenergetics and, therefore, for the very survival of cells. Thirty-five years of research on TFAM structure and function generated a considerable body of experimental evidence, some of which remains to be fully reconciled. Recent advancements allowed an unprecedented glimpse into the structure of TFAM complexed with promoter DNA and TFAM within the open promoter complexes. These novel insights, however, raise new questions about the function of this remarkable protein. In our review, we compile the available literature on TFAM structure and function and provide some critical analysis of the available data.

The C-Terminal Tail of Mitochondrial Transcription Factor A Is Dispensable for Mitochondrial DNA Replication and Transcription In Situ

Mitochondrial transcription factor A (TFAM) is one of the widely studied but still incompletely understood mitochondrial protein, which plays a crucial role in the maintenance and transcription of mitochondrial DNA (mtDNA). The available experimental evidence is often contradictory in assigning the same function to various TFAM domains, partly owing to the limitations of those experimental systems. Recently, we developed the GeneSwap approach, which enables in situ reverse genetic analysis of mtDNA replication and transcription and is devoid of many of the limitations of the previously used techniques. Here, we utilized this approach to analyze the contributions of the TFAM C-terminal (tail) domain to mtDNA transcription and replication. We determined, at a single amino acid (aa) resolution, the TFAM tail requirements for in situ mtDNA replication in murine cells and established that tail-less TFAM supports both mtDNA replication and transcription. Unexpectedly, in cells expressing either C-terminally truncated murine TFAM or DNA-bending human TFAM mutant L6, HSP1 transcription was impaired to a greater extent than LSP transcription. Our findings are incompatible with the prevailing model of mtDNA transcription and thus suggest the need for further refinement.

Adenovirus protein VII binds the A-box of HMGB1 to repress interferon responses

Viruses hijack host proteins to promote infection and dampen host defenses. Adenovirus encodes the multifunctional protein VII that serves both to compact viral genomes inside the virion and disrupt host chromatin. Protein VII binds the abundant nuclear protein high mobility group box 1 (HMGB1) and sequesters HMGB1 in chromatin. HMGB1 is an abundant host nuclear protein that can also be released from infected cells as an alarmin to amplify inflammatory responses. By sequestering HMGB1, protein VII prevents its release, thus inhibiting downstream inflammatory signaling. However, the consequences of this chromatin sequestration on host transcription are unknown. Here, we employ bacterial two-hybrid interaction assays and human cell biological systems to interrogate the mechanism of the protein VII-HMGB1 interaction. HMGB1 contains two DNA binding domains, the A- and B-boxes, that bend DNA to promote transcription factor binding while the C-terminal tail regulates this interaction. We demonstrate that protein VII interacts directly with the A-box of HMGB1, an interaction that is inhibited by the HMGB1 C-terminal tail. By cellular fractionation, we show that protein VII renders A-box containing constructs insoluble, thereby acting to prevent their release from cells. This sequestration is not dependent on HMGB1's ability to bind DNA but does require post-translational modifications on protein VII. Importantly, we demonstrate that protein VII inhibits expression of interferon β, in an HMGB1- dependent manner, but does not affect transcription of downstream interferon- stimulated genes. Together, our results demonstrate that protein VII specifically harnesses HMGB1 through its A-box domain to depress the innate immune response and promote infection.

Two high-mobility group domains of MHG1 are necessary to maintain mtDNA in Neurospora crassa

The mhg1 (NCU02695/ada-23) gene encodes the mitochondrial high-mobility group box (HMG-box or HMGB) protein in Neurospora crassa. The mhg1 KO strain (mhg1^KO) has mitochondrial DNA (mtDNA) instability and a short lifespan; however, the function of MHG1 remains unclear. To investigate the role of this protein in the maintenance of mtDNA, domain deleted MHG1 proteins were expressed in the mhg1^KO strain, and their effects were analyzed. We identified two putative HMG-domains, HMGBI and HMGBII. Although deletion of the HMG-box did not abolish MHG1's mitochondrial localization, the mhg1^KO phenotype of a severe growth defect and a high sensitivity to mutagens could not be restored by introduction of HMG-box deleted mhg1 gene into the KO strain. It was indicated that recombinant full-length MHG1, i.e., mitochondrial targeting sequence (MTS) containing protein, did not exhibit explicit DNA binding, whereas the MHG1 protein truncated for the MTS sequence did in vitro by an electrophoretic mobility shift assay. Furthermore, recombinant MHG1 protein lacking MTS and HMG-domains, either HMGBI or HMGBII, had DNA affinity and an altered band shift pattern compared with MTS-truncated MHG1 protein. These results suggest that cleavage of MTS and appropriate DNA binding via HMG-domains are indispensable for maintaining mtDNA in N. crassa.

