Translational Imaging Spectroscopy for Proximal Sensing.

Proximal sensing as the near field counterpart of remote sensing offers a broad variety of applications. Imaging spectroscopy in general and translational laboratory imaging spectroscopy in particular can be utilized for a variety of different research topics. Geoscientific applications require a precise pre-processing of hyperspectral data cubes to retrieve at-surface reflectance in order to conduct spectral feature-based comparison of unknown sample spectra to known library spectra. A new pre-processing chain called GeoMAP-Trans for at-surface reflectance retrieval is proposed here as an analogue to other algorithms published by the team of authors. It consists of a radiometric, a geometric and a spectral module. Each module consists of several processing steps that are described in detail. The processing chain was adapted to the broadly used HySPEX VNIR/SWIR imaging spectrometer system and tested using geological mineral samples. The performance was subjectively and objectively evaluated using standard artificial image quality metrics and comparative measurements of mineral and Lambertian diffuser standards with standard field and laboratory spectrometers. The proposed algorithm provides highly qualitative results, offers broad applicability through its generic design and might be the first one of its kind to be published. A high radiometric accuracy is achieved by the incorporation of the Reduction of Miscalibration Effects (ROME) framework. The geometric accuracy is higher than 1 µpixel. The critical spectral accuracy was relatively estimated by comparing spectra of standard field spectrometers to those from HySPEX for a Lambertian diffuser. The achieved spectral accuracy is better than 0.02% for the full spectrum and better than 98% for the absorption features. It was empirically shown that point and imaging spectrometers provide different results for non-Lambertian samples due to their different sensing principles, adjacency scattering impacts on the signal and anisotropic surface reflection properties.

Negative regulation of mitochondrial transcription by mitochondrial topoisomerase I.

Mitochondrial topoisomerase I is a genetically distinct mitochondria-dedicated enzyme with a crucial but so far unknown role in the homeostasis of mitochondrial DNA metabolism. Here, we present data suggesting a negative regulatory function in mitochondrial transcription or transcript stability. Deficiency or depletion of mitochondrial topoisomerase I increased mitochondrial transcripts, whereas overexpression lowered mitochondrial transcripts, depleted respiratory complexes I, III and IV, decreased cell respiration and raised superoxide levels. Acute depletion of mitochondrial topoisomerase I triggered neither a nuclear mito-biogenic stress response nor compensatory topoisomerase IIß upregulation, suggesting the concomitant increase in mitochondrial transcripts was due to release of a local inhibitory effect. Mitochondrial topoisomerase I was co-immunoprecipitated with mitochondrial RNA polymerase. It selectively accumulated and rapidly exchanged at a subset of nucleoids distinguished by the presence of newly synthesized RNA and/or mitochondrial RNA polymerase. The inactive Y559F-mutant behaved similarly without affecting mitochondrial transcripts. In conclusion, mitochondrial topoisomerase I dampens mitochondrial transcription and thereby alters respiratory capacity. The mechanism involves selective association of the active enzyme with transcriptionally active nucleoids and a direct interaction with mitochondrial RNA polymerase. The inhibitory role of topoisomerase I in mitochondrial transcription is strikingly different from the stimulatory role of topoisomerase I in nuclear transcription.

Target genes of Topoisomerase IIß regulate neuronal survival and are defined by their chromatin state.

Topoisomerases are essential for DNA replication in dividing cells, but their genomic targets and function in postmitotic cells remain poorly understood. Here we show that a switch in the expression from Topoisomerases IIα (Top2α) to IIß (Top2ß) occurs during neuronal differentiation in vitro and in vivo. Genome-scale location analysis in stem cell-derived postmitotic neurons reveals Top2ß binding to chromosomal sites that are methylated at lysine 4 of histone H3, a feature of regulatory regions. Indeed Top2ß-bound sites are preferentially promoters and become targets during the transition from neuronal progenitors to neurons, at a time when cells exit the cell cycle. Absence of Top2ß protein or its activity leads to changes in transcription and chromatin accessibility at many target genes. Top2ß deficiency does not impair stem cell properties and early steps of neuronal differentiation but causes premature death of postmitotic neurons. This neuronal degeneration is caused by up-regulation of Ngfr p75, a gene bound and repressed by Top2ß. These findings suggest a chromatin-based targeting of Top2ß to regulatory regions in the genome to govern the transcriptional program associated with neuronal differentiation and longevity.

Localization of MLH3 at the centrosomes.

