Plant mitochondria possess a gene expression system in which post-transcriptional events, such as transcript end maturation and turnover mechanisms play a key role in regulating the transcriptome. In addition, during early developmental stages of embryo germination, differing transcript profiles have been seen. This research focuses on two loci in wheat mitochondria, ccmFn-rps1 and rpl5-Psirps14-cox3, to elucidate the transcription and post-transcriptional events involved in their expression. Northern analysis of the ccmFN-rps1 genes during early seed-to-seedling development reveals a 3.2 kb primary transcript and a 2.7 kb bicistronic mRNA. A 0.7 kb monocistronic rps1 mRNA is detectable up to 2d but there is no detectable monocistronic ccmFN transcript during the stