Behavior of mitochondria in synchronized cells of Chlamydomonas reinhardtii (Chlorophyta)

Ultrastructural observations of mitochondrial morphology through the life cycle of the brown alga, Mutiomo cylindricus (Cutleriaceae, Tilopteridales)

Mitochondrial morphology varies according to development and the physiological conditions of the cell. Here, we performed electron tomography using serial sections to analyze the number, individual volume, and morphological complexity of mitochondria in the cells across two generations in the life cycle of the brown alga Mutimo cylindricus. This species shows a heteromorphic alternation of generations between the macroscopic gametophyte and the crustose sporophyte during its life cycle and displays anisogamous sexual reproduction. We observed the mitochondria in the vegetative cells of gametophytes and sporophytes to mainly show tubular or discoidal shapes with high morphological complexity. The morphology of the mitochondria in the male and female gametes changed to a nearly spherical or oval shape from a tubular or discoidal shape before release. In this species, degradation of the paternal mitochondria was observed in the zygote 2 h after fertilization. Morphological changes in the mitochondria were not observed until 6 h after fertilization. Twenty-four-hour-old zygotes before and after cytokinesis showed a similar number of mitochondria as 6-h-old zygotes; however, the volume and morphological complexity increased. The results indicated that the maternal mitochondria did not undergo fission or fusion until this stage. Based on the analysis results of the number and total volume of mitochondria before and after the release of the gametes, it is possible that the mitochondria in the female gametes fuse immediately before release.

Determinants, and implications, of the shape and size of thylakoids and cristae

Thylakoids are flattened sacs isolated from other membranes; cristae are attached to the rest of the inner mitochondrial membrane by the crista junction, but the crista lumen is separated from the intermembrane space. The shape of thylakoids and cristae involves membranes with small (5-30 nm) radii of curvature. While the mechanism of curvature is not entirely clear, it seems to be largely a function of Curt proteins in thylakoids and Mitochondrial Organising Site and Crista Organising Centre proteins and oligomeric F_OF₁ ATP synthase in cristae. A subordinate, or minimal, role is attributable to lipids with areas of their head group area greater (convex leaflet) or smaller (concave leaflet) than the area of the lipid tail; examples of the latter group are monogalactosyldiglyceride in thylakoids and cardiolipin in cristae. The volume per unit area on the lumen side of the membrane is less than that of the chloroplast stroma or cyanobacterial cytosol for thylakoids, and mitochondrial matrix for cristae. A low volume per unit area of thylakoids and cristae means a small lumen width that is the average of wider spaces between lipid parts of the membranes and the narrower gaps dominated by extra-membrane components of transmembrane proteins. These structural constraints have important implications for the movement of the electron carriers plastocyanin and cytochrome c₆ (thylakoids) and cytochrome c (cristae) and hence the separation of the membrane-associated electron donors to, and electron acceptors from, these water-soluble electron carriers. The donor/acceptor pairs, are the cytochrome fb₆Fe_nh complex and P₇₀₀⁺ in thylakoids, and Complex III and Complex IV of cristae. The other energy flux parallel to the membranes is that of the proton motive force generated by redox-powered H⁺ pumps into the lumen to the proton motive force use in ATP synthesis by H⁺ flux from the lumen through the ATP synthase. For both the electron transport and proton motive force movement, concentration differences of reduced and oxidised electron carriers and protonated and deprotonated pH buffers are involved. The need for diffusion along a congested route of these energy transfer agents may limit the separation of sources and sinks parallel to the membranes of thylakoids and cristae.

Improved Algal Toxicity Test System for Robust Omics-Driven Mode-of-Action Discovery in Chlamydomonas reinhardtii

Algae are key components of aquatic food chains. Consequently, they are internationally recognised test species for the environmental safety assessment of chemicals. However, existing algal toxicity test guidelines are not yet optimized to discover molecular modes of action, which require highly-replicated and carefully controlled experiments. Here, we set out to develop a robust, miniaturised and scalable Chlamydomonas reinhardtii toxicity testing approach tailored to meet these demands. We primarily investigated the benefits of synchronised cultures for molecular studies, and of exposure designs that restrict chemical volatilisation yet yield sufficient algal biomass for omics analyses. Flow cytometry and direct-infusion mass spectrometry metabolomics revealed significant and time-resolved changes in sample composition of synchronised cultures. Synchronised cultures in sealed glass vials achieved adequate growth rates at previously unachievably-high inoculation cell densities, with minimal pH drift and negligible chemical loss over 24-h exposures. Algal exposures to a volatile test compound (chlorobenzene) yielded relatively high reproducibility of metabolic phenotypes over experimental repeats. This experimental test system extends existing toxicity testing formats to allow highly-replicated, omics-driven, mode-of-action discovery.