Mutations in human DNA mismatch repair (MMR) genes are commonly associated with hereditary nonpolyposis colorectal cancer (HNPCC). MLH1 protein heterodimerizes with PMS2, PMS1, and MLH3 to form MutLα, MutLß, and MutLγ, respectively. We reported recently stable expression of GFP-linked MLH3 in human cell lines. Monitoring these cell lines during the cell cycle using live cell imaging combined with confocal microscopy, we detected accumulation of MLH3 at the centrosomes. Fluorescence recovery after photobleaching (FRAP) revealed high mobility and fast exchange rates at the centrosomes as it has been reported for other DNA repair proteins. MLH3 may have a role in combination with other repair proteins in the control of centrosome numbers.

Stable expression of MutLγ in human cells reveals no specific response to mismatched DNA, but distinct recruitment to damage sites.

The human DNA mismatch repair (MMR) gene family comprises four MutL paralogues capable of forming heterodimeric MutLα (MLH1-PMS2), MutLß (MLH1-PMS1), and MutLγ (MLH1-MLH3) protein complexes. Human MutL subunits PMS2 and MLH3 contain an evolutionarily conserved amino acid motif DQHA(X)2E(X)4E identified as an endonucleolytic domain capable of incising a defective DNA strand. PMS2 of MutLα is generally accepted to be the sole executor of endonucleolytic activity, but since MLH3 was shown to be able to perform DNA repair at low levels in vitro, our aim was to investigate whether or not MLH3 is activated as a backup under MutLα-deficient conditions. Here, we report stable expression of GFP-tagged MLH3 in the isogenic cell lines 293 and 293T which are functional or defective for MLH1 expression, respectively. As expected, MLH3 formed dimeric complexes with endogenous and recombinant MLH1. MutLγ dimers were recruited to sites of DNA damage induced by UVA micro-irradiation as shown for MutLα. Surprisingly, splicing variant MLH3Δ7 lacking the endonucleolytic motif displayed congruent foci formation, implying that recruitment is not necessarily representing active DNA repair. As an alternative test for repair enzyme activity, we combined alkylation-directed DNA damage with comet formation assays. While recombinant MutLα led to full recovery of DNA damage response in MMR deficient cells, expression of MutLγ or single MLH3 failed to do so. These experiments show recruitment and persistence of MutLγ-heterodimers at UVA-induced DNA lesions. However, we demonstrate that in a MutLα-deficient background no DNA repair-specific function carried out by MutLγ can be detected in living cells.

Adaptation of topoisomerase I paralogs to nuclear and mitochondrial DNA.

Topoisomerase I is essential for DNA metabolism in nuclei and mitochondria. In yeast, a single topoisomerase I gene provides for both organelles. In vertebrates, topoisomerase I is divided into nuclear and mitochondrial paralogs (Top1 and Top1mt). To assess the meaning of this gene duplication, we targeted Top1 to mitochondria or Top1mt to nuclei. Overexpression in the fitting organelle served as control. Targeting of Top1 to mitochondria blocked transcription and depleted mitochondrial DNA. This was also seen with catalytically inactive Top1 mutants, but not with Top1mt overexpressed in mitochondria. Targeting of Top1mt to the nucleus revealed that it was much less able to interact with mitotic chromosomes than Top1 overexpressed in the nucleus. Similar experiments with Top1/Top1mt hybrids assigned these functional differences to structural divergences in the DNA-binding core domains. We propose that adaptation of this domain to different chromatin environments in nuclei and mitochondria has driven evolutional development and conservation of organelle-restricted topoisomerase I paralogs in vertebrates.

53BP1 and MDC1 foci formation in HT-1080 cells for low- and high-LET microbeam irradiations.

