The Replication, Differentiation, and Inheritance of Plastids with Emphasis on the Concept of Organelle Nuclei

A chloroplast protein homologous to the eubacterial topological specificity factor minE plays a role in chloroplast division

We report the identification of a nucleus-encoded minE gene, designated AtMinE1, of Arabidopsis. The encoded AtMinE1 protein possesses both N- and C-terminal extensions, relative to the eubacterial and algal chloroplast-encoded MinE proteins. The N-terminal extension functioned as a chloroplast-targeting transit peptide, as revealed by a transient expression assay using an N terminus:green fluorescent protein fusion. Histochemical beta-glucuronidase staining of transgenic Arabidopsis lines harboring an AtMinE1 promoter::uidA reporter fusion unveiled specific activation of the promoter in green tissues, especially at the shoot apex, which suggests a requirement for cell division-associated AtMinE1 expression for proplastid division in green tissues. In addition, we generated transgenic plants overexpressing a full-length AtMinE1 cDNA and examined the subcellular structures of those plants. Giant heteromorphic chloroplasts were observed in transgenic plants, with a reduced number per cell, whereas mitochondrial morphology remained similar to that of wild-type plants. Taken together, these observations suggest that MinE is the third conserved component involved in chloroplast division.

THE PLASTID DIVISION MACHINE

Plastid division is essential for the maintenance of plastid populations in cells undergoing division and for the accumulation of large chloroplast numbers in photosynthetic tissues. Although the mechanisms mediating plastid division are poorly understood, ultrastructural studies imply this process is accomplished by a dynamic macromolecular machine organized into ring structures at the plastid midpoint. A key component of the engine that powers this machine is the motor-like protein FtsZ, a cytoskeletal GTPase of endosymbiotic origin that forms a ring at the plastid division site, similar to the function of its prokaryotic relatives in bacterial cytokinesis. This review considers the phylogenetic distribution and structural properties of two recently identified plant FtsZ protein families in the context of their distinct roles in plastid division and describes current evidence regarding factors that govern their placement at the division site. Because of their evolutionary and mechanistic relationship, the process of bacterial cell division provides a valuable, though incomplete, paradigm for understanding plastid division in plants.

Visualization of an FtsZ ring in chloroplasts of Lilium longiflorum leaves

FtsZ is a bacterial division protein which forms a ring at the leading edge of the cell division site. To date, a hypothesis that the plant FtsZ forms the same structure in chloroplast division is proposed, but has not been demonstrated yet. In this study, recombinant LlFtsZ (Lilium longiflorum FtsZ) protein was produced from a previously isolated ftsZ cDNA clone [Mori and Tanaka (2000) Protoplasma 214: 57] and used to raise polyclonal anti-LlFtsZ antibodies in rabbits. In immunoblot analysis with the total protein extracted from L. longiflorum leaves, purified antibodies specifically recognized LlFtsZ whose molecular mass was approximately 43 kDa. This size corresponded to that of the recombinant LlFtsZ protein lacking N-terminal sequence, which suggests that the full-length LlFtsZ translation product has a putative N-terminal signal peptide. Moreover, fluorescent and electron microscopy revealed that the anti-LlFtsZ antibodies recognized ring structures at stromal side of the constriction point of dividing chloroplasts. Here, we show direct evidence that FtsZ ring is involved in chloroplast division.

Was the evolution of plastid genetic machinery discontinuous?

The plastid nucleoid consists of plastid DNA and various, mostly uncharacterized, DNA-binding proteins. The plastid DNA undoubtedly originated from an ancestral cyanobacterial genome, but the origin of the nucleoid proteins appears complex. Initial biochemical analysis of these proteins, as well as comparative genome informatics, suggest that proteins of eukaryotic origin replaced most of the original prokaryotic proteins during the evolution of plastids in the lineage of green plants.