Novel interaction interfaces mediate the interaction between the NEIL1 DNA glycosylase and mitochondrial transcription factor A

The maintenance of human mitochondrial DNA (mtDNA) is critical for proper cellular function as damage to mtDNA, if left unrepaired, can lead to a diverse array of pathologies. Of the pathways identified to participate in DNA repair within the mitochondria, base excision repair (BER) is the most extensively studied. Protein-protein interactions drive the step-by-step coordination required for the successful completion of this pathway and are important for crosstalk with other mitochondrial factors involved in genome maintenance. Human NEIL1 is one of seven DNA glycosylases that initiates BER in both the nuclear and mitochondrial compartments. In the current work, we scrutinized the interaction between NEIL1 and mitochondrial transcription factor A (TFAM), a protein that is essential for various aspects of mtDNA metabolism. We note, for the first time, that both the N- and C- terminal domains of NEIL1 interact with TFAM revealing a unique NEIL1 protein-binding interface. The interaction between the two proteins, as observed biochemically, appears to be transient and is most apparent at concentrations of low salt. The presence of DNA (or RNA) also positively influences the interaction between the two proteins, and molar mass estimates indicate that duplex DNA is required for complex formation at higher salt concentrations. Hydrogen deuterium exchange mass spectrometry data reveal that both proteins exchange less deuterium upon DNA binding, indicative of an interaction, and the addition of NEIL1 to the TFAM-DNA complex alters the interaction landscape. The transcriptional activity of TFAM appears to be independent of NEIL1 expression under normal cellular conditions, however, in the presence of DNA damage, we observe a significant reduction in the mRNA expression of TFAM-transcribed mitochondrial genes in the absence of NEIL1. Overall, our data indicate that the interaction between NEIL1 and TFAM can be modulated by local environment such as salt concentrations, protein availability, the presence of nucleic acids, as well as the presence of DNA damage.

A minimal motif for sequence recognition by mitochondrial transcription factor A (TFAM)

Mitochondrial transcription factor A (TFAM) plays a critical role in mitochondrial transcription initiation and mitochondrial DNA (mtDNA) packaging. Both functions require DNA binding, but in one case TFAM must recognize a specific promoter sequence, while packaging requires coating of mtDNA by association with non sequence-specific regions. The mechanisms by which TFAM achieves both sequence-specific and non sequence-specific recognition have not yet been determined. Existing crystal structures of TFAM bound to DNA allowed us to identify two guanine-specific interactions that are established between TFAM and the bound DNA. These interactions are observed when TFAM is bound to both specific promoter sequences and non-sequence specific DNA. These interactions are established with two guanine bases separated by 10 random nucleotides (GN₁₀G). Our biochemical results demonstrate that the GN₁₀G consensus is essential for transcriptional initiation and contributes to facilitating TFAM binding to DNA substrates. Furthermore, we report a crystal structure of TFAM in complex with a non sequence-specific sequence containing a GN₁₀G consensus. The structure reveals a unique arrangement in which TFAM bridges two DNA substrates while maintaining the GN₁₀G interactions. We propose that the GN₁₀G consensus is key to facilitate the interaction of TFAM with DNA.

Mitochondrial Transcription Factor A Binds to and Promotes Mutagenic Transcriptional Bypass of O4-Alkylthymidine Lesions

O2- and O4-alkylated thymidine lesions are known to be poorly repaired and persist in mammalian tissues. To understand how mammalian cells sense the presence and regulate the repair of these lesions, we employed a quantitative proteomic method to discover regioisomeric O2- and O4-n-butylthymidine (O2- and O4-nBudT)-binding proteins. We were able to identify 21 and 74 candidate DNA damage recognition proteins for O2-nBudT- and O4-nBudT-bearing DNA probes, respectively. Among these proteins, DDB1 and DDB2 selectively bind to O2-nBudT-containing DNA, whereas three high-mobility group (HMG) proteins (i.e., HMGB1, HMGB2, and mitochondrial transcription factor A (TFAM)) exhibit preferential binding to O4-nBudT-bearing DNA. We further demonstrated that TFAM binds directly and selectively with O4-alkyldT-harboring DNA, and the binding capacity depends mainly on the HMG box-A domain of TFAM. We also found that TFAM promotes transcriptional mutagenesis of O4-nBudT and O4-pyridyloxobutylthymidine, which is a DNA adduct induced by tobacco-specific N-nitrosamines, in vitro and in human cells. Together, we explored, for the first time, the interaction proteomes of O-alkyldT lesions, and our study expanded the functions of TFAM by revealing its capability in the recognition of O4-alkyldT-bearing DNA and uncovering its modulation of transcriptional mutagenesis of these lesions in human cells.

Crystal structure of the HMG domain of human BAF57 and its interaction with four-way junction DNA

The SWI/SNF chromatin remodeling complex plays important roles in gene regulation and it is classified as the SWI/SNF complex in yeast and BAF complex in vertebrates. BAF57, one of the subunits that forms the chromatin remodeling complex core, is well conserved in the BAF complex of vertebrates, which is replaced by bap111 in the Drosophila BAP complex and does not have a counterpart in the yeast SWI/SNF complex. This suggests that BAF57 is a key component of the chromatin remodeling complex in higher eukaryotes. BAF57 contains a HMG domain, which is widely distributed among various proteins and functions as a DNA binding motif. Most proteins with HMG domain bind to four-way junction (4WJ) DNA. Here, we report the crystal structure of the HMG domain of BAF57 (BAF57^HMG) at a resolution of 2.55 Å. The structure consists of three α-helices and adopts an L-shaped form. The overall structure is stabilized by a hydrophobic core, which is formed by hydrophobic residues. The binding affinity between BAF57^HMG and 4WJ DNA is determined as a 295.83 ± 1.05 nM using a fluorescence quenching assay, and the structure revealed 4WJ DNA binding site of BAF57^HMG. Our data will serve structural basis in understanding the roles of BAF57 during chromatin remodeling process.