An improved assessment of the biological effects and related risks of low doses of ionizing radiation is currently an important issue in radiation biology. Irradiations using microbeams are particularly well suited for precise and localized dose depositions, whereas recombinant cell lines with fluorescent proteins allow the live observation of radiation-induced foci. Living cells of the fibrosarcoma cell line HT-1080 stably expressing 53BP1 or full-length reconstituted MDC1 fused to Green Fluorescent Protein (GFP) were irradiated with protons and α-particles of linear energy transfers (LETs) of 15 and 75 keV/µm, respectively. Using a microbeam, the irradiations were carried out in line patterns, which facilitated the discrimination between undefined background and radiation-induced foci. As expected, foci formation and respective kinetics from α-particle irradiations with a high LET of 75 keV/µm could be detected in a reliable manner by both fusion proteins, as reported previously. Colocalization of γ-H2AX foci confirmed the DSB nature of the detected foci. As a novel result, the application of protons with low LET of 15 keV/µm generated 53BP1- and MDC1-mediated foci of almost equal size and slightly different kinetics. This new data expands the capability of 53BP1 and wild-type MDC1 on visible foci formation in living cells after irradiation with low-LET particles. Furthermore, the kinetics in HT-1080 cells for α-particle irradiation show a delay of about 20 s for 53BP1 foci detection compared to wild-type MDC1, confirming the hierarchical assembly of both proteins. Preliminary data for proton irradiations are shown and also these indicate a delay for 53BP1 versus MDC1.

Subcellular Localization and Mitotic Interactome Analyses Identify SIRT4 as a Centrosomally Localized and Microtubule Associated Protein.

The stress-inducible and senescence-associated tumor suppressor SIRT4, a member of the family of mitochondrial sirtuins (SIRT3, SIRT4, and SIRT5), regulates bioenergetics and metabolism via NAD+-dependent enzymatic activities. Next to the known mitochondrial location, we found that a fraction of endogenous or ectopically expressed SIRT4, but not SIRT3, is present in the cytosol and predominantly localizes to centrosomes. Confocal spinning disk microscopy revealed that SIRT4 is found during the cell cycle dynamically at centrosomes with an intensity peak in G2 and early mitosis. Moreover, SIRT4 precipitates with microtubules and interacts with structural (α,ß-tubulin, γ-tubulin, TUBGCP2, TUBGCP3) and regulatory (HDAC6) microtubule components as detected by co-immunoprecipitation and mass spectrometric analyses of the mitotic SIRT4 interactome. Overexpression of SIRT4 resulted in a pronounced decrease of acetylated α-tubulin (K40) associated with altered microtubule dynamics in mitotic cells. SIRT4 or the N-terminally truncated variant SIRT4(ΔN28), which is unable to translocate into mitochondria, delayed mitotic progression and reduced cell proliferation. This study extends the functional roles of SIRT4 beyond mitochondrial metabolism and provides the first evidence that SIRT4 acts as a novel centrosomal/microtubule-associated protein in the regulation of cell cycle progression. Thus, stress-induced SIRT4 may exert its role as tumor suppressor through mitochondrial as well as extramitochondrial functions, the latter associated with its localization at the mitotic spindle apparatus.

Dynamics of human DNA topoisomerases IIalpha and IIbeta in living cells.

DNA topoisomerase (topo) II catalyses topological genomic changes essential for many DNA metabolic processes. It is also regarded as a structural component of the nuclear matrix in interphase and the mitotic chromosome scaffold. Mammals have two isoforms (alpha and beta) with similar properties in vitro. Here, we investigated their properties in living and proliferating cells, stably expressing biofluorescent chimera of the human isozymes. Topo IIalpha and IIbeta behaved similarly in interphase but differently in mitosis, where only topo IIalpha was chromosome associated to a major part. During interphase, both isozymes joined in nucleolar reassembly and accumulated in nucleoli, which seemed not to involve catalytic DNA turnover because treatment with teniposide (stabilizing covalent catalytic DNA intermediates of topo II) relocated the bulk of the enzymes from the nucleoli to nucleoplasmic granules. Photobleaching revealed that the entire complement of both isozymes was completely mobile and free to exchange between nuclear subcompartments in interphase. In chromosomes, topo IIalpha was also completely mobile and had a uniform distribution. However, hypotonic cell lysis triggered an axial pattern. These observations suggest that topo II is not an immobile, structural component of the chromosomal scaffold or the interphase karyoskeleton, but rather a dynamic interaction partner of such structures.

C-terminal regions of topoisomerase IIalpha and IIbeta determine isoform-specific functioning of the enzymes in vivo.