Subdiffraction-resolution live-cell imaging for visualizing thylakoid membranes

The chloroplast is the chlorophyll-containing organelle that produces energy through photosynthesis. Within the chloroplast is an intricate network of thylakoid membranes containing photosynthetic membrane proteins that mediate electron transport and generate chemical energy. Historically, electron microscopy (EM) has been a powerful tool for visualizing the macromolecular structure and organization of thylakoid membranes. However, an understanding of thylakoid membrane dynamics remains elusive because EM requires fixation and sectioning. To improve our knowledge of thylakoid membrane dynamics we need to consider at least two issues: (i) the live-cell imaging conditions needed to visualize active processes in vivo; and (ii) the spatial resolution required to differentiate the characteristics of thylakoid membranes. Here, we utilize three-dimensional structured illumination microscopy (3D-SIM) to explore the optimal imaging conditions for investigating the dynamics of thylakoid membranes in living plant and algal cells. We show that 3D-SIM is capable of examining broad characteristics of thylakoid structures in chloroplasts of the vascular plant Arabidopsis thaliana and distinguishing the structural differences between wild-type and mutant strains. Using 3D-SIM, we also visualize thylakoid organization in whole cells of the green alga Chlamydomonas reinhardtii. These data reveal that high light intensity changes thylakoid membrane structure in C. reinhardtii. Moreover, we observed the green alga Chromochloris zofingiensis and the moss Physcomitrella patens to show the applicability of 3D-SIM. This study demonstrates that 3D-SIM is a promising approach for studying the dynamics of thylakoid membranes in photoautotrophic organisms during photoacclimation processes.

The Chlamydomonas cell cycle

The position of Chlamydomonas within the eukaryotic phylogeny makes it a unique model in at least two important ways: as a representative of the critically important, early-diverging lineage leading to plants; and as a microbe retaining important features of the last eukaryotic common ancestor (LECA) that has been lost in the highly studied yeast lineages. Its cell biology has been studied for many decades and it has well-developed experimental genetic tools, both classical (Mendelian) and molecular. Unlike land plants, it is a haploid with very few gene duplicates, making it ideal for loss-of-function genetic studies. The Chlamydomonas cell cycle has a striking temporal and functional separation between cell growth and rapid cell division, probably connected to the interplay between diurnal cycles that drive photosynthetic cell growth and the cell division cycle; it also exhibits a highly choreographed interaction between the cell cycle and its centriole-basal body-flagellar cycle. Here, we review the current status of studies of the Chlamydomonas cell cycle. We begin with an overview of cell-cycle control in the well-studied yeast and animal systems, which has yielded a canonical, well-supported model. We discuss briefly what is known about similarities and differences in plant cell-cycle control, compared with this model. We next review the cytology and cell biology of the multiple-fission cell cycle of Chlamydomonas. Lastly, we review recent genetic approaches and insights into Chlamydomonas cell-cycle regulation that have been enabled by a new generation of genomics-based tools.

Regulation of CCM genes in Chlamydomonas reinhardtii during conditions of light-dark cycles in synchronous cultures

We have investigated transcript level changes of CO(2)-concentrating mechanism (CCM) genes during light-dark (12 h:12 h) cycles in synchronized Chlamydomonas reinhardtii at air-level CO(2). CCM gene transcript levels vary at various times of light-dark cycles, even at same air-level CO(2). Transcripts of inorganic carbon transporter genes (HLA3, LCI1, CCP1, CCP2 and LCIA) and mitochondrial carbonic anhydrase genes (CAH4 and CAH5) are up regulated in light, following which their levels decline in dark. Contrastingly, transcripts of chloroplast carbonic anhydrases namely CAH6, CAH3 and LCIB are up regulated in dark. CAH3 and LCIB transcript levels reached maximum by the end of dark, followed by high expression into early light period. In contrast, CAH6 transcript level stayed high in dark, followed by high level even in light. Moreover, the up regulation of transcripts in dark was undone by high CO(2), suggesting that the dark induced CCM transcripts were regulated by CO(2) even in dark when CCM is absent. Thus while the CAH3 transcript level modulations appear not to positively correlate with that of CCM, the protein regulation matched with CCM status: in spite of high transcript levels in dark, CAH3 protein reached peak level only in light and localized entirely to pyrenoid, a site functionally relevant for CCM. Moreover, in dark, CAH3 protein level not only reduced but also the protein localized as a diffused pattern in chloroplast. We propose that transcription of most CCM genes, followed by protein level changes including their intracellular localization of a subset is subject to light-dark cycles.

