Genetics and genomics of chloroplast biogenesis [Zea mays L.]
2005
Alsheikh, M. (Iowa State Univ. of Science and Technology, Ames (USA). Dept. of Genetics Development and Cell Biology) | Rodermel, S. (Iowa State Univ. of Science and Technology, Ames (USA). Dept. of Genetics Development and Cell Biology)
Plastids are derived from an endosymbiotic event in which a cyanobacterial-like prokaryote was engulfed by a proto-eukaryotic cell. Subsequently, genes were transferred from the DNA of the symbiont to that of the nucleus, resulting in a reduction in size of the protoorganellar genome. The presence of organelle and nuclear genetic compartments in the same cell fueled the evolution of mechanisms to coordinate gene expression between plastid and nucleus. Maize mutants have long provided an ideal model for addressing these mechanisms, which include anterograde (nucleus-to-plastid) and retrograde (plastid-to-nucleus) controls. Maize has also played a central role in understanding the structure and function of plastid genome. For instance, the maize plastid genome was the first for which a restriction map was derived. It was subsequently found that plastid DNAs are highly conserved in size and sequence content among higher plants. However, the early notion that plastid DNA is a circular molecule might need to be revised in favour of linear forms. Maize plastid genes were among the first to be cloned and sequenced. These include the genes for the large subunit of Rubisco (rbcL) and the DI protein of PSII (psbA). It is now known that plastid genome contains a complete set of ribosomal RNA genes (23S, 16S, 5S and 4.5S rRNAs) and a full complement of 30 tRNA genes. Most of the about 70 protein coding genes in maize plastid genome are devoted to photosynthetic functions, such as genes for subunits of photosynthetic complexes. Yet, a large number also code for proteins necessary for protein synthesis - ribosomal subunits, RNA polymerase subunits - and other plastid functions, such as the ClpP1 subunit of the Clp protease. The prokaryotic nature of the plastid chromosome has allowed the development of efficient chloroplast transformation strategies for several plant and algal species. However, to date, maize has remained refractory to chloroplast transformation
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