The 70-kDa major DNA-compacting protein of the chloroplast nucleoid is sulfite reductase

The chloroplast nucleoid is a complex of chloroplast DNA and various, mostly uncharacterized proteins. An abundant 70-kDa protein of the isolated nucleoids of pea chloroplasts was identified as sulfite reductase by N-terminal sequence analysis as well as immunoblot analysis, spectrophotometry and enzyme activity analysis. Recombinant maize sulfite reductase was indeed able to compact chloroplast DNA and to form nucleoid-like particles in vitro. The role of sulfite reductase in the structural organization of the nucleoid is discussed.

A homologue of the bacterial cell division site-determining factor MinD mediates placement of the chloroplast division apparatus

BACKGROUND

Chloroplast division in plant cells occurs by binary fission, yielding two daughter plastids of equal size. Previously, we reported that two Arabidopsis homologues of FtsZ, a bacterial protein that forms a cytokinetic ring during cell division, are essential for plastid division in plants, and may be involved in the formation of plastid-dividing rings on both the stromal and cytosolic surfaces of the chloroplast envelope membranes. In bacteria, positioning of the FtsZ ring at the center of the cell is mediated in part by the protein MinD. Here, we identified AtMinD1, an Arabidopsis homologue of MinD, and investigated whether positioning of the plastid-division apparatus at the plastid midpoint might involve a mechanism similar to that in bacteria.

RESULTS

Sequence analysis and in vitro chloroplast import experiments indicated that AtMinD1 contains a transit peptide that targets it to the chloroplast. Transgenic Arabidopsis plants with reduced AtMinD1 expression exhibited variability in chloroplast size and number and asymmetrically constricted chloroplasts, strongly suggesting that the plastid-division machinery is misplaced. Overexpression of AtMinD1 inhibited chloroplast division. These phenotypes resemble those of bacterial mutants with altered minD expression.

CONCLUSIONS

Placement of the plastid-division machinery at the organelle midpoint requires a plastid-targeted form of MinD. The results are consistent with a model whereby assembly of the division apparatus is initiated inside the chloroplast by the plastidic form of FtsZ, and suggest that positioning of the cytosolic components of the apparatus is specified by the position of the plastidic components.

The active digestion of uniparental chloroplast DNA in a single zygote of Chlamydomonas reinhardtii is revealed by using the optical tweezer

The non-Mendelian inheritance of organelle genes is a phenomenon common to almost all eukaryotes, and in the isogamous alga Chlamydomonas reinhardtii, chloroplast (cp) genes are transmitted from the mating type positive (mt(+)) parent. In this study, the preferential disappearance of the fluorescent cp nucleoids of the mating type negative (mt(-)) parent was observed in living young zygotes. To study the change in cpDNA molecules during the preferential disappearance, the cpDNA of mt(+) or mt(-) origin was labeled separately with bacterial aadA gene sequences. Then, a single zygote with or without cp nucleoids was isolated under direct observation by using optical tweezers and investigated by nested PCR analysis of the aadA sequences. This demonstrated that cpDNA molecules are digested completely during the preferential disappearance of mt(-) cp nucleoids within 10 min, whereas mt(+) cpDNA and mitochondrial DNA are protected from the digestion. These results indicate that the non-Mendelian transmission pattern of organelle genes is determined immediately after zygote formation.

Elasmobranch color change: A short review and novel data on hormone regulation

Skins of Potamotrygon reticulatus are light in color in vitro, exhibiting punctate melanophores. Alpha-Melanocyte stimulating hormone (EC(50) = 4.58 x 10(-9) M) and prolactin (EC(50) = 1.44 x 10(-9) M) darken the skins in a dose-dependent manner. The endothelins ET-1, ET-2 and ET-3, and the purines, ATP, and uracil triphosphate (UTP) were not able to induce either skin lightening or darkening. Forskolin and the calcium ionophore A23187 promoted a dose-dependent darkening response, whereas N(2), 2'-O-dibutyryl guanosine 3'-5'-cyclic monophosphate (db cyclic GMP), phorbol-12-myristate-13-acetate (TPA), and 1-oleoyl-2-acetyl-sn-glycerol (OAG) were ineffective. The maximal response obtained with the calcium ionophore A23187 was only 76% of maximal darkening. These results indicate that the cyclic adenosine 3'-5'-monophosphate (cAMP) pathway is probably involved in the pigment dispersion of P. reticulatus melanophores. Other experiments should be done to further investigate how cytosolic calcium may be physiologically increased, and the existence of a putative cross-talk between calcium and cAMP signals. In conclusion, the only hormones effective on P. reticulatus melanophores were prolactin and alpha-MSH. No aggregating agent has been shown to antagonize these actions. Prolactin effect on elasmobranch melanophores adds a novel physiological role to this ancient hormone. J. Exp. Zool. 284:485-491, 1999.