Enhanced genetic tools for engineering multigene traits into green algae

Transgenic microalgae have the potential to impact many diverse biotechnological industries including energy, human and animal nutrition, pharmaceuticals, health and beauty, and specialty chemicals. However, major obstacles to sophisticated genetic and metabolic engineering in algae have been the lack of well-characterized transformation vectors to direct engineered gene products to specific subcellular locations, and the inability to robustly express multiple nuclear-encoded transgenes within a single cell. Here we validate a set of genetic tools that enable protein targeting to distinct subcellular locations, and present two complementary methods for multigene engineering in the eukaryotic green microalga Chlamydomonas reinhardtii. The tools described here will enable advanced metabolic and genetic engineering to promote microalgae biotechnology and product commercialization.

Iron sparing and recycling in a compartmentalized cell

This review focuses on economizing, prioritizing and recycling iron in Chlamydomonas, a reference organism for discovering mechanisms of acclimation to poor iron nutrition in the plant lineage. The metabolic flexibility of Chlamydomonas offers a unique opportunity to distinguish the impact of iron nutrition on photosynthetic versus respiratory metabolism, and the contribution of subcellular compartments to iron storage and mobilization. Mechanisms of iron sparing include down regulation of protein abundance by transcript reduction or protein degradation. Two well-studied examples of hierarchical iron allocation are the maintenance of FeSOD in the plastid and heterotrophic metabolism in acetate-grown cells at the expense of photosynthetic metabolism. The latter implicates the existence of a pathway for inter-compartment iron recycling when access to iron becomes limiting.

Visualization of microbodies in Chlamydomonas reinhardtii

In Chlorophycean algal cells, these organelles are generally called microbodies because they lack the enzymes found in the peroxisomes of higher plants. Microbodies in some algae contain fewer enzymes than the peroxisomes of higher plants, and some unicellular green algae in Chlorophyceae such as Chlamydomonas reinhardtii do not possess catalase, an enzyme commonly found in peroxisomes. Thus, whether microbodies in Chlorophycean algae are similar to the peroxisomes of higher plants, and whether they use a similar transport mechanism for the peroxisomal targeting signal (PTS), remain unclear. To determine whether the PTS is present in the microbodies of Chlorophycean algae, and to visualize the microbodies in Chlamydomonas cells, we examined the sub-cellular localization of green fluorescent proteins (GFP) fused to several PTS-like sequences. We detected GFP compartments that were spherical with a diameter of 0.3-1.0 μm in transgenic Chlamydomonas. Comparative analysis of the character of GFP-compartments observed by fluorescence microscopy and that of microbodies by electron microscopy indicated that the compartments were one and the same. The result also showed that the microbodies in Chlorophycean cells have a similar transport mechanism to that of peroxisomes of higher plants.

Reconstruction of a human mitochondrial complex I mutation in the unicellular green alga Chlamydomonas

Defects in complex I (NADH:ubiquinone oxidoreductase (EC 1.6.5.3)) are the most frequent cause of human respiratory disorders. The pathogenicity of a given human mitochondrial mutation can be difficult to demonstrate because the mitochondrial genome harbors large numbers of polymorphic base changes that have no pathogenic significance. In addition, mitochondrial mutations are usually found in the heteroplasmic state, which may hide the biochemical effect of the mutation. We propose that the unicellular green alga Chlamydomonas could be used to study such mutations because (i) respiratory complex-deficient mutants are viable and mitochondrial mutations are found in the homoplasmic state, (ii) transformation of the mitochondrial genome is feasible, and (iii) Chlamydomonas complex I is similar to that of humans. To illustrate this proposal, we introduced a Leu157Pro substitution into the Chlamydomonas ND4 subunit of complex I in two recipient strains by biolistic transformation, demonstrating that site-directed mutagenesis of the Chlamydomonas mitochondrial genome is possible. This substitution did not lead to any respiratory enzyme defects when present in the heteroplasmic state in a patient with chronic progressive external ophthalmoplegia. When present in the homoplasmic state in the alga, the mutation does not prevent assembly of whole complex I (950 kDa) and the NADH dehydrogenase activity of the peripheral arm of the complex is mildly affected. However, the NADH:duroquinone oxidoreductase activity is strongly reduced, suggesting that the substitution could affect binding of ubiquinone to the membrane domain. The in vitro defects correlate with a decrease in dark respiration and growth rate in vivo.

