Plant mitochondrial L2 ribosomal protein genes and their expression.
|Title:||Plant mitochondrial L2 ribosomal protein genes and their expression.|
|Abstract:||The endosymbiotic hypothesis for the origin of mitochondria invokes the transfer of many genes from the respiring bacterium to the nucleus of the host cell early in the symbiotic association. Recent evidence suggests that such transfer is an ongoing evolutionary process in the plant lineage. This project focuses on the location and expression of genes coding for the mitochondrial (mt) large ribosomal subunit protein L2 in different plants to gain knowledge about their function and evolution. L2 is known to be one of the most conservative ribosomal proteins present in prokaryotes and eukaryotes. The rice mt genome was found to have a single copy of a functional rpl2 gene with a group II intron whereas wheat has only a truncated rpl2 pseudogene corresponding to the extreme carboxyl terminus and no rpl2 homologous sequences were detected in the soybean mt genome. Nuclear-located transferred mt rpl2 copies have been identified in wheat and they are found to be transcribed. An RNA-mediated gene transfer is predicted since it lacks the mt intron at the expected splice site. In rice, the mt rpl2 gene is transcribed, spliced and edited. The mRNA levels of the rice mt rpl2 are very low compared to typical respiratory chain genes and have higher transcript accumulation in the germinating embryo stage than seedlings. The transcript profile is complex and different between stages indicating different processing pathways in germinating embryos and seedlings. Low steady state rpl2 mRNA levels compared to excised introns indicate a rapid turnover of mRNA and a stable excised form of intron. However the excised introns appear to be present as non-lariat forms by RT-PCR experiments. This observation suggests an initial hydrolysis step in the splicing reaction. Phylogenetic analyses on derived L2 and S12 protein sequences of mitochondria and other compartments indicate a rapid evolution of mt L2 proteins in comparison to mt S12. The transfer of mt genes to the nucleus points to an accelerated rate of evolution of L2 proteins. The codon usage pattern of the transferred wheat nuclear-located mt rpl2 sequences is in agreement with an adaptation to the pattern of other wheat nuclear genes. This study illustrates the recent evolutionary transfer of functional mt genes to the nucleus. It also indicates that the transferred genes may be modified to a certain extent during or after transfer to the new environment. The presence of a mitochondrial as well as a transferred nuclear copy may reflect the transition stage in a gene transfer event or the possibility of maintenance of the mt copy for specialized functional regulation during different developmental stages. (Abstract shortened by UMI.)|
|Collection||Thèses, 1910 - 2010 // Theses, 1910 - 2010|