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Chloroplasts, Genetics of
Chloroplasts, G enetics of 337 result of plastid ribosome deficiency or of absence of carotenoids, which leads to photobleaching of the plastids in the light. Recent work in Chlamydomonas strongly suggests that some of the plastid-derived factors involved in this chloroplast±nuclear crosstalk are a chlorophyll precursor, Mg-protoporphyrin IX methylester, and its immediate precursor.
Flagellar Assembly and Function The flagellar system of C. reinhardtii has proven to be particularly well suited for the study of microtubule assembly and function, and motility. The reason is that flagellar biosynthesis can be readily synchronized and numerous mutants affected in the function and assembly of the flagellar apparatus have been isolated. Both mutants with abnormal or no motility and those deficient in flagellar assembly have been characterized. Besides the major flagellar a and b tubulins, as many as 250±300 distinct polypeptides can be resolved in the flagellae. Analysis of many paralyzed mutants has revealed deficiencies in sets of polypeptides corresponding to distinct flagellar protein complexes. Because flagellar structure has been conserved throughout evolution, results obtained with Chlamydomonas are relevant for understanding human diseases. These include primary ciliary dyskinesia, which affects cilia motility; polycystic kidney disease, some forms of which involve a defect in assembly of the primary cilia; and retinitis pigmentosa, which causes blindness due to retinal degeneration and involves a defect in transport of proteins through the connecting cilium of the photoreceptor cells. Several of the Chlamydomonas flagellar proteins are remarkably similar to human proteins associated with some of these diseases. It is thus apparent that the use of C. reinhardtii as a model system is not restricted to photosynthesis and chloroplast biogenesis, but can also be extended for the understanding of human diseases associated with flagellar or ciliary dysfunction.
Further Reading
Curry AM and Rosenbaum JL (1991) Flagellar radial spoke: a model molecular genetic system for studying organelle assembly. Cell Motility Cytoskeleton 24: 224±232. Harris EH (1989) The Chlamydomonas Sourcebook. San Diego, CA: Academic Press. Rochaix JD, Goldschmidt-Clermont M and Merchant S (eds) (1998) The molecular biology of chloroplasts and mitochondria in Chlamydomonas. In Govindjee (ed.) Advances in Photosynthesis, vol. 7. Dordrecht/Boston/London: Kluwer Academic.
See also: Chloroplasts, Genetics of; Photosynthesis, Genetics of
Chlamydomonas, Historical Model See: Photosynthesis, Genetics of
Chloroplasts, Genetics of B D Dyer Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.1493
Photosynthesizers convert carbon dioxide into sugar using light energy captured by the green pigment, chlorophyll. They comprise most of the biomass on Earth and are at the base of almost all ecological communities, deep sea vents being a notable exception. Photosynthesizers are found in the two major divisions of organisms, the prokaryotes and the eukaryotes, and are distinguished by a lack of nucleus and other cell compartments in the former and the presence of a nucleus and compartments such as mitochondria and chloroplasts in the latter. Chloroplasts are the site where photosynthesis occurs in eukaryotes, in particular in plants and in algae. An evolutionary link between prokaryotes and eukaryotes is that chloroplasts are former prokaryotic (cyanobacterial) symbionts, acquired about 212 billion years ago by an ancestral eukaryote. Chloroplasts are now well-integrated, permanent residents of their hosts, although they still retain some of their prokaryotic characteristics. This includes a genome with the semiautonomous capabilities of replication, transcription, and translation. In general, photosynthetic symbioses are quite common, perhaps due to the obvious advantages to acquiring a food-generating partner. It appears that chloroplasts (sometimes generically called plastids) evolved several times. Among the earliest extant lineages of photosynthesizers are the euglenoids. Green algae (Chlorophytes, including Chlamydomonas) seem to have acquired their chloroplasts later and subsequently some of this lineage gave rise to plants.
Chloroplast Genomes Compared to their cyanobacterial ancestors, chloroplasts have lost most of their genes. Algae and plant chloroplasts have only a few hundred kilobases of DNA in circular genomes present in multiple copies with about 100 genes. Parasitic plants that have secondarily lost the ability to photosynthesize have even smaller chloroplast genomes as in Epifagus with
338
C h r i s t m a s Di s e a s e
70 kb of DNA and 42 genes. An even greater loss of genes is found unexpectedly in the remnant chloroplasts of all apicomplexans which are obligate parasites. For example Plasmodium which causes malaria has a plastid genome of 35 kb. Sequence analyses suggest that the apicomplexans evolved from photosynthetic dinoflagellates. For additional details on loss of genes see Mitochondria and Symbionts, Genetics of.
Shared Coding Complete loss of redundant and extraneous genes seems to occur easily in close symbioses, perhaps because a streamlining of the genome confers a replicative advantage. In addition intracellular horizontal transfer of genes may occur, facilitated by the proximity of the chloroplast, mitochondrial, and nuclear genes. The direction of transfer is strongly biased toward the nucleus, although chloroplast to mitochondria transfers are also noted. A result of horizontal transfer is a shared coding for some essential chloroplast structures including the ribosomes. This makes the relationships even more obligate among the various genomes of eukaryotic cells. For additional details see Mitochondria and Symbionts, Genetics of.