Rewiring and regulation of cross-compartmentalized metabolism in protists

Plastid acquisition, endosymbiotic associations, lateral gene transfer, organelle degeneracy or even organelle loss influence metabolic capabilities in many different protists. Thus, metabolic diversity is sculpted through the gain of new metabolic functions and moderation or loss of pathways that are often essential in the majority of eukaryotes. What is perhaps less apparent to the casual observer is that the sub-compartmentalization of ubiquitous pathways has been repeatedly remodelled during eukaryotic evolution, and the textbook pictures of intermediary metabolism established for animals, yeast and plants are not conserved in many protists. Moreover, metabolic remodelling can strongly influence the regulatory mechanisms that control carbon flux through the major metabolic pathways. Here, we provide an overview of how core metabolism has been reorganized in various unicellular eukaryotes, focusing in particular on one near universal catabolic pathway (glycolysis) and one ancient anabolic pathway (isoprenoid biosynthesis). For the example of isoprenoid biosynthesis, the compartmentalization of this process in protists often appears to have been influenced by plastid acquisition and loss, whereas for glycolysis several unexpected modes of compartmentalization have emerged. Significantly, the example of trypanosomatid glycolysis illustrates nicely how mathematical modelling and systems biology can be used to uncover or understand novel modes of pathway regulation.

Serial section immunoelectron microscopy of algal cells

Electron microscopy when combined with immunogold labeling provides a 2D image of intracellular protein distribution. Cells are however 3D structures. We describe a method of serial section immunogold electron microscopy that allows a 3D cellular image to be reconstructed from a series of electron micrographs. Cells are fixed to preserve cellular ultrastructure and they are embedded in plastic allowing ultrathin sections to be obtained. The ribbon of ultrathin serial sections produced as the microtome sequentially cuts through the sample is labeled with a monospecific antibody to the protein of interest and then with protein-A gold making the antigen-antibody complex visible in the electron microscope. A common field of view from each serial section is photographed in the electron microscope. Using image analysis software, each digitized micrograph is sequentially aligned; immunolabel and cellular structures of interest are traced onto each micrograph; the micrographs are stacked; and the structures of interest are rendered as solid surfaces producing a 3D image of protein distribution within the cell.

A proteomic survey of Chlamydomonas reinhardtii mitochondria sheds new light on the metabolic plasticity of the organelle and on the nature of the alpha-proteobacterial mitochondrial ancestor

Mitochondria play a key role in the life and death of eukaryotic cells, yet the full spectrum of mitochondrial functions is far from being fully understood, especially in photosynthetic organisms. To advance our understanding of mitochondrial functions in a photosynthetic cell, an extensive proteomic survey of Percoll-purified mitochondria from the metabolically versatile, hydrogen-producing green alga Chlamydomonas reinhardtii was performed. Different fractions of purified mitochondria from Chlamydomonas cells grown under aerobic conditions were analyzed by nano-liquid chromatography-electrospray ionization-mass spectrometry after protein separation on sodium dodecyl sulfate polyacrylamide gel electrophoresis or on blue-native polyacrylamide gel electrophoresis. Of the 496 nonredundant proteins identified, 149 are known or predicted to reside in other cellular compartments and were thus excluded from the molecular and evolutionary analyses of the Chlamydomonas proteome. The mitochondrial proteome of the photosynthetic alga reveals important lineage-specific differences with other mitochondrial proteomes, reflecting the high metabolic diversity of the organelle. Some mitochondrial metabolic pathways in Chlamydomonas appear to combine typical mitochondrial enzymes and bacterial-type ones, whereas others are unknown among mitochondriate eukaryotes. The comparison of the Chlamydomonas proteins to their identifiable homologs predicted from 354 sequenced genomes indicated that Arabidopsis is the most closely related nonalgal eukaryote. Furthermore, this phylogenomic analysis shows that free-living alpha-proteobacteria from the metabolically versatile orders Rhizobiales and Rhodobacterales better reflect the gene content of the ancestor of the chlorophyte mitochondria than parasitic alpha-proteobacteria with reduced and specialized genomes.

