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1.
We have determined the nucleotide sequence of a 7 kb (1 kb = 10(3) base-pairs) region that includes the entire small single-copy region (SSC) of the plastid genome of Epifagus virginiana, a non-photosynthetic, parasitic flowering plant. The SSC (4.8 kb) is considerably smaller than those of photosynthetic plants due to the complete deletion of all photosynthetic, chlororespiratory and ribosomal protein genes. This leaves only two genes: a protein gene of 1738 codons whose product is unlikely to be involved in bioenergetic processes and a leucine tRNA gene (trn(LUAG)). Both genes span junctions between the inverted repeat and the SSC, with the consequence that the terminal 20 base-pairs of the repeat is transcribed in both directions and functions both as the 3' end of the tRNA gene and as an internal segment of orf1738. We find that the region of tobacco plastid DNA homologous to Epifagus orf1738 contains a single open reading frame (ORF) of 1901 codons rather than the three ORFs of 1244, 273 and 228 codons originally reported. However, we confirm that the equivalent region of the bryophyte Marchantia contains two genes (1068 and 464 codons) corresponding to the N and C-terminal portions of the dicot protein. In contrast, rice plastid DNA contains a severely truncated pseudogene at this locus.  相似文献   

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Plastids of non-photosynthetic cells or tissues, such as chromoplasts or leukoplasts, which develop during the course of ontogenetic differentiation contain DNA which is identical to chloroplast DNA with respect to size, organization and gene content. Also in ribosome-deficient bleached plastids, produced in leaves by experimental treatments or mutation, chloroplast DNA remains unaltered. The chloroplast DNA of various bleached mutant strains of Euglena has suffered major deletions or rearrangements, but is, however, never totally lost. Also leukoplasts of parasitic higher plants contain DNA. In the organellar DNA of several parasitic plants photosynthetic genes are conserved. In the heterotrophic flagellate Astasia and in the holoparasite Epifagus virginiana (Orobanchaceae) the size of the plastid DNA is greatly reduced by major deletions and most or all photosynthetic genes or genes related to the chloroplastic respiratory chain are lost. The residual plastid genomes have, however, retained genes for RNAs, tRNAs and ribosomal polypeptides and these are transcribed, although plastidic RNA-polymerase genes are lost in Epifagus. These findings demand the existence of a nuclear-encoded RNA-polymerase. The relevance of the conservation of plastid DNA and of plastidic gene expression in non-photosynthetic cells is discussed, remains, however, at present elusive. Open reading frames of unknown function might be of particular significance for non-photosynthetic plastids.  相似文献   

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A J Lohan  K H Wolfe 《Genetics》1998,150(1):425-433
The plastid genome of the nonphotosynthetic parasitic plant Epifagus virginiana contains only 17 of the 30 tRNA genes normally found in angiosperm plastid DNA. Although this is insufficient for translation, the genome is functional, so import of cytosolic tRNAs into plastids has been suggested. This raises the question of whether the tRNA genes that remain in E. virginiana plastid DNA are active or have just fortuitously escaped deletion. We report the sequences of 20 plastid tRNA loci from Orobanche minor, which shares a nonphotosynthetic ancestor with E. virginiana. The two species have 9 intact tRNA genes in common, the others being defunct in one or both species. The intron-containing trnLUAA gene is absent from E. virginiana, but it is intact, transcribed, and spliced in O. minor. The shared intact genes are better conserved than intergenic sequences, which indicates that these genes are being maintained by natural selection and, therefore, must be functional. For the most part, the tRNA species conserved in nonphotosynthetic plastids are also those that have never been found to be imported in plant mitochondria, which suggests that the same rules may govern tRNA import in the two organelles. A small photosynthesis gene, psbI, is still intact in O. minor, and computer simulations show that some small nonessential genes have an appreciable chance of escaping deletion.  相似文献   