PfKsgA1 functions as a transcription initiation factor and interacts with the N-terminal region of the mitochondrial RNA polymerase of Plasmodium falciparum

The small mitochondrial genome (mtDNA) of the malaria parasite is known to transcribe its genes polycistonically, although promoter element(s) have not yet been identified. An unusually large Plasmodium falciparum candidate mitochondrial phage-like RNA polymerase (PfmtRNAP) with an extended N-terminal region is encoded by the parasite nuclear genome. Using specific antibodies against the enzyme, we established that PfmtRNAP was targeted exclusively to the mitochondrion and interacted with mtDNA. Phylogenetic analysis showed that it is part of a separate apicomplexan clade. A search for PfmtRNAP-associated transcription initiation factors using sequence homology and in silico protein-protein interaction network analysis identified PfKsgA1. PfKsgA1 is a dual cytosol- and mitochondrion-targeted protein that functions as a small subunit rRNA dimethyltransferase in ribosome biogenesis. Chromatin immunoprecipitation showed that PfKsgA1 interacts with mtDNA, and in vivo crosslinking and pull-down experiments confirmed PfmtRNAP-PfKsgA1 interaction. The ability of PfKsgA1 to serve as a transcription initiation factor was demonstrated by complementation of yeast mitochondrial transcription factor Mtf1 function in Rpo41-driven in vitro transcription. Pull-down experiments using PfKsgA1 and PfmtRNAP domains indicated that the N-terminal region of PfmtRNAP interacts primarily with the PfKsgA1 C-terminal domain with some contacts being made with the linker and N-terminal domain of PfKsgA1. In the absence of full-length recombinant PfmtRNAP, solution structures of yeast mitochondrial RNA polymerase Rpo41 complexes with Mtf1 or PfKsgA1 were determined by small-angle X-ray scattering. Protein interaction interfaces thus identified matched with those reported earlier for Rpo41-Mtf1 interaction and overlaid with the PfmtRNAP-interfacing region identified experimentally for PfKsgA1. Our results indicate that in addition to a role in mitochondrial ribosome biogenesis, PfKsgA1 has an independent function as a transcription initiation factor for PfmtRNAP.

Mitochondrial HMG-Box Containing Proteins: From Biochemical Properties to the Roles in Human Diseases

Mitochondrial DNA (mtDNA) molecules are packaged into compact nucleo-protein structures called mitochondrial nucleoids (mt-nucleoids). Their compaction is mediated in part by high-mobility group (HMG)-box containing proteins (mtHMG proteins), whose additional roles include the protection of mtDNA against damage, the regulation of gene expression and the segregation of mtDNA into daughter organelles. The molecular mechanisms underlying these functions have been identified through extensive biochemical, genetic, and structural studies, particularly on yeast (Abf2) and mammalian mitochondrial transcription factor A (TFAM) mtHMG proteins. The aim of this paper is to provide a comprehensive overview of the biochemical properties of mtHMG proteins, the structural basis of their interaction with DNA, their roles in various mtDNA transactions, and the evolutionary trajectories leading to their rapid diversification. We also describe how defects in the maintenance of mtDNA in cells with dysfunctional mtHMG proteins lead to different pathologies at the cellular and organismal level.

Mitochondrial Protein Synthesis and mtDNA Levels Coordinated through an Aminoacyl-tRNA Synthetase Subunit

The aminoacylation of tRNAs by aminoacyl-tRNA synthetases (ARSs) is a central reaction in biology. Multiple regulatory pathways use the aminoacylation status of cytosolic tRNAs to monitor and regulate metabolism. The existence of equivalent regulatory networks within the mitochondria is unknown. Here, we describe a functional network that couples protein synthesis to DNA replication in animal mitochondria. We show that a duplication of the gene coding for mitochondrial seryl-tRNA synthetase (SerRS2) generated in arthropods a paralog protein (SLIMP) that forms a heterodimeric complex with a SerRS2 monomer. This seryl-tRNA synthetase variant is essential for protein synthesis and mitochondrial respiration. In addition, SLIMP interacts with the substrate binding domain of the mitochondrial protease LON, thus stimulating proteolysis of the DNA-binding protein TFAM and preventing mitochondrial DNA (mtDNA) accumulation. Thus, mitochondrial translation is directly coupled to mtDNA levels by a network based upon a profound structural modification of an animal ARS.

In vitro studies on non-canonical DNA binding specificities of KAP6 and HMO1 and mechanistic insights into DNA bound and unbinding dynamics of KAP6

High mobility group box (HMGB) members are DNA binding proteins with varied functions present across kingdoms. The mechanism by which HMGBs with varying number of HMG boxes are able to carry out similar functions, are poorly understood. Moreover, how non-canonical DNAs are recognized by HMGB proteins is not clear. To address these, we carried out detailed biochemical and computational studies to characterize two HMGB members- Kinetoplast associated protein (KAP6) of Trypanosoma and High mobility group protein 1 (HMO1) from yeast. Here, we report that KAP6 binds non-canonical DNAs tighter than B-form DNA. Among non-canonical DNAs, KAP6 has the highest affinity for splayed and flap structures, but least for Holliday Junction (HJ). In contrast, HMO1 binds tighter to HJ. Computational analysis show that the secondary structural elements involved in DNA interaction are conserved in HMGB members KAP6 and mitochondrial transcription factor A. Simulation analyses revealed that the ~90° bend in DNA induced by KAP6 HMG box is a result of two ~45° bends, by Helix 1 and Helix 2 of the protein. Our data also suggests that the orthologs of HMO1 and KAP6 are oligomers in solution, which could be necessary for their functioning such as DNA bending and looping.