Temporal and spatial coordination of cells with their plastid component

Careful coordination of cell multiplication with plastid multiplication and partition at cytokinesis is required to maintain the universal presence of plastids in the major photosynthetic lines of evolution. However, no cell cycle control points are known that might underlie this coordination. We review common properties, and their variants, of plastids and plastid DNA in germline, multiplying, and mature cells of phyla capable of photosynthesis. These suggest a basic level of control dictated perhaps by the same mechanisms that coordinate cell size with the nuclear ploidy level. No protein synthesis within the plastid appears to be necessary for this system to operate successfully at the level that maintains the presence of plastids in cells. A second, and superimposed, level of controls dictates expansion of the plastid in both size and number in response to signals associated with differentiation and with the environment. We also compare the germane properties of plastids with those of mitochondria. With the advent of genomics and new cell and molecular techniques, the players in these control mechanisms should now be identifiable.

Do plastid envelope membranes play a role in the expression of the plastid genome?

A unique biochemical machinery is present within the two envelope membranes surrounding plastids (Joyard et al., Plant Physiol. 118 (1998) 715-723) that reflects the stage of development of the plastid and the specific metabolic requirements of the various tissues. Envelope membranes are the site for the synthesis and metabolism of specific lipids. They are also the site of transport of metabolites, proteins and information between plastids and surrounding cellular compartments. For instance, a complex machinery for the import of nuclear-encoded plastid proteins is rapidly being elucidated. The functional studies of plastid envelope membranes result in the characterization of an increasing number of envelope proteins with unexpected functions. For instance, recent experiments have demonstrated that envelope membranes bind specifically to plastid genetic systems, the nucleoids surrounded by plastid ribosomes. At early stages of plastid differentiation, the inner envelope membrane contains a unique protein (named PEND protein) that binds specifically to plastid DNA. This tight connection suggests that the PEND protein is at least involved in partitioning the plastid DNA to daughter plastids during division. The PEND protein can also provide a physical support for replication and transcription. In addition, factors involved in the control of plastid protein synthesis can become associated to envelope membranes. This was shown for a protein homologous to the E. coli ribosome recycling factor and for the stabilizing factors of some specific chloroplast mRNAs encoding thylakoid membrane proteins. In fact, the envelope membranes together with the plastid DNA are the two essential constituents of plastids that confer identity to plastids and their interactions are becoming uncovered through molecular as well as cytological studies. In this review, we will focus on these recent observations (which are consistent with the endosymbiotic origin of plastids) and we discuss possible roles for the plastid envelope in the expression of plastid genome.

Decrease in mitochondrial DNA and concurrent increase in plastid DNA in generative cells of Pharbitis nil during pollen development

The amount of organellar DNA in a generative cell of Pharbitis nil was observed when squashed pollen grains collected on the day of flowering were stained with the DNA-specific fluorochrome 4',6-diamidino-2-phenylindole (DAPI). Using both DAPI-fluorescence microscopy and electron microscopy, observation of the same thin section of Technovit 7100 resin-embedded material revealed that all of the organellar DNA in mature generative cells is plastid DNA, and there is no mitochondrial DNA. During pollen development, we observed organellar DNA in fluorescence microscopic images using double-staining with DAPI and 3,3'-dihexyloxacarbocyanine iodide (DiOC6) and quantified the DNA using a video-intensified microscope photon counting system (VIMPCS). In the vegetative cells, the amounts of both mitochondrial and plastid DNA progressively decreased and had disappeared by 2 days before flowering. In the generative cells, mitochondrial DNA disappeared sooner than in the vegetative cells, indicating a more active mechanism for the decrease in mitochondrial DNA in the generative cells. In contrast, plastid DNA in the generative cells increased markedly. The DNA content per plastid was at a minimum value (corresponding to one copy of the plastid genome) 7 days before flowering, but it increased to a maximum value (corresponding to over 10 copies of the plastid genome) 2 days before flowering. Similar results were also obtained with immunogold electron microscopy using an anti-DNA antibody. These results suggest that the DNA content of mitochondria and plastids in P. nil is controlled independently during pollen development.