Variations on Genetic Code and Editing Unlike mitochondria, chloroplasts seem to adhere to the genetic code, at least among those that have been examined so far. However, some chloroplast sequences do undergo some editing of mRNA, in particular, conversions of C to U. The purpose of editing, convoluted as it is, appears to be a means of regulating and modifying transcription. For more details on editing see Mitochondria, RNA Editing in Plants.
Recombination of Chloroplast DNA A wide range of mutant chloroplast genes including antibiotic sensitivities and pigment alterations may be observed to recombine in those algae and plants in which gametes are of similar size. For example, Chlamydomonas has been frequently used to demonstrate recombination.
Maternal Inheritance There is considerable variation in the plants and algae in respect to gamete size. In some cases maternal gametes (ova) are much larger than the paternal ones (pollen) and contribute entirely or almost entirely to the chloroplasts of the zygote. This means that maternal inheritance of chloroplast mutations can occur in some plants. Often such inheritance is manifested
by variegation as in chloroplast mutants that fail to produce chlorophyll, yielding a splotchy phenotype of green and colorless areas on the plant. Completely colorless plants generally fail to reproduce, so a mixed population of chloroplasts is more likely to be inherited.
Further Reading
Dyer B and Obar R (1994) Tracing the History of Eukaryotic Cells. New York: Columbia University Press. Gillham N (1994) Organelle Genes and Genomes. New York: Oxford University Press. Margulis L (1993) Symbiosis in Cell Evolution. New York: WH Freeman.
See also: Mitochondria; RNA Editing in Plants; Symbionts, Genetics of
Christmas Disease F Gianelli Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.0196
Christmas disease is the name given to the type of hemophilia caused by deficiency of coagulation factor IX. The term originates from the surname (Christmas) of the first patient found to suffer from this type of hemophilia. Christmas disease is synonymous with hemophilia B. See also: Hemophilia
Chromatid J Y Lee and T L Orr-Weaver Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.0198
A chromatid is one of the replicated copies of a chromosome. Identical sister chromatids are produced as a result of DNA replication. In contrast, homologous chromosomes derive from either the mother or the father of the organism, and although they contain the same set of genes, they usually have genetic differences. Sister chromatids are physically attached all along their lengths and particularly at the centromeres. This cohesion between the chromatids is established while the DNA is being replicated and is mediated by several proteins, some of which constitute the cohesin complex. Sister chromatid cohesion is essential for the movement of the chromosomes to the metaphase
Flagellar Assembly and Function The flagellar system of C. reinhardtii has proven to be particularly well suited for the study of microtubule assembly and function, and motility. The reason is that flagellar biosynthesis can be readily synchronized and numerous mutants affected in the function and assembly of the flagellar apparatus have been isolated. Both mutants with abnormal or no motility and those deficient in flagellar assembly have been characterized. Besides the major flagellar a and b tubulins, as many as 250±300 distinct polypeptides can be resolved in the flagellae. Analysis of many paralyzed mutants has revealed deficiencies in sets of polypeptides corresponding to distinct flagellar protein complexes. Because flagellar structure has been conserved throughout evolution, results obtained with Chlamydomonas are relevant for understanding human diseases. These include primary ciliary dyskinesia, which affects cilia motility; polycystic kidney disease, some forms of which involve a defect in assembly of the primary cilia; and retinitis pigmentosa, which causes blindness due to retinal degeneration and involves a defect in transport of proteins through the connecting cilium of the photoreceptor cells. Several of the Chlamydomonas flagellar proteins are remarkably similar to human proteins associated with some of these diseases. It is thus apparent that the use of C. reinhardtii as a model system is not restricted to photosynthesis and chloroplast biogenesis, but can also be extended for the understanding of human diseases associated with flagellar or ciliary dysfunction.
Further Reading
Curry AM and Rosenbaum JL (1991) Flagellar radial spoke: a model molecular genetic system for studying organelle assembly. Cell Motility Cytoskeleton 24: 224±232. Harris EH (1989) The Chlamydomonas Sourcebook. San Diego, CA: Academic Press. Rochaix JD, Goldschmidt-Clermont M and Merchant S (eds) (1998) The molecular biology of chloroplasts and mitochondria in Chlamydomonas. In Govindjee (ed.) Advances in Photosynthesis, vol. 7. Dordrecht/Boston/London: Kluwer Academic.