Topoisomerase II removes supercoils and catenanes generated during DNA metabolic processes such as transcription and replication. Vertebrate cells express two genetically distinct isoforms (alpha and beta) with similar structures and biochemical activities but different biological roles. Topoisomerase IIalpha is essential for cell proliferation, whereas topoisomerase IIbeta is required only for aspects of nerve growth and brain development. To identify the structural features responsible for these differences, we exchanged the divergent C-terminal regions (CTRs) of the two human isoforms (alpha 1173-1531 and beta 1186-1621) and tested the resulting hybrids for complementation of a conditional topoisomerase IIalpha knockout in human cells. Proliferation was fully supported by all enzymes bearing the alpha CTR. The alpha CTR also promoted chromosome binding of both enzyme cores, and was by itself chromosome-bound, suggesting a role in enzyme targeting during mitosis. In contrast, enzymes bearing the beta CTR supported proliferation only rarely and when expressed at unusually high levels. A similar analysis of the divergent N-terminal regions (alpha 1-27 and beta 1-43) revealed no role in isoform-specific functions. Our results show that it is the CTRs of human topoisomerase II that determine their isoform-specific functions in proliferating cells. They also indicate persistence of some functional redundancy between the two isoforms.

Rapid and prolonged stalling of human DNA topoisomerase I in UVA-irradiated genomic areas.

DNA topoisomerase I appears to be involved in DNA damage and repair in a complex manner. The enzyme is required for DNA maintenance and repair, but it may also damage DNA through its covalently DNA-bound, catalytic intermediate. The latter mechanism plays a role in tumor cell killing by camptothecins, but seems also involved in oxidative cell killing and certain stages of apoptosis. Stalling and/or suicidal DNA cleavage of topoisomerase I adjacent to nicks and modified DNA bases has been demonstrated in vitro. Here, we investigate the enzyme's interactions with UVA-induced DNA lesions inside living cells. We irradiated cells expressing GFP-tagged topoisomerase I with an UVA laser focused through a confocal microscope at confined areas of the nuclei. At irradiated sites, topoisomerase I accumulated within seconds, and accumulation lasted for more than 90 min. This effect was apparently due to reduced mobility, although the enzyme was not immobilized at the irradiated nuclear sites. Similar observations were made with mutant versions of topoisomerase I lacking the active site tyrosine or the N-terminal domain, but not with the N-terminal domain alone. Thus, accumulation of topoisomerase I at UVA-modified DNA sites is most likely due to non-covalent binding to damaged DNA, and not suicidal cleavage of such lesions. The rapid onset of accumulation suggests that topoisomerase I functions in this context as a component of DNA damage recognition and/or a cofactor of fast DNA-repair processes. However, the prolonged duration of accumulation suggests that it is also involved in more long-termed processes.

Residues 190-210 of human topoisomerase I are required for enzyme activity in vivo but not in vitro.

DNA-topoisomerase I (topo I) unwinds the DNA- double helix by cutting one strand and allowing rotation of the other. In vitro, this function does not require the N-terminal domain of the enzyme, which is believed to regulate cellular properties. To assess this role, we studied the cellular distribution and mobility of green fluorescent protein-chimera of human topo I lacking either the entire N-terminal domain or a portion of it. We find that topo I truncated up to position 210 is not stabilized by camptothecin in covalent DNA-complexes inside a living cell, whereas in vitro it retains full DNA-relaxation activity, and is targeted by camptothecin in the usual manner. This difference is not shared with a fragment lacking the N-terminal domain up to position 190, indicating that residues 190-210 play a crucial role for the activity of the enzyme in its physiological environment, but not in vitro. Since it is impossible to discriminate in vivo whether this region is required for topo I to form covalent DNA intermediates in the cell, or just for camptothecin to bind and stabilize such complexes, we could not explain precisely these cellular observations. However, inactivity in vivo of the enzyme lacking this region is indicated by a lesser cytotoxicity.

Spatio-temporal regulation of the human licensing factor Cdc6 in replication and mitosis.

To maintain genome stability, the thousands of replication origins of mammalian genomes must only initiate replication once per cell cycle. This is achieved by a strict temporal separation of ongoing replication in S phase, and the formation of pre-replicative complexes in the preceding G1 phase, which "licenses" each origin competent for replication. The contribution of the loading factor Cdc6 to the timing of the licensing process remained however elusive due to seemingly contradictory findings concerning stabilization, degradation and nuclear export of Cdc6. Using fluorescently tagged Cdc6 (Cdc6-YFP) expressed in living cycling cells, we demonstrate here that Cdc6-YFP is stable and chromatin-associated during mitosis and G1 phase. It undergoes rapid proteasomal degradation during S phase initiation followed by active export to the cytosol during S and G2 phases. Biochemical fractionation abolishes this nuclear exclusion, causing aberrant chromatin association of Cdc6-YFP and, likely, endogenous Cdc6, too. In addition, we demonstrate association of Cdc6 with centrosomes in late G2 and during mitosis. These results show that multiple Cdc6-regulatory mechanisms coexist but are tightly controlled in a cell cycle-specific manner.