DNA specificities modulate the binding of human transcription factor A to mitochondrial DNA control region

Human mitochondrial DNA (h-mtDNA) codes for 13 subunits of the oxidative phosphorylation pathway, the essential route that produces ATP. H-mtDNA transcription and replication depends on the transcription factor TFAM, which also maintains and compacts this genome. It is well-established that TFAM activates the mtDNA promoters LSP and HSP1 at the mtDNA control region where DNA regulatory elements cluster. Previous studies identified still uncharacterized, additional binding sites at the control region downstream from and slightly similar to LSP, namely sequences X and Y (Site-X and Site-Y) (Fisher et al., Cell 50, pp 247-258, 1987). Here, we explore TFAM binding at these two sites and compare them to LSP by multiple experimental and in silico methods. Our results show that TFAM binding is strongly modulated by the sequence-dependent properties of Site-X, Site-Y and LSP. The high binding versatility of Site-Y or the considerable stiffness of Site-X tune TFAM interactions. In addition, we show that increase in TFAM/DNA complex concentration induces multimerization, which at a very high concentration triggers disruption of preformed complexes. Therefore, our results suggest that mtDNA sequences induce non-uniform TFAM binding and, consequently, direct an uneven distribution of TFAM aggregation sites during the essential process of mtDNA compaction.

Upregulation of Mitochondrial Transcription Factor A Promotes the Repairment of Renal Tubular Epithelial Cells in Sepsis by Inhibiting Reactive Oxygen Species-Mediated Toll-Like Receptor 4/p38MAPK Signaling

BACKGROUND

Mitochondrial transcription factor A (TFAM) plays multiple pathophysiologic roles in mitochondrial DNA (mtDNA) maintenance. However, the role of TFAM in sepsis-induced acute kidney injury (AKI) remains largely unknown.

METHODS

Lipopolysaccharide (LPS) treatment of HK-2 cells mimics the in vitro model of AKI inflammation. pcDNA3.1 plasmid was used to construct pcDNA3.1-TFAM. sh-TFAM-543, sh-TFAM-717, sh-TFAM-765, sh-TFAM-904 and pcDNA3.1-TFAM were transfected into HK-2 cells using Lipofectamine 2000. MtDNA transcriptional levels were detected by quantitative real-time polymerase chain reaction (qRT-PCR). 3-(4,5)-dimethylthiahiazo (-z-y1)-3,5-di-phenytetrazoliumromide (MTT) assay was performed to assess the cell viability. Changes in reactive oxygen species (ROS) and mitochondrial membrane potential in HK-2 cells were detected using the corresponding kits. Immunofluorescence experiment was used to investigate the displacement of TFAM. mRNA and protein expression levels of TFAM and its related genes were measured by qRT-PCR and western blot respectively. Mice in sepsis were administered cecal ligation and puncture surgery.

RESULTS

LPS treatment was a non-lethal influencing factor, leading to the upregulation of ROS levels and downregulation of mtDNA copy number and NADH dehydrogenase subunit-1 (ND1) expression, and caused damage to the mitochondria. As the LPS treatment time increased, TFAM was displaced from the periphery of the nucleus to cytoplasm. TFAM reduced ROS and P38MAPK levels by inhibiting toll-like receptor 4 (TLR4) expression, ultimately inhibiting inflammation and repairing mtDNA.

CONCLUSIONS

Our results indicate that TFAM repairs mtDNA by blocking the TLR4/ROS/P38MAPK signaling pathway in inflammatory cells, thereby repairing septic tubular epithelial cells, and TFAM may serve as a new target for sepsis therapy.

Organization of DNA in Mammalian Mitochondria

As with all organisms that must organize and condense their DNA to fit within the limited volume of a cell or a nucleus, mammalian mitochondrial DNA (mtDNA) is packaged into nucleoprotein structures called nucleoids. In this study, we first introduce the general modes of DNA compaction, especially the role of the nucleoid-associated proteins (NAPs) that structure the bacterial chromosome. We then present the mitochondrial nucleoid and the main factors responsible for packaging of mtDNA: ARS- (autonomously replicating sequence-) binding factor 2 protein (Abf2p) in yeast and mitochondrial transcription factor A (TFAM) in mammals. We summarize the single-molecule manipulation experiments on mtDNA compaction and visualization of mitochondrial nucleoids that have led to our current knowledge on mtDNA compaction. Lastly, we discuss the possible regulatory role of DNA packaging by TFAM in DNA transactions such as mtDNA replication and transcription.

Cytosine methylation of mitochondrial DNA at CpG sequences impacts transcription factor A DNA binding and transcription

In eukaryotes, cytosine methylation of nuclear DNA at CpG sequences (5mCpG) regulates epigenetic inheritance through alterations in chromatin structure. However, mitochondria lack nucleosomal chromatin, therefore the molecular mechanisms by which 5mCpG influences mitochondria must be different and are as yet unknown. Mitochondrial Transcription Factor A (TFAM) is both the primary DNA-compacting protein in the mitochondrial DNA (mtDNA) nucleoid and a transcription-initiation factor. TFAM must encounter hundreds of CpGs in mtDNA, so the occurrence of 5mCpG has the potential to impact TFAM-DNA recognition. We used biophysical approaches to determine whether 5mCpG alters any TFAM-dependent activities. 5mCpG in the heavy strand promoter (HSP1) increased the binding affinity of TFAM and induced TFAM multimerization with increased cooperativity compared to nonmethylated DNA. However, 5mCpG had no apparent effect on TFAM-dependent DNA compaction. Additionally, 5mCpG had a clear and context-dependent effect on transcription initiating from the three mitochondrial promoters. Taken together, our findings demonstrate that 5mCpG in the mitochondrial promoter region does impact TFAM-dependent activities in vitro.