Characterization of Chlamydomonas reinhardtii zygote-specific cDNAs that encode novel proteins containing ankyrin repeats and WW domains

Genes that are expressed only in the young zygote are considered to be of great importance in the development of an isogamous green alga, Chlamydomonas reinhardtii. Clones representing the Zys3 gene were isolated from a cDNA library prepared using zygotes at 10 min after fertilization. Sequencing of Zys3 cDNA clones resulted in the isolation of two related molecular species. One of them encoded a protein that contained two kinds of protein-to-protein interaction motifs known as ankyrin repeats and WW domains. The other clone lacked the ankyrin repeats but was otherwise identical. These mRNA species began to accumulate simultaneously in cells beginning 10 min after fertilization, and reached maximum levels at about 4 h, after which time levels decreased markedly. Genomic DNA gel-blot analysis indicated that Zys3 was a single-copy gene. The Zys3 proteins exhibited parallel expression to the Zys3 mRNAs at first, appearing 2 h after mating, and reached maximum levels at more than 6 h, but persisted to at least 1 d. Immunocytochemical analysis revealed their localization in the endoplasmic reticulum, which suggests a role in the morphological changes of the endoplasmic reticulum or in the synthesis and transport of proteins to the Golgi apparatus or related vesicles.

Improved sensitivity for high resolution in situ hybridization using resin extraction of methyl methacrylate embedded material

An in situ hybridization procedure resulting in both high resolution and sensitivity was established by using the removable methyl methacrylate resin, Technovit 9100. Young bicellular pollen of tobacco (Nicotiana tabacum L. SR-1) was embedded in Technovit 9100 resin and sectioned. The resin was extracted with (2-methoxyethyl)-acetate followed by in situ hybridization with cRNA probes to detect cytoplasmic 18S/25S rRNA. Signal intensity obtained by this procedure was approximately twice as great as that obtained by an earlier procedure using Technovit 7100, a glycol methacrylate resin that cannot be removed from sections. This improvement in sensitivity made it possible to observe subcellular localization of small amounts of RNA as revealed by visualization of plastid 23S rRNA in a generative cell of Plumbago auriculata pollen.

The biparental transmission of the mitochondrial genome in Chlamydomonas reinhardtii visualized in living cells

In the isogamous green alga Chlamydomonas reinhardtii, the chloroplast genome is transmitted from the mt+ parent, while the mitochondrial genes are believed to be inherited from the mt- parent. Chloroplast nucleoids have been visualized by DAPI (4,6-diamidino-2-phenylindole) staining, and the preferential digestion of the mt- chloroplast nucleoids has been observed in young zygotes. However, the mitochondrial nucleoids have never been visualized, and their behavior is only deduced from genetic and biochemical studies. We discovered that the mitochondrial and chloroplast genomes can be visualized simultaneously in living cells, using the fluorescent dye SYBR Green I. The ability to visualize the mitochondrial and chloroplast genome in vivo permits the direct observation of the number, distribution and behavior of the chloroplast and mitochondrial nucleoids in young zygotes. Using this method, the biparental transmission of the mitochondrial genome was revealed.

Zero-length protein-nucleic acid crosslinking by radical-generating coordination complexes as a probe for analysis of protein-DNA interactions in vitro and in vivo

Redox-active coordination complexes such as 1,10-phenanthroline-Cu(II) (OP-Cu) and bleomycin-Fe(III) are commonly used as "chemical nucleases" to introduce single-strand breaks in nucleic acids. Here we report that under certain conditions these complexes may crosslink proteins to nucleic acids. In vitro experiments suggest that proteins are crosslinked to DNA by a mechanism similar to dimethyl sulfate-induced crosslinking. Furthermore, we demonstrate that the OP-Cu complex can generate protein-DNA crosslinks in mammalian cells in vivo. By combining the OP-Cu crosslinking and a "protein shadow" hybridization assay we identify proteins interacting with DNA in isolated pea chloroplasts and show that this methodology can be applied to detect DNA-binding proteins on specific DNA sequences either in vitro or in vivo.