Inferring a system model of subcellular topoisomerase II beta localization dynamics.

Measuring the mobility of proteins in living cells has become critical to many studies in cell biology and forms the basis for discussion on sub-cellular dynamics. Increasingly localization networks are being put together into compartment models to represent the exchange of molecules, represented mathematically as ordinary differential equations (ODE). The set-up is based on published literature, the "knowledge" of the investigator and 3D visualization of the data. Here we demonstrate this method on the example of a simple distribution model of the molecule Topoisomerase II beta (Topo II beta), nuclear protein that modifies DNA topology. It is found in high concentration in the nucleolus and diffuse in the nucleoplasm, demonstrating a non-membranous inhomogeneity in its distribution. We expand on the simple model by adding additional components to fit fluorescence recovery after photobleaching (FRAP) experiments for protein (GFP) labeled Topo II beta to measure its mobility. This model is then validated by comparing it with alternative scenarios and shown to have predictive power.

Impact of human autoantibodies on ß1-adrenergic receptor conformation, activity, and internalization.

AIMS: Autoantibodies against second extracellular loops of ß(1)-adrenergic receptors frequent in dilated cardiomyopathy confer myocardial dysfunction presumably via cAMP stimulation. Here, we investigate the autoantibody impact on receptor conformation and function. METHODS AND RESULTS: IgG was prepared from patients with dilated cardiomyopathy, matched healthy donors (10 each) or commercial IgG preparations (2). IgG binding to ß(1)-adrenergic receptor peptides was detected in 5 of 10 patients and 2 of 10 controls. IgG colocalization with the native receptor was detected in 8 of 10 patients and 1 of 10 controls (10 of 10 patients and 7 of 10 controls at >30 mg IgG/L). All IgGs exhibiting receptor colocalization triggered changes in receptor conformation (determined with fluorescent sensors) not stringently correlated to cAMP stimulation, suggesting the induction of more or less active receptor conformations. Receptor-activating IgG was detected in 8 of 10 patients but only 1 of 10 controls. In addition, IgG from 8 of 10 patients and 3 of 10 controls attenuated receptor internalization (measured by total internal reflection fluorescence microscopy). IgG-inducing inactive receptor conformations had no effect on subsequent cAMP stimulation by isoproterenol. IgG-inducing active receptor conformations dampened or augmented subsequent cAMP stimulation by isoproterenol, depending on whether receptor internalization was attenuated or not. Corresponding IgG effects on the basal beating rate and chronotropic isoproterenol response of embryonic human cardiomyocytes were observed. CONCLUSIONS: (i) Autoantibodies trigger conformation changes in the ß(1)-adrenergic receptor molecule. (ii) Some also attenuate receptor internalization. (iii) Combinations thereof increase the basal beating rate of cardiomyocytes and optionally entail dampening of their chronotropic catecholamine responses. (iv) The latter effects seem specific for patient autoantibodies, which also have higher levels.

Genotoxicity of dietary, environmental and therapeutic topoisomerase II poisons is uniformly correlated to prolongation of enzyme DNA residence.

SCOPE: DNA damage by genistein and etoposide is determined by the half-life of topoisomerase II-DNA linkage induced [Bandele O. J. and Osheroff N., Biochemistry 2008, 47, 11900]. Here, we test whether this applies generally to dietary flavonoids and therapeutic compounds enhancing topoisomerase II-DNA cleavage (Topo II poisons). METHODS AND RESULTS: We compared the impact of Topo II poisons on DNA residence kinetics of biofluorescent human topoisomerases IIα and IIß (delineating duration of the DNA-linked enzyme state) with histone 2AX phosphorylation (delineating DNA damage response). Prolongation of topoisomerase II-DNA residence was correlated to DNA damage response, whereas topoisomerase II-DNA linkage was not. Catalytic inhibitors stabilizing topoisomerase II on unbroken DNA also exhibited such a correlation, albeit at a lower level of DNA damage response. Therapeutic Topo II poisons had stronger and more durable effects on enzyme II DNA residence and elicited stronger DNA damage responses than natural or dietary ones. CONCLUSIONS: Topoisomerase II-mediated DNA damage appears related to the prolongation of enzyme DNA residence more than to enzyme-DNA cleavage. Due to this reason, genistein and other tested natural and dietary Topo II poisons have a much lower genotoxic potential than therapeutic ones under the conditions of equal topoisomerase II-DNA linkage.