Structural basis of mitochondrial transcription

The mitochondrial genome is transcribed by a single-subunit DNA-dependent RNA polymerase (mtRNAP) and its auxiliary factors. Structural studies have elucidated how mtRNAP cooperates with its dedicated transcription factors to direct RNA synthesis: initiation factors TFAM and TFB2M assist in promoter-DNA binding and opening by mtRNAP while the elongation factor TEFM increases polymerase processivity to the levels required for synthesis of long polycistronic mtRNA transcripts. Here, we review the emerging body of structural and functional studies of human mitochondrial transcription, provide a molecular movie that can be used for teaching purposes and discuss the open questions to guide future directions of investigation.

Acetylation and phosphorylation of human TFAM regulate TFAM-DNA interactions via contrasting mechanisms

Mitochondrial transcription factor A (TFAM) is essential for the maintenance, expression and transmission of mitochondrial DNA (mtDNA). However, mechanisms for the post-translational regulation of TFAM are poorly understood. Here, we show that TFAM is lysine acetylated within its high-mobility-group box 1, a domain that can also be serine phosphorylated. Using bulk and single-molecule methods, we demonstrate that site-specific phosphoserine and acetyl-lysine mimics of human TFAM regulate its interaction with non-specific DNA through distinct kinetic pathways. We show that higher protein concentrations of both TFAM mimics are required to compact DNA to a similar extent as the wild-type. Compaction is thought to be crucial for regulating mtDNA segregation and expression. Moreover, we reveal that the reduced DNA binding affinity of the acetyl-lysine mimic arises from a lower on-rate, whereas the phosphoserine mimic displays both a decreased on-rate and an increased off-rate. Strikingly, the increased off-rate of the phosphoserine mimic is coupled to a significantly faster diffusion of TFAM on DNA. These findings indicate that acetylation and phosphorylation of TFAM can fine-tune TFAM-DNA binding affinity, to permit the discrete regulation of mtDNA dynamics. Furthermore, our results suggest that phosphorylation could additionally regulate transcription by altering the ability of TFAM to locate promoter sites.

Protein Flexibility and Synergy of HMG Domains Underlie U-Turn Bending of DNA by TFAM in Solution

Human mitochondrial transcription factor A (TFAM) distorts DNA into a U-turn, as shown by crystallographic studies. The relevance of this U-turn is associated with transcription initiation at the mitochondrial light strand promoter (LSP). However, it has not been yet discerned whether a tight U-turn or an alternative conformation, such as a V-shape, is formed in solution. Here, single-molecule FRET experiments on freely diffusing TFAM/LSP complexes containing different DNA lengths show that a DNA U-turn is induced by progressive and cooperative binding of the two TFAM HMG-box domains and the linker between them. SAXS studies further show compaction of the protein upon complex formation. Finally, molecular dynamics simulations reveal that TFAM/LSP complexes are dynamic entities, and the HMG boxes induce the U-turn against the tendency of the DNA to adopt a straighter conformation. This tension is resolved by reversible unfolding of the linker, which is a singular mechanism that allows a flexible protein to stabilize a tight bending of DNA.

The human mitochondrial transcription factor A is a versatile G-quadruplex binding protein

The ability of the guanine-rich strand of the human mitochondrial DNA (mtDNA) to form G-quadruplex structures (G4s) has been recently highlighted, suggesting potential functions in mtDNA replication initiation and mtDNA stability. G4 structures in mtDNA raise the question of their recognition by factors associated with the mitochondrial nucleoid. The mitochondrial transcription factor A (TFAM), a high-mobility group (HMG)-box protein, is the major binding protein of human mtDNA and plays a critical role in its expression and maintenance. HMG-box proteins are pleiotropic sensors of DNA structural alterations. Thus, we investigated and uncovered a surprising ability of TFAM to bind to DNA or RNA G4 with great versatility, showing an affinity similar than to double-stranded DNA. The recognition of G4s by endogenous TFAM was detected in mitochondrial extracts by pull-down experiments using a G4-DNA from the mtDNA conserved sequence block II (CSBII). Biochemical characterization shows that TFAM binding to G4 depends on both the G-quartets core and flanking single-stranded overhangs. Additionally, it shows a structure-specific binding mode that differs from B-DNA, including G4-dependent TFAM multimerization. These TFAM-G4 interactions suggest functional recognition of G4s in the mitochondria.

The self-association of HMGB1 and its possible role in the binding to DNA and cell membrane receptors

High mobility group box 1 (HMGB1), a chromatin protein, interacts with DNA and controls gene expression. However, when HMGB1 is released from apoptotic or damaged cells, it triggers proinflammatory reactions by interacting with various receptors, mainly receptor for advanced glycation end-products (RAGE) and toll-like receptors (TLRs). The self-association of HMGB1 has been found to be crucial for its DNA-related biological functions. It is influenced by several factors, such as ionic strength, pH, specific divalent metal cations, redox environment and acetylation. This self-association may also play a role in the interaction with RAGE and TLRs and the concomitant inflammatory responses. Future studies should address the potential role of HMGB1 self-association on its interactions with DNA, RAGE and TLRs, as well as the influence of physicochemical factors in different cellular environments on these interactions.