LOCALIZATION OF OXYTETRACYCLINE IN CHLAMYDOMONAS REINHARDTII (CHLOROPHYCEAE)(1)

Oxytetracycline (OTC) is an important antimicrobial used in aquaculture. However, residues of OTC have been isolated from nontarget aquatic organisms, sediments, and water located near aquaculture facilities. Identifying OTC in plant material is particularly difficult due to interference from pigments and polyphenol substances but is important especially for algae since they are a primary food source for fish in early life stages. In this study, we describe the effect of OTC (0.1, 1, 10, 25, 50, 100 μg · mL(-1) ) on cell growth, and the localization of OTC (0, 1, 25, 100 μg · mL(-1) ) in vacuoles of Chlamydomonas reinhardtii P. A. Dang. (wildtype, ATCC 18798). We also present a method for semiquantifying OTC in living cells using fluorescent microscopy and Adobe Photoshop. We exposed algal cells to OTC and sampled after 2 or 7 d exposure. On day 7, OTC significantly inhibited algal growth at 1, 10, 25, 50, and 100 μg · mL(-1) . When viewed with fluorescent microscopy, cells exposed to the 25 and 100 μg · mL(-1) contained yellow fluorescent areas, ≤1 μm in diameter that were easily discernable against the red fluorescence of the intracellular chl. The fluorescent areas corresponded to small spherical vacuoles (i.e., polyphosphate bodies that contain calcium and magnesium complexed with polyphosphate) seen in the cells by LM. Since OTC has a high affinity for divalent cations, we suggest that OTC is localized in these vacuoles.

Complete elimination of maternal mitochondrial DNA during meiosis resulting in the paternal inheritance of the mitochondrial genome in Chlamydomonas species

The non-Mendelian inheritance of organellar DNA is common in most plants and animals. In the isogamous green alga Chlamydomonas species, progeny inherit chloroplast genes from the maternal parent, as paternal chloroplast genes are selectively eliminated in young zygotes. Mitochondrial genes are inherited from the paternal parent. Analogically, maternal mitochondrial DNA (mtDNA) is thought to be selectively eliminated. Nevertheless, it is unclear when this selective elimination occurs. Here, we examined the behaviors of maternal and paternal mtDNAs by various methods during the period between the beginning of zygote formation and zoospore formation. First, we observed the behavior of the organelle nucleoids of living cells by specifically staining DNA with the fluorochrome SYBR Green I and staining mitochondria with 3,3'-dihexyloxacarbocyanine iodide. We also examined the fate of mtDNA of male and female parental origin by real-time PCR, nested PCR with single zygotes, and fluorescence in situ hybridization analysis. The mtDNA of maternal origin was completely eliminated before the first cell nuclear division, probably just before mtDNA synthesis, during meiosis. Therefore, the progeny inherit the remaining paternal mtDNA. We suggest that the complete elimination of maternal mtDNA during meiosis is the primary cause of paternal mitochondrial inheritance.

Shared molecular characteristics of successfully transformed mitochondrial genomes in Chlamydomonas reinhardtii

Three types of respiratory deficient mitochondrial strains have been reported in Chlamydomonas reinhardtii: a deficiency due to (i) two base substitutions causing an amino acid change in the apocytochrome b (COB) gene (i.e., strain named dum-15), (ii) one base deletion in the COXI gene (dum-19), or (iii) a large deletion extending from the left terminus of the genome to somewhere in the COB gene (dum-1, -14, and -16). We found that these respiratory deficient strains of C. reinhardtii can be divided into two groups: strains that are constantly transformable and those could not be transformed in our experiments. All transformable mitochondrial strains were limited to the type that has a large deletion in the left arm of the genome. For these mitochondria, transformation was successful not only with purified intact mitochondrial genomes but also with DNA-constructs containing the compensating regions. In comparison, mitochondria of all the non-transformable strains have both of their genome termini intact, leading us to speculate that mitochondria lacking their left genome terminus have unstable genomes and might have a higher potential for recombination. Analysis of mitochondrial gene organization in the resulting respiratory active transformants was performed by DNA sequencing and restriction enzyme digestion. Such analysis showed that homologous recombination occurred at various regions between the mitochondrial genome and the artificial DNA-constructs. Further analysis by Southern hybridization showed that the wild-type genome rapidly replaces the respiratory deficient monomer and dimer mitochondrial genomes, while the E. coli vector region of the artificial DNA-construct likely does not remain in the mitochondria.