XRCC4 controls nuclear import and distribution of Ligase IV and exchanges faster at damaged DNA in complex with Ligase IV.

Non-homologous end-joining (NHEJ) is one major pathway for the repair of double-stranded DNA breaks in mammals. Following break recognition, alignment and processing, broken DNA ends are finally rejoined by the essential DNA Ligase IV. In the cell, Ligase IV is unable to function without its constitutive interaction partner XRCC4 and becomes unstable when it is missing, and it has been assumed that XRCC4 may also be required for recruitment of Ligase IV to repair sites. To investigate the function of complex formation between both proteins directly in the living cell, we stably expressed them as bio-fluorescent fusion proteins in human HT-1080 cell clones. Ligase IV or XRCC4 were expressed either alone or both were co-expressed at a roughly equimolar ratio. Labelled proteins were overexpressed manifold in comparison to endogenously expressed proteins. We show that over-expressed Ligase IV was only partially imported into the nucleus and showed a diffuse distribution there, whereas XRCC4 expressed alone was entirely nuclear with a distinct exclusion from nucleoli. When Ligase IV was co-expressed with XRCC4, both proteins formed the natural complex, and Ligase IV was not only efficiently imported but also resembled the sub-nuclear distribution of XRCC4. In addition, Ligase IV, when in complex with XRCC4, acquired a delayed nuclear reimport after mitotic cell division of XRCC4. We further determined by photobleaching the kinetics with which the proteins exchange at UVA laser-irradiated nuclear sites between damage-bound and diffusing states. We found that the dynamic exchange rate of the Ligase IV/XRCC4 complex at micro-irradiated sites was faster than that of XRCC4 expressed alone. In summary, our findings demonstrate a novel function of XRCC4 in controlling nuclear import and sub-nuclear distribution of Ligase IV, and they suggest that XRCC4 modulates the dynamic interaction of the Ligase IV/XRCC4 complex with the NHEJ machinery at double-stranded DNA breaks.

Repair of DNA damage induced by the mycotoxin alternariol involves tyrosyl-DNA phosphodiesterase 1.

Alternariol (AOH) was reported recently to act as a topoisomerase poison. To underline the relevance of topoisomerase targeting for the genotoxic properties of AOH, we addressed the question whether human tyrosyl-DNA phosphodiesterase 1 (TDP1), an enzyme vital to the repair of covalent DNA-topoisomerase adducts, affects AOH-mediated genotoxicity. The relevance of TDP1 activity on AOH-induced genotoxicity was investigated by the comet assay in human cells overexpressing GFP chimera of TDP1 or the inactive mutant TDP1(H263A) as well as in cells subjected to siRNA-mediated knock-down of endogenous TDP1. Cells overexpressing TDP1 exhibited significantly less DNA damage after treatment with AOH in comparison to cells expressing the inactive mutant TDP1(H263A). In accordance with these results, levels of AOH inducing DNA strand breaks were increased in TDP1-suppressed cells in comparison to cells transfected with control siRNA. The specific topoisomerase poisons camptothecin and etoposide caused comparable effects, underlining that TDP1 plays an important role in the repair of topoisomerase-mediated DNA damage. In summary, the repair enzyme TDP1 was identified as a factor for the modulation of AOH-mediated DNA damage in human cells.

Mitotic chromosomes are constrained by topoisomerase II-sensitive DNA entanglements.

We have analyzed the topological organization of chromatin inside mitotic chromosomes. We show that mitotic chromatin is heavily self-entangled through experiments in which topoisomerase (topo) II is observed to reduce mitotic chromosome elastic stiffness. Single chromosomes were relaxed by 35% by exogenously added topo II in a manner that depends on hydrolysable adenosine triphosphate (ATP), whereas an inactive topo II cleavage mutant did not change chromosome stiffness. Moreover, experiments using type I topos produced much smaller relaxation effects than topo II, indicating that chromosome relaxation by topo II is caused by decatenation and/or unknotting of double-stranded DNA. In further experiments in which chromosomes are first exposed to protease to partially release protein constraints on chromatin, ATP alone relaxes mitotic chromosomes. The topo II-specific inhibitor ICRF-187 blocks this effect, indicating that it is caused by endogenous topo II bound to the chromosome. Our experiments show that DNA entanglements act in concert with protein-mediated compaction to fold chromatin into mitotic chromosomes.