Oligomerization of Hmo1 mediated by box A is essential for DNA binding in vitro and in vivo

Hmo1, a member of HMGB family proteins in Saccharomyces cerevisiae, binds to and regulates the transcription of genes encoding ribosomal RNA and ribosomal proteins. The functional motifs of Hmo1 include two HMG-like motifs, box A and box B, and a C-terminal tail. To elucidate the molecular roles of the HMG-like boxes in DNA binding in vivo, we analyzed the DNA-binding activity of various Hmo1 mutants using ChIP or reporter assays that enabled us to conveniently detect Hmo1 binding to the promoter of RPS5, a major target gene of Hmo1. Our mutational analyses showed that box B is a bona fide DNA-binding motif and that it also plays other important roles in cell growth. However, box A, especially its first α-helix, contributes to DNA binding of Hmo1 by inducing self-assembly of Hmo1. Intriguingly, box A mediated formation of oligomers of more than two proteins on DNA in vivo. Furthermore, duplication of the box B partially alleviates the requirement for box A. These findings suggest that the principal role of box A is to assemble multiple box B in the appropriate orientation, thereby stabilizing the binding of Hmo1 to DNA and nucleating specific chromosomal architecture on its target genes.

Mitochondrial Matrix Processes

Mitochondria possess their own genome that, despite its small size, is critically important for their functioning, as it encodes several dozens of RNAs and proteins. All biochemical processes typical for bacterial and nuclear DNA are described in mitochondrial matrix: replication, repair, recombination, and transcription. Commonly, their mechanisms are similar to those found in bacteria, but they are characterized by several unique features. In this review, we provide an overall description of mitochondrial matrix processes paying special attention to the typical features of such mechanisms.

Mutations in TFAM, encoding mitochondrial transcription factor A, cause neonatal liver failure associated with mtDNA depletion

In humans, mitochondrial DNA (mtDNA) depletion syndromes are a group of genetically and clinically heterogeneous autosomal recessive disorders that arise as a consequence of defects in mtDNA replication or nucleotide synthesis. Clinical manifestations are variable and include myopathic, encephalomyopathic, neurogastrointestinal or hepatocerebral phenotypes. Through clinical exome sequencing, we identified a homozygous missense variant (c.533C>T; p.Pro178Leu) in mitochondrial transcription factor A (TFAM) segregating in a consanguineous kindred of Colombian-Basque descent in which two siblings presented with IUGR, elevated transaminases, conjugated hyperbilirubinemia and hypoglycemia with progression to liver failure and death in early infancy. Results of the liver biopsy in the proband revealed cirrhosis, micro- and macrovesicular steatosis, cholestasis and mitochondrial pleomorphism. Electron microscopy of muscle revealed abnormal mitochondrial morphology and distribution while enzyme histochemistry was underwhelming. Electron transport chain testing in muscle showed increased citrate synthase activity suggesting mitochondrial proliferation, while respiratory chain activities were at the lower end of normal. mtDNA content was reduced in liver and muscle (11% and 21% of normal controls respectively). While Tfam mRNA expression was upregulated in primary fibroblasts, Tfam protein level was significantly reduced. Furthermore, functional investigations of the mitochondria revealed reduced basal respiration and spare respiratory capacity, decreased mtDNA copy number and markedly reduced nucleoids. TFAM is essential for transcription, replication and packaging of mtDNA into nucleoids. Tfam knockout mice display embryonic lethality secondary to severe mtDNA depletion. In this report, for the first time, we associate a homozygous variant in TFAM with a novel mtDNA depletion syndrome.

Mechanism of Mitochondrial Transcription Factor A Attenuation of CpG-Induced Antibody Production

Mitochondrial transcription factor A (TFAM) had previously been shown to act as a damage associated molecular pattern with the ability to enhance CpG-A phosphorothioate oligodeoxynucleotide (ODN)-mediated stimulation of IFNα production from human plasmacytoid dendritic cells. Examination of the mechanism by which TFAM might influence CpG ODN mediated innate immune responses revealed that TFAM binds directly, tightly and selectively to the structurally related CpG-A, -B, and -C ODN. TFAM also modulated the ability of the CpG-B or -C to stimulate the production of antibodies from human B cells. TFAM showed a dose-dependent modulation of CpG-B, and -C -induced antibody production from human B cells in vitro, with enhancement of high dose and inhibition of low doses of CpG stimulation. This effect was linked to the ability of TFAM to directly inhibit the binding of CpG ODNs to B cells, in a manner consistent with the relative binding affinities of TFAM for the ODNs. These data suggest that TFAM alters the free concentration of the CpG available to stimulate B cells by sequestering this ODN in a TFAM-CpG complex. Thus, TFAM has the potential to decrease the pathogenic consequences of exposure to natural CpG-like hypomethylated DNA in vivo, as well as such as that found in traumatic injury, infection, autoimmune disease and during pregnancy.

Yeast mitochondrial HMG proteins: DNA-binding properties of the most evolutionarily divergent component of mitochondrial nucleoids

Comparative biochemical analysis of mtHMG proteins from distantly related yeast species revealed that they exhibit a preference for recombination/replication intermediates. We discuss how these biochemical characteristics relate to the role of mtHMG proteins in mtDNA compaction and evolution.