Cyanidioschyzon merolae genome. A tool for facilitating comparable studies on organelle biogenesis in photosynthetic eukaryotes

The ultrasmall unicellular red alga Cyanidioschyzon merolae lives in the extreme environment of acidic hot springs and is thought to retain primitive features of cellular and genome organization. We determined the 16.5-Mb nuclear genome sequence of C. merolae 10D as the first complete algal genome. BLASTs and annotation results showed that C. merolae has a mixed gene repertoire of plants and animals, also implying a relationship with prokaryotes, although its photosynthetic components were comparable to other phototrophs. The unicellular green alga Chlamydomonas reinhardtii has been used as a model system for molecular biology research on, for example, photosynthesis, motility, and sexual reproduction. Though both algae are unicellular, the genome size, number of organelles, and surface structures are remarkably different. Here, we report the characteristics of double membrane- and single membrane-bound organelles and their related genes in C. merolae and conduct comparative analyses of predicted protein sequences encoded by the genomes of C. merolae and C. reinhardtii. We examine the predicted proteins of both algae by reciprocal BLASTP analysis, KOG assignment, and gene annotation. The results suggest that most core biological functions are carried out by orthologous proteins that occur in comparable numbers. Although the fundamental gene organizations resembled each other, the genes for organization of chromatin, cytoskeletal components, and flagellar movement remarkably increased in C. reinhardtii. Molecular phylogenetic analyses suggested that the tubulin is close to plant tubulin rather than that of animals and fungi. These results reflect the increase in genome size, the acquisition of complicated cellular structures, and kinematic devices in C. reinhardtii.

An anaplerotic role for mitochondrial carbonic anhydrase in Chlamydomonas reinhardtii

Previous studies of the mitochondrial carbonic anhydrase (mtCA) of Chlamydomonas reinhardtii showed that expression of the two genes encoding this enzyme activity required photosynthetically active radiation and a low CO(2) concentration. These studies suggested that the mtCA was involved in the inorganic carbon-concentrating mechanism. We have now shown that the expression of the mtCA at low CO(2) concentrations decreases when the external NH(4)(+) concentration decreases, to the point of being undetectable when NH(4)(+) supply restricts the rate of photoautotrophic growth. The expression of mtCA can also be induced at supra-atmospheric partial pressure of CO(2) by increasing the NH(4)(+) concentration in the growth medium. Conditions that favor mtCA expression usually also stimulate anaplerosis. We therefore propose that the mtCA is involved in supplying HCO(3)(-) for anaplerotic assimilation catalyzed by phosphoenolpyruvate carboxylase, which provides C skeletons for N assimilation under some circumstances.

Use of Chlamydomonas reinhardtii mutants for anticancer drug screening

We investigated the possibility of utilizing alga cells instead of mammalian cells for the screening of anticancer drugs. The alga cells grow in synthetic media whereas the mammalian cells require complex and more expensive media along with heavy investment and manpower. To assess the validity of this new approach, analysis of growth inhibition by antitumor agents was carried out jointly on a wall-less (cw15) mutant of Chlamydomonas reinhardtii, that obviates the problem of drug uptake, and the murine leukemic cell line L1210, commonly used for anticancer drug screening. The presence of the topoisomerases I and II (approximately 97 and approximately 2 x 170 kDa, respectively) in the nuclear extracts of C. reinhardtii and their possible role as targets of the drugs was also investigated. Concentrated extracts were separated into >100 and <100 kDa fractions and their topoisomerase I and II activities were measured on relaxation of supercoiled plasmid DNA, decatenation of the catenated kinetoplast DNA and cleavage of plasmid DNA. Our results do not show significant difference in growth inhibition by antitumorals between the wall-less mutant of the alga and the murine leukemic cell line L1210. We noted that alga cells were inhibited by antibiotics that target gyrase, a bacterial variant of topoisomerase II which is also found in chloroplasts. At the molecular level, the alga nuclear fractions, >100 and <100 kDa, displayed the same activities as the mammalian enzymes topoisomerases I and II, respectively, and were blocked by the same poisons. We concluded that the wall-less cw15 mutant of C. reinhardtii could advantageously replace mammalian cells in the screening of the anticancer drugs. The alga enzymes could also provide an opportunity to delineate the phylogeny of the topoisomerase superfamily.