The division apparatus of plastids and mitochondria

Mitochondria and plastids in eukaryotic cells contain distinct genomes and multiply in the cytoplasm by binary division of preexisting organelles. Mitochondrial and plastid nuclei are easily visualized as compartments in the matrix of organelles by high-resolution fluorescence microscopy and by immunoelectron microscopy using anti-DNA antibodies. Plastid and mitochondrial division can be clearly separated into two main events: division of the organelle nuclei, and then division of the rest of the organelles, the process of organellokinesis (mitochondriokinesis and plastidokinesis). The mechanical apparatus that regulates organellokinesis has remained undetermined. In 1986, the plastid-dividing apparatus (PD ring) for plastidokinesis was first identified by us in the primitive red alga Cyanidium caldarium RK-1. The PD ring is located in the cytoplasm outside the organelle envelope at the constricted isthmus of dividing organelles and has subsequently been found in all eukaryotic plants examined. We were also the first to identify the mitochondrion-dividing apparatus (MD ring) for mitochondriokinesis in the unicellular red alga Cyanidioschyzon merolae in 1993. Eukaryotic cell division is therefore controlled by at least three dividing apparata (rings), a contractile ring, an MD ring, and a PD ring, while bacterial division is controlled by a single bacterial contractile FtsZ ring. The aims of this review are to present the fine structure, process of formation, and contraction of the organelle-dividing apparatus, focusing on evolutionary conservation and diversion from the bacterial contractile ring.

The recombinant product of the Chryptomonas phi plastid gene hlpA is an architectural HU-like protein that promotes the assembly of complex nucleoprotein structures

The HlpA protein which is encoded by the hlpA gene in the plastid genome of the cryptomonad alga Chryptomonas phi is structurally related to the non-sequence-specific DNA-binding and DNA-bending HU family of chromatin-associated proteins. The expression of the HlpA protein complements the mutant phenotype of Bacillus subtilis cells impaired in the Hbsu protein (B. subtilis HU), as measured by the resistance of the cells to methylmethane sulphonate. To analyse the interactions of HlpA with DNA, we expressed the protein in Escherichia coli and purified it to homogeneity. HlpA interacts preferentially with four-way junction DNA or DNA minicircles, when compared with linear DNA, recognising DNA structure. HlpA and E. coli HU display comparable affinities for all types of DNA tested; however, HlpA exhibits a stronger tendency to self-associate in the presence of DNA. Accordingly, HlpA oligomerises more readily than HU in protein crosslinking experiments. In the presence of topoisomerase I, HlpA constrains negative superhelical turns in closed circular plasmid DNA. The HlpA protein mediates the joining of distant recombination sites into a complex nucleoprotein structure, as judged by beta-mediated site-specific recombination. The results presented provide evidence that HlpA is a functional plastid equivalent of nuclear and mitochondrial HMG1-like proteins and bacterial HU proteins.

Complete nucleotide sequence of the chloroplast genome from the green alga Chlorella vulgaris: the existence of genes possibly involved in chloroplast division

The complete nucleotide sequence of the chloroplast genome (150,613 bp) from the unicellular green alga Chlorella vulgaris C-27 has been determined. The genome contains no large inverted repeat and has one copy of rRNA gene cluster consisting of 16S, 23S, and 5S rRNA genes. It contains 31 tRNA genes, of which the tRNALeu(GAG) gene has not been found in land plant chloroplast DNAs analyzed so far. Sixty-nine protein genes and eight ORFs conserved with those found in land plant chloroplasts have also been found. The most striking is the existence of two adjacent genes homologous to bacterial genes involved in cell division, minD and minE, which are arranged in the same order in Escherichia coli. This finding suggests that the mechanism of chloroplast division is similar to bacterial division. Other than minD and minE homologues, genes encoding ribosomal proteins L5, L12, L19, and S9 (rpl5, rpl12, rpl19, and rps9); a chlorophyll biosynthesis Mg chelating subunit (chlI); and elongation factor EF-Tu (tufA), which have not been reported from land plant chloroplast DNAs, are present in this genome. However, many of the new chloroplast genes recently found in red and brown algae have not been found in C. vulgaris. Furthermore, this algal species possesses two long ORFs related to ycf1 and ycf2 that are exclusively found in land plants. These observations suggest that C. vulgaris is closer to land plants than to red and brown algae.

