Translational Machinery in Plant Organelles
Chloroplasts and mitochondria have their own distinctive translation systems to direct synthesis of proteins encoded by the organellar genomes. The machinery, which includes ribosomal RNAs, ribosomal proteins, transfer RNAs, translation factors and aminoacyl-tRNA synthetases, is partly specified by organellar genes and partly by nuclear genes. The latter give rise to proteins synthesized in the cytoplasm (by the third distinct translation system in plant cells) and then imported into the appropriate organelle. The chloroplast and mitochondrial translation systems in flowering plants each are comprised of more than 100 different components, so that their complexity in fact exceeds the number of different protein-coding genes that they decode. The organellar systems retain traces of their endosymbiotic bacterial ancestry, the chloroplast to a greater extent than the mitochondrion; however, they have acquired a number of novel features and unique complexities. Of particular note is the mosaic composition of the organellar translation apparatus, arising in part from the recent intracellular movement of genetic information, as well as the sharing of certain nuclear gene products. More specifically, ribosomal protein genes have migrated from the organelle to the nucleus, chloroplast transfer RNA gene copies have moved to the mitochondrion, cytosol-type transfer RNAs are imported for use in the mitochondrion, and nuclear-encoded aminoacyl-tRNA synthetases are dual targeted to both the chloroplast and mitochondrion. In addition, there is evidence for intercellular plant-to-plant horizontal movement of mitochondrial ribosomal protein genes during recent evolution.
KeywordsMaize Codon Photosynthesis Cytosol NADH
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