Yeast mtDNA is compacted into nucleoprotein structures called mitochondrial nucleoids (mt-nucleoids). The principal mediators of nucleoid formation are mitochondrial high-mobility group (HMG)-box containing (mtHMG) proteins. Although these proteins are some of the fastest evolving components of mt-nucleoids, it is not known whether the divergence of mtHMG proteins on the level of their amino acid sequences is accompanied by diversification of their biochemical properties. In the present study we performed a comparative biochemical analysis of yeast mtHMG proteins from Saccharomyces cerevisiae (ScAbf2p), Yarrowia lipolytica (YlMhb1p) and Candida parapsilosis (CpGcf1p). We found that all three proteins exhibit relatively weak binding to intact dsDNA. In fact, ScAbf2p and YlMhb1p bind quantitatively to this substrate only at very high protein to DNA ratios and CpGcf1p shows only negligible binding to dsDNA. In contrast, the proteins exhibit much higher preference for recombination intermediates such as Holliday junctions (HJ) and replication forks (RF). Therefore, we hypothesize that the roles of the yeast mtHMG proteins in maintenance and compaction of mtDNA in vivo are in large part mediated by their binding to recombination/replication intermediates. We also speculate that the distinct biochemical properties of CpGcf1p may represent one of the prerequisites for frequent evolutionary tinkering with the form of the mitochondrial genome in the CTG-clade of hemiascomycetous yeast species.

Distinct structural features of TFAM drive mitochondrial DNA packaging versus transcriptional activation

TFAM (transcription factor A, mitochondrial) is a DNA-binding protein that activates transcription at the two major promoters of mitochondrial DNA (mtDNA)--the light strand promoter (LSP) and the heavy strand promoter 1 (HSP1). Equally important, it coats and packages the mitochondrial genome. TFAM has been shown to impose a U-turn on LSP DNA; however, whether this distortion is relevant at other sites is unknown. Here we present crystal structures of TFAM bound to HSP1 and to nonspecific DNA. In both, TFAM similarly distorts the DNA into a U-turn. Yet, TFAM binds to HSP1 in the opposite orientation from LSP explaining why transcription from LSP requires DNA bending, whereas transcription at HSP1 does not. Moreover, the crystal structures reveal dimerization of DNA-bound TFAM. This dimerization is dispensable for DNA bending and transcriptional activation but is important in DNA compaction. We propose that TFAM dimerization enhances mitochondrial DNA compaction by promoting looping of the DNA.

Mitochondrial Transcription Factor A (TFAM) Binds to RNA Containing 4-Way Junctions and Mitochondrial tRNA

Mitochondrial DNA (mtDNA) is maintained within nucleoprotein complexes known as nucleoids. These structures are highly condensed by the DNA packaging protein, mitochondrial Transcription Factor A (TFAM). Nucleoids also include RNA, RNA:DNA hybrids, and are associated with proteins involved with RNA processing and mitochondrial ribosome biogenesis. Here we characterize the ability of TFAM to bind various RNA containing substrates in order to determine their role in TFAM distribution and function within the nucleoid. We find that TFAM binds to RNA-containing 4-way junctions but does not bind appreciably to RNA hairpins, internal loops, or linear RNA:DNA hybrids. Therefore the RNA within nucleoids largely excludes TFAM, and its distribution is not grossly altered with removal of RNA. Within the cell, TFAM binds to mitochondrial tRNAs, consistent with our RNA 4-way junction data. Kinetic binding assays and RNase-insensitive TFAM distribution indicate that DNA remains the preferred substrate within the nucleoid. However, TFAM binds to tRNA with nanomolar affinity and these complexes are not rare. TFAM-immunoprecipitated tRNAs have processed ends, suggesting that binding is not specific to RNA precursors. The amount of each immunoprecipitated tRNA is not well correlated with tRNA celluar abundance, indicating unequal TFAM binding preferences. TFAM-mt-tRNA interaction suggests potentially new functions for this protein.

Structural models of mammalian mitochondrial transcription factor B2

Mammalian mitochondrial DNA (mtDNA) encodes 13 core proteins of oxidative phosphorylation, 12S and 16S ribosomal RNAs, and 22 transfer RNAs. Mutations and deletions of mtDNA and/or nuclear genes encoding mitochondrial proteins have been implicated in a wide range of diseases. Thus, cell survival and health of the organism require some steady-state level of the mitochondrial genome and its expression. In mammalian systems, the mitochondrial transcription factor B2 (mtTFB2 or TFB2M) is indispensable for transcription initiation. TFB2M along with two other proteins, mitochondrial RNA polymerase (mtRNAP or POLRMT) and mitochondrial transcription factor A (mtTFA or TFAM), are key components of the core mitochondrial transcription apparatus. Structural information for POLRMT and TFAM from humans is available; however, there is no available structure for TFB2M. In the present study, three-dimensional structure of TFB2M from humans was modeled using a combination of homology modeling and small-angle X-ray scattering (SAXS). The TFB2M structural model adds substantively to our understanding of TFB2M function. An explanation for the low or absent RNA methyltransferase activity is provided. A putative nucleic acid-binding site is revealed. The amino and carboxy termini, while likely lacking defined secondary structure, appear to adopt compact, globular conformations, thus "capping" the ends of the protein. Finally, sites of interaction of TFB2M with other factors, protein and/or nucleic acid, are suggested by the identification of species-specific clusters on the surface of the protein.

