| Research Abstract |
Chloroplasts are geneticall controlled not only by their own genome but also by numerous nuclear genes. Nuclear encoded proteins are synthesized in cytoplasm as prcursors and then delivered into the organella. While the chloroplast genes are expressed within the chloroplasts via their own machinery for protein biogenesis, although the machinery itself is also composed partly of the nuclear gene products. As no RNA may be delivered from cytoplasm, all the necessary RNAs ought to be generated within the chloroplasts.
To clarify the genetic share of chloroplast genome in plant cells, we have determined the entire chloroplast DNA sequence (121,024 bp) from a liverwort, Marchantia polymorpha, by using cultured green cells, and deduced the gene organization. We could detect 136 possible genes; i.e. 8 for two sets of rRNAs, 36 for 31 tRNA species, 91 for presumptive proteins or ORFs, and 1 for tRNA pseudogene. The universal codons are used throughout the genome and the 31 species of tRNA must
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translate them properly. Introns were detected in 20 genes, mostly of group <II> except one group <I> . About half of the chloroplast genes are concerned with the basic machinery for gene expression, including 9 rpl, 11 rps, 4 rpo, i infA and 44 RNA genes for rRNA and tRNA. These have been identified by comparing amino acid sequences with homologous genes in E. coli( resulting even in a similar order), showing a prokaryotic feature in the basic machinery. Ob-viously these genes are not enough to complete the transcription-translation machinery, and many remainder proteins must be supplied by the nuclear genes. Similarly, none of the protein complexes of photosynthetic and other apparatus are completed without nuclear encoded proteins. Since the allocation of each component in a complex to either chloroplast or nuclear genomes, which has been elucidated in liverwort, appears to be general among higher plants, the basis of present genetic system of chloroplasts in green plants has to be established before the blanching of Bryophytina and Tracheophytina, and from this unified single origin the present variety of chloroplast genomes in green plants may be evolved. Less
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