The Organization of the Maize Plastid Chromosome：P...
The maize chloroplast chromosome，in common with many other chloroplast DNAs，is a circle comprised of two large inverted repeated segments separated by two stretches of unique sequences.A number of genes have been mapped on the chromosome and have been sequenced to reveal features of the chromosome and of plastid genes.These genes have a mixture of prokaryotic and eukaryotic features.
此文发表于1984 年“ICN-UCLA Symposium”.“Biosynthesis of the Photosynthetic Apparatus：Molecular Biology，Development and Regulation”Alan R.Liss，Ind.，N.Y.，14：257-272
Authors'present addresses：Enno T.Krebbcr's present address is Max-Planck-Institur für Züchtungsforschung.Abtcilung Saedler.D-5000 Koln 30.Federal Republic of Germany；Ulrich Kück's present address is lehrstuhl für Allgemeine Botanik.Rhur-Universirat Bochum.postfach 10 21 48.D-4630 Bochum，Federal Republic of Germany；Ignacio M.Larrinua's present address is Lilly Research Laboratories MC625.307 East McCarty Street，Indianapolis.Indiana 46285；Karen M.T.Muskavitch's present address is Department of Biology.Indiana University.Jordan Hall.Bloomiington，Indiana 47405；Rudi Schantz's present address js Universite Louis Pasteur.Laboratoire Dc Physiologie Vegegale 28.Rue Goethc.67083.Strasbourg CEDER.Francc；Andre A.Stcinmetz's present address is IBMC.15 rue Renc Descartes，F-67084.Strasbourg.CEDEX.France；Yu-Shung.Zhu's present address is Department of Microbiology.University of Illinois at Urbana-Champaign，131 Burrill Hall.407 South Goodwin Avenuc.Urbana.Illinois 61801.
Rcccivcd Scptcmbcr 16.1983.
@1984 Alan R.Liss，Inc.
Key words：maize plastids，transcription，Shine-Dalgarno sequences，promoters
Two maize plastid developmental programs in which the abundance of mRNAs vary are discussed.In differentiation of bundle sheath and mesophyll cells，the gene for the large subunit of ribulose bisphosphate carboxylase is transcribed little if at all in mesophyll cells but abundantly in bundle sheath cells.In the development of chloroplasts from etioplasts in response to illumination，extensive analyses have shown that five regions of the chromosome totaling about 19%of the total unique sequences are represented by increased amounts of RNA starting within 2 h after dark-grown seedlings are illuminated and continuing for 10—20 h.For three of the five photogene-containing regions，very large transcripts are seen together with a number of classes of smaller transcripts.The relationships between the larger and smaller transcripts is not known.In some cases，eg.the gene for large subunit of ribulose bisphosphate carboxylase，the distribution of transcript sizes differs at various stages of greening.(https://www.daowen.com)
One approach to analyzing mechanisms of regulation of transcription has been the study of the action of the maize plastid DNA-dependent RNA polymerase in vitro.This system preferentially transcribes certain chloroplast genes over others from supercoiled chimeric plasmids comprised of maize plastid DNA sequences cloned into bacterial plasmid vehicles，and promises to be useful for studying DNA sequences that may regulate expression as well as polypeptides that may interact with templates or the polymerase to alter the array of transcripts in the plastid during stages in a developmental program.
It has been known since early this century that greening of plants，ie，developments of plastids，is affected by some genes transmitted in a Mendelian manner，but there are in addition uniparentally transmitted genetic factors that regulate greening.The latter genes were said to be in the“plastome”，the former in the genome（ie，the nuclear genome）.The bulk of this report deals with properties of the plastid genetic material，primarily of Zea mays，discovered during the past few years using recombinant DNA technology and the techniques of molecular biology.
In the early 1960s ways that three nuclear genes affect plastid development were determined.A mutation at the yellow stripe I（ysl）locus of maize results in，as the gene designation indicates，yellow-striped plants.The lesion is in the uptake of iron by roots［1］.The phenotype matches symptoms of moderate iron deficiency in maize.The effects of a recessive mutation in barley xantha 23 can be overcome by administration of leucine to the seedling，and barley mutant albina 7 becomes phenotypically normal when aspartic acid is administered［2］.These few examples establish the principle that chloroplast development depends upon the adequacy of the culture medium，ie，the composition of the cytoplasm in which it grows，and that starvation retards plastid development-mutations that reduce the levels of plastid nutrients in the cell below acceptable limits，and block normal plastid development.It is useful to remember that the plastids of barley xantha 23，barley albina 7 and ysl，are different from one another and yet all are capable of developing to maturity when the plant is supplied with the appropriate material.The differences in appearance of the plastids suggests that starvation for one nutrient does not have the same effect as starvation for another and that the production of certain plastid elements may be affected differently from others by the absence of a particular essential nutrient.
Combinations of transmission genetics and analyses of the gene products have been used to locate genes for chloroplast ribosomal proteins in Chlamydomonas and of the genes for the targe and small subunits of RUBISCO（ribulose bisphosphate carboxylase）.The results revealed an unexpected intimacy between nuclear and chloroplast gene products in multimeric components of the plastid.Some mutations to erythromycin-resistance in Chlamydomonas reinhardi were transmitted in a Mendalian manner and judged to be in nuclear genes；one was transmitted uniparentally as expected for an organefle gene［3，4］.The nuclear mutation was shown to occur at the locus cry M-I of nuclear linkage group Ⅺ and this locus was shown to code for protein 6 of the large subunit of the chloroplast ribosome［5，6］.Related data showed that some chloroplast ribosomal proteins of Chlamydomonas were products of the nuclear genome and others of the plastid genome.Wildman and co-workers［7，8］found，as a result of crosses between various species of Nicotiana and analyses of the subunits of RUBISCO，that the gene for the small subunit was transmitted according to the rules of Mendel and thus was judged to be a nuclear gene，while the gene for the large subunit was transmitted uniparentally and thus was judged to be a plastid gene.Realization that genes for polypeptides of multimeric plastid components are dispersed among the separate compartmentalized genomes of the eukaryotic cell called for serious consideration and analyses of possible mechanisms for distribution of genes among multiple genomes during the evolution of eukaryotic cells［9-11］and for the study of mechanisms for integration of the expression of dispersed genes.
One long-term goal of the study of the molecular developmental biology of plastids is to understand mechanisms for the integration of expression of plastid and nuclear genes for subunits of multimeric plastid components.Another goal of this work is to learn details of the structures of proteins of the photosynthetic apparatus and their functional relationships to one another.To proceed toward either of these goals.it is necessary to understand the structures of plastid genes and how their expression is regulated as well as the structures of nuclear genes for plastid components and how their expression is regulated.
The opportunity to study the molecular developmental biology of plastids followed from the firm identification of DNA and ribosomes in plastids in the early 1960s and the emergence of recombinant DNA technology in the mid-1970s.Physical mapping and cloning of chloroplast genes could be carried out with any eukaryotic green plantand it has been done with many by now.The particular case of Zea mays.with which the present report is mainly concerned，serves as an example.As outlined below，genes for stable and abundant RNAs such as rRNAs and tRNAs that can be purified relatively easily have been mapped by various molecular hybridization techniques.On the other hand，a major approach employed to locate genes coding for proteins has been to express a segment of cloned plastid DNA（cloned in E coli）in vitro using a transcription-translation system and identifying the protein immunologically or by other means［12，13］.By now enough plastid genes have been mapped and sequenced to reveal a number of features of plastid chromosome and gene strucrure as well as to provide well-characterized templates for studying the regulation of transcription in vivo and in vitro.