Cytokinin affects nuclear- and plastome-encoded energy-converting plastid enzymes

Cytokinins induce two specific morphological alterations in mosses: (i) the differentiation of a tip-growing cell into a three-faced apical cell (the so-called bud), and (ii) the division of chloroplasts. In a developmental mutant of the moss Physcomitrella patens (Hedw.) B.S.G. (mutant PC22) impeded in both cellular differentiation (bud production) and chloroplast division, addition of cytokinin (N6-delta 2-isopentenyladenine) led to bud production after 3 d in the wild type and after 7 d in the mutant. Hormone induced a division of the mutant macrochloroplasts starting within 24 h and ongoing for 72 h. During this period the abundances of several plastid proteins changed in both genotypes as judged by two-dimensional-protein gel electrophoresis, silver staining and subsequent quantification with novel computer software. Eight of these polypeptides were isolated independently, subjected to microsequencing and thus identified, resulting in the first protein sequence data from a moss. Three polypeptides (24 kDa, 22 kDa, 20 kDa) were found to be homologous to enhancer protein OEE2 of the oxygen-evolving complex, four to represent isoforms of phosphoglycerate kinase (EC 2.7.2.3), and one was identified as the beta-chain of chloroplast ATPase (EC 3.6.1.34). Possible involvement of these key enzymes of the chloroplast energy-conversion machinery in organelle division and in cellular differentiation is discussed. Further sequence information was obtained from both subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39). Amounts of these polypeptides were not appreciably affected by cytokinin in moss chloroplasts.

Evidence for UV-B-induced DNA degradation in Euglena gracilis mediated by activation of metal-dependent nucleases

It is demonstrated that in vivo irradiation with artificial UV-B for several hours significantly reduces the amount of large DNA extractable from immobilized Euglena in comparison with non-irradiated controls. This UV-B effect can be eliminated by a drastic reduction of the divalent ion concentration in the extracellular medium, i.e. the substitution of the culture medium by Tris-buffered agarose. Moreover, in vitro degradation of large DNA is demonstrated for crude protein extracts isolated from non-irradiated or UV-B-irradiated Euglena. The nuclease activity is shown for both crude protein extracts and purified nucleases; in both cases, two protein bands possessing nuclease activity are obtained with apparent molecular masses of 26 and 40 kDa and their activity is inhibited by specific nuclease inhibitors, i.e. aurintricarboxylic acid and ATP, applied at a concentration as low as 10(-8) M. Moreover, in vitro, nuclease activity clearly depends on the pH, with an optimum around pH 4.5, and on the ion composition of the extracellular medium. A strong stimulating effect is shown for Ca2+ with an optimum around 10(-4) M; this effect is potentiated by Zn2+ and Mn2+, but strongly counteracted by Mg2+ and the calmodulin inhibitors trifluoperazine and N- (6-aminohexyl)-5-chloro-1-naphthalenesulphonamide (W5). These results favour the concept which explains the lethal UV-B effect on Euglena as arising from a change in the general metabolic state of the cell and an activation of a DNA-degrading system, i.e. activation of metal-dependent nucleases (U.K. Tirlapur, D.-P. Häder and R. Scheuerlein, UV-B mediated damage in the photosynthetic flagellate, Euglena gracilis, studied by image analysis, Beitr. Biol. Pflanzen, 67 (1992) 305-317).