The strictly aerobic yeast Yarrowia lipolytica tolerates loss of a mitochondrial DNA-packaging protein

Mitochondrial DNA (mtDNA) is highly compacted into DNA-protein structures termed mitochondrial nucleoids (mt-nucleoids). The key mt-nucleoid components responsible for mtDNA condensation are HMG box-containing proteins such as mammalian mitochondrial transcription factor A (TFAM) and Abf2p of the yeast Saccharomyces cerevisiae. To gain insight into the function and organization of mt-nucleoids in strictly aerobic organisms, we initiated studies of these DNA-protein structures in Yarrowia lipolytica. We identified a principal component of mt-nucleoids in this yeast and termed it YlMhb1p (Y. lipolytica mitochondrial HMG box-containing protein 1). YlMhb1p contains two putative HMG boxes contributing both to DNA binding and to its ability to compact mtDNA in vitro. Phenotypic analysis of a Δmhb1 strain lacking YlMhb1p resulted in three interesting findings. First, although the mutant exhibits clear differences in mt-nucleoids accompanied by a large decrease in the mtDNA copy number and the number of mtDNA-derived transcripts, its respiratory characteristics and growth under most of the conditions tested are indistinguishable from those of the wild-type strain. Second, our results indicate that a potential imbalance between subunits of the respiratory chain encoded separately by nuclear DNA and mtDNA is prevented at a (post)translational level. Third, we found that mtDNA in the Δmhb1 strain is more prone to mutations, indicating that mtHMG box-containing proteins protect the mitochondrial genome against mutagenic events.

A disorder-to-order structural transition in the COOH-tail of Fz4 determines misfolding of the L501fsX533-Fz4 mutant

Frizzled 4 belongs to the superfamily of G protein coupled receptors. The unstructured cytosolic tail of the receptor is essential for its activity. The mutation L501fsX533 in the fz4 gene results in a new COOH-tail of the receptor and causes a form of Familial exudative vitreoretinopathy. Here we show that the mutated tail is structured. Two amphipathic helices, displaying affinity for membranes and resembling the structure of Influenza Hemagglutinin fusion peptide, constitute the new fold. This tail induces the aggregation of the receptor in the Endoplasmic Reticulum and it is sufficient to block the export to the Golgi of a chimeric VSVG protein containing the mutated tail. Affecting the tail's structure, net charge or amphipathicity relocates the mutated Fz4 receptor to the Plasma Membrane. Such disorder-to-order structural transition was never described in GPCRs and opens a new scenario on the possible effect of mutations on unstructured regions of proteins.

A novel intermediate in transcription initiation by human mitochondrial RNA polymerase

The mitochondrial genome is transcribed by a single-subunit T7 phage-like RNA polymerase (mtRNAP), structurally unrelated to cellular RNAPs. In higher eukaryotes, mtRNAP requires two transcription factors for efficient initiation-TFAM, a major nucleoid protein, and TFB2M, a transient component of mtRNAP catalytic site. The mechanisms behind assembly of the mitochondrial transcription machinery and its regulation are poorly understood. We isolated and identified a previously unknown human mitochondrial transcription intermediate-a pre-initiation complex that includes mtRNAP, TFAM and promoter DNA. Using protein-protein cross-linking, we demonstrate that human TFAM binds to the N-terminal domain of mtRNAP, which results in bending of the promoter DNA around mtRNAP. The subsequent recruitment of TFB2M induces promoter melting and formation of an open initiation complex. Our data indicate that the pre-initiation complex is likely to be an important target for transcription regulation and provide basis for further structural, biochemical and biophysical studies of mitochondrial transcription.

Mitochondrial transcription factor A regulated ionizing radiation-induced mitochondrial biogenesis in human lung adenocarcinoma A549 cells

Mitochondrial transcription factor A (TFAM), the first well-characterized transcription factor from vertebrate mitochondria, is closely related to mitochondrial DNA (mtDNA) maintenance and repair. Recent evidence has shown that the ratio of mtDNA to nuclearDNA (nDNA) is increased in both human cells and murine tissues after ionizing radiation (IR). However, the underlying mechanism has not as yet been clearly identified. In the present study, we demonstrated that in human lung adenocarcinoma A549 cells, expression of TFAM was upregulated, together with the increase of the relative mtDNA copy number and cytochrome c oxidase (COX) activity after α-particle irradiation. Furthermore, short hairpin RNA (shRNA)-mediated TFAM knockdown inhibited the enhancement of the relative mtDNA copy number and COX activity caused by α-particles. Taken together, our data suggested that TFAM plays a crucial role in regulating mtDNA amplification and mitochondrial biogenesis under IR conditions.

Reversed-phase ion-pair liquid chromatography method for purification of duplex DNA with single base pair resolution

Obtaining quantities of highly pure duplex DNA is a bottleneck in the biophysical analysis of protein–DNA complexes. In traditional DNA purification methods, the individual cognate DNA strands are purified separately before annealing to form DNA duplexes. This approach works well for palindromic sequences, in which top and bottom strands are identical and duplex formation is typically complete. However, in cases where the DNA is non-palindromic, excess of single-stranded DNA must be removed through additional purification steps to prevent it from interfering in further experiments. Here we describe and apply a novel reversed-phase ion-pair liquid chromatography purification method for double-stranded DNA ranging in lengths from 17 to 51 bp. Both palindromic and non-palindromic DNA can be readily purified. This method has the unique ability to separate blunt double-stranded DNA from pre-attenuated (n-1, n-2, etc) synthesis products, and from DNA duplexes with single base pair overhangs. Additionally, palindromic DNA sequences with only minor differences in the central spacer sequence of the DNA can be separated, and the purified DNA is suitable for co-crystallization of protein–DNA complexes. Thus, double-stranded ion-pair liquid chromatography is a useful approach for duplex DNA purification for many applications.