See also: Chloroplasts, Genetics of; Photosynthesis, Genetics of
Chlamydomonas, Historical Model See: Photosynthesis, Genetics of
Chloroplasts, Genetics of B D Dyer Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.1493
Photosynthesizers convert carbon dioxide into sugar using light energy captured by the green pigment, chlorophyll. They comprise most of the biomass on Earth and are at the base of almost all ecological communities, deep sea vents being a notable exception. Photosynthesizers are found in the two major divisions of organisms, the prokaryotes and the eukaryotes, and are distinguished by a lack of nucleus and other cell compartments in the former and the presence of a nucleus and compartments such as mitochondria and chloroplasts in the latter. Chloroplasts are the site where photosynthesis occurs in eukaryotes, in particular in plants and in algae. An evolutionary link between prokaryotes and eukaryotes is that chloroplasts are former prokaryotic (cyanobacterial) symbionts, acquired about 212 billion years ago by an ancestral eukaryote. Chloroplasts are now well-integrated, permanent residents of their hosts, although they still retain some of their prokaryotic characteristics. This includes a genome with the semiautonomous capabilities of replication, transcription, and translation. In general, photosynthetic symbioses are quite common, perhaps due to the obvious advantages to acquiring a food-generating partner. It appears that chloroplasts (sometimes generically called plastids) evolved several times. Among the earliest extant lineages of photosynthesizers are the euglenoids. Green algae (Chlorophytes, including Chlamydomonas) seem to have acquired their chloroplasts later and subsequently some of this lineage gave rise to plants.
Chloroplast Genomes Compared to their cyanobacterial ancestors, chloroplasts have lost most of their genes. Algae and plant chloroplasts have only a few hundred kilobases of DNA in circular genomes present in multiple copies with about 100 genes. Parasitic plants that have secondarily lost the ability to photosynthesize have even smaller chloroplast genomes as in Epifagus with
338
C h r i s t m a s Di s e a s e
70 kb of DNA and 42 genes. An even greater loss of genes is found unexpectedly in the remnant chloroplasts of all apicomplexans which are obligate parasites. For example Plasmodium which causes malaria has a plastid genome of 35 kb. Sequence analyses suggest that the apicomplexans evolved from photosynthetic dinoflagellates. For additional details on loss of genes see Mitochondria and Symbionts, Genetics of.
Shared Coding Complete loss of redundant and extraneous genes seems to occur easily in close symbioses, perhaps because a streamlining of the genome confers a replicative advantage. In addition intracellular horizontal transfer of genes may occur, facilitated by the proximity of the chloroplast, mitochondrial, and nuclear genes. The direction of transfer is strongly biased toward the nucleus, although chloroplast to mitochondria transfers are also noted. A result of horizontal transfer is a shared coding for some essential chloroplast structures including the ribosomes. This makes the relationships even more obligate among the various genomes of eukaryotic cells. For additional details see Mitochondria and Symbionts, Genetics of.
Variations on Genetic Code and Editing Unlike mitochondria, chloroplasts seem to adhere to the genetic code, at least among those that have been examined so far. However, some chloroplast sequences do undergo some editing of mRNA, in particular, conversions of C to U. The purpose of editing, convoluted as it is, appears to be a means of regulating and modifying transcription. For more details on editing see Mitochondria, RNA Editing in Plants.
Recombination of Chloroplast DNA A wide range of mutant chloroplast genes including antibiotic sensitivities and pigment alterations may be observed to recombine in those algae and plants in which gametes are of similar size. For example, Chlamydomonas has been frequently used to demonstrate recombination.
Maternal Inheritance There is considerable variation in the plants and algae in respect to gamete size. In some cases maternal gametes (ova) are much larger than the paternal ones (pollen) and contribute entirely or almost entirely to the chloroplasts of the zygote. This means that maternal inheritance of chloroplast mutations can occur in some plants. Often such inheritance is manifested
by variegation as in chloroplast mutants that fail to produce chlorophyll, yielding a splotchy phenotype of green and colorless areas on the plant. Completely colorless plants generally fail to reproduce, so a mixed population of chloroplasts is more likely to be inherited.
Further Reading
Dyer B and Obar R (1994) Tracing the History of Eukaryotic Cells. New York: Columbia University Press. Gillham N (1994) Organelle Genes and Genomes. New York: Oxford University Press. Margulis L (1993) Symbiosis in Cell Evolution. New York: WH Freeman.
See also: Mitochondria; RNA Editing in Plants; Symbionts, Genetics of
Christmas Disease F Gianelli Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.0196
Christmas disease is the name given to the type of hemophilia caused by deficiency of coagulation factor IX. The term originates from the surname (Christmas) of the first patient found to suffer from this type of hemophilia. Christmas disease is synonymous with hemophilia B. See also: Hemophilia
Chromatid J Y Lee and T L Orr-Weaver Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.0198
A chromatid is one of the replicated copies of a chromosome. Identical sister chromatids are produced as a result of DNA replication. In contrast, homologous chromosomes derive from either the mother or the father of the organism, and although they contain the same set of genes, they usually have genetic differences. Sister chromatids are physically attached all along their lengths and particularly at the centromeres. This cohesion between the chromatids is established while the DNA is being replicated and is mediated by several proteins, some of which constitute the cohesin complex. Sister chromatid cohesion is essential for the movement of the chromosomes to the metaphase