Molecular and cellular mechanisms of mitochondrial nuclear division and mitochondriokinesis

Our present understanding of mitochondrial division can be summarized as follows: Mitochondria contain a specific genome, synthesize their own DNA, and multiply semi-autonomously. Strands of mitochondrial DNA (mt-DNA) in the in vivo organelles of all eukaryotes are organized to form mitochondrial nuclei (nucleoids) (mt-nuclei) with specific proteins including a histone-like protein and transcription factors at the central region of the mitochondrion. We can easily observe the mt-nucleus in vivo mitochondria in various organisms such as fungi, algae, plants, and animals by using high-resolution epifluorescence microscopy. Therefore, the process of mitochondrial division can be clearly separated into two main events: division of the mt-nuclei and mitochondriokinesis analogous to cytokinesis. Mitochondria undergo binary division which is accompanied by the division of the mt-nucleus. A remarkable characteristic of mitochondrial multiplication during the mitochondrial life cycle is that mitochondria can multiply the mt-chromosome by endoduplication until 50-100 copies are present. Mitochondria can then divide without mitochondrial DNA synthesis to eventually contain 1-5 copies of the mt-chromosome. This characteristic phenomenon can be observed during cell differentiation, such as during the formation of plasmodia and sclerotia of Physarum polycephalum and during embryogenesis and the formation of meristematic tissues in plants. The mitochondrial chromosome has a mitochondrial "kinetochore (centromere)" which is A-T rich and contains specific sequences such as topoisomerase binding sites, tandem repeats, and inverted repeats. A bridge of proteins may exist between the kinetochore DNA and membrane systems. Mitochondrial chromosomes can divide according to the growth of a membrane system between the kinetochores. Mitochondriokinesis progresses steadily along with mitochondrial nuclear division. As the membrane at the equatorial region of a mitochondrion contracts, the neck of the cleavage furrow narrows, and eventually the daughter mitochondria are separated. An actin-like protein may power mitochondriokinesis by separating the daughter mitochondria. In general, mitochondriokinesis occurs by contraction rather than by partition of the inner membrane.

A mating type-linked gene cluster expressed in Chlamydomonas zygotes participates in the uniparental inheritance of the chloroplast genome

A characteristic feature of early zygote development in Chlamydomonas is the selective degradation of chloroplast DNA from the mating type minus parent. The zygote-specific gene cluster ezy-1 is linked to the mating type locus and is transcribed almost immediately upon zygote formation. We show here that the acidic Ezy-1 polypeptide is rapidly transported to both the plus and minus chloroplasts, where it interacts with each chloroplast nucleoid. Expression of ezy-1 is selectively inhibited when plus, but not minus, gametes are briefly ultraviolet irradiated just prior to mating, a treatment known to disrupt the uniparental inheritance of chloroplast traits. We propose that the Ezy-1 polypeptide participates in the destruction of the minus chloroplast DNA in zygotes and thus the uniparental inheritance of chloroplast traits. The ezy-1 gene represents a valuable molecular probe for dissecting mechanisms underlying organelle inheritance.

Four steps to two sexes

Four steps through which parasitic intracellular symbionts could bring about the evolution of two sexes are considered. In the first step, a primitive host population has biparental cytoplasmic inheritance and lacks gametic differentiation: parasitic cytoplasmic elements readily invade and spread by vertical transmission through such host populations, even if they have major deleterious effects on their hosts. The second step leads to the establishment of a nuclear mutant in the host (locus A) that prevents inheritance of the cytoplasm in gametes in which it occurs. This mutant comes to equilibrium at an intermediate frequency, because a double dose of symbionts is more deleterious than a single dose, and zygotes lacking cytoplasm from both gametes are inviable. The third step involves the spread of a mutant at another nuclear locus (B), causing self-incompatibility of gametes in which it occurs. If this is closely linked to locus A, the mutant may become established by preventing the deleterious gamete unions. The mutant at locus B must, however, start both with an appreciable frequency and be in gametic disequilibrium with locus A. In the fourth step a second mutation causing self-incompatibility occurs at locus B. This allele spreads by becoming associated with the other allele at locus A, eventually leaving the population with two gamete types, or sexes, one predominantly transmitting the cytoplasm, and the other eliminating it. It is argued that this is a feasible mechanism for the origin of two sexes.