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The nucleotide sequence of a 7.4 kb region containing the entire plastid ribosomal RNA operon of the nongreen parasitic plant Epifagus virginiana has been determined. Analysis of the sequence indicates that all four rRNA genes are intact and almost certainly functional. In contrast, the split genes for tRNAIle and tRNAAla present in the 16S-23S rRNA spacer region have become pseudogenes, and deletion upstream of the 16S rRNA gene has removed a tRNAVal gene and most of the promoter region for the rRNA operon. The rate of nucleotide substitution in 16S and 23S rRNAs is several times higher in Epifagus than in tobacco, a related photosynthetic plant. Possible reasons for this, including relaxed translational constraints, are discussed.  相似文献   

7.
Summary Albino rice plants derived from pollen contain plastid genomes that have suffered large-scale deletions. From the roots of albino plants, we obtained several calli containing homogeneous plastid DNA differing in the size and position of the deletion. Southern blotting and pulsed field gel electrophoresis experiments revealed that the DNAs were linear molecules having a hairpin structure at both termini, existing as monomers (19 kb) or dimers, trimers and tetramers linked to form head-to-head and tail-to-tail multimers. This characteristic form is similar to that of the vaccinia virus, in which the replication origin is thought to lie at or near the hairpin termini. Furthermore, polymerase chain reaction experiments revealed complete loss of the ribosomal RNA genes of the plastid DNA. The results suggest that plant cells can grow without translation occurring in plastids. All of the deleted plastid DNAs commonly retained the region containing the tRNAGlu gene (trnE), which is essential for biosynthesis of porphyrin. As porphyrin is the precursor of heme for mitochondria and other organelles, it is considered thattrnE on the remnant plastid genome may be transcribed by an RNA polymerase encoded on nuclear DNA.  相似文献   

8.
We have used heterologous probes to investigate the degree of sequence conservation in the plastid genome ofConopholis americana, a totally achlorophyllous angiosperm which exists as a root parasite on red oaks. AlthoughConopholis is completely nonphotosynthetic, it retains a plastid genome in which certain regions, including that which contains the ribosomal RNA genes, are highly conserved. Other regions, including those containing the genes for numerous photosynthesis proteins, are either absent or highly divergent. We also find that the 16S and 23S ribosomal genes of theConopholis plastid are transcribed and processed, implying a potentially functional genetic apparatus. These results are in agreement with findings reported recently for a related root parasite,Epifagus virginiana (de Pamphilis and Palmer, 1990). Furthermore, the plastid genome is maintained in high copy number in fruit tissue, whereas mature seeds have an approximately 10-fold lower copy number.  相似文献   

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Chlamydomonas reinhardtii is a unicellular eukaryotic alga possessing a single chloroplast that is widely used as a model system for the study of photosynthetic processes. This report analyzes the surprising structural and evolutionary features of the completely sequenced 203,395-bp plastid chromosome. The genome is divided by 21.2-kb inverted repeats into two single-copy regions of approximately 80 kb and contains only 99 genes, including a full complement of tRNAs and atypical genes encoding the RNA polymerase. A remarkable feature is that >20% of the genome is repetitive DNA: the majority of intergenic regions consist of numerous classes of short dispersed repeats (SDRs), which may have structural or evolutionary significance. Among other sequenced chlorophyte plastid genomes, only that of the green alga Chlorella vulgaris appears to share this feature. The program MultiPipMaker was used to compare the genic complement of Chlamydomonas with those of other chloroplast genomes and to scan the genomes for sequence similarities and repetitive DNAs. Among the results was evidence that the SDRs were not derived from extant coding sequences, although some SDRs may have arisen from other genomic fragments. Phylogenetic reconstruction of changes in plastid genome content revealed that an accelerated rate of gene loss also characterized the Chlamydomonas/Chlorella lineage, a phenomenon that might be independent of the proliferation of SDRs. Together, our results reveal a dynamic and unusual plastid genome whose existence in a model organism will allow its features to be tested functionally.  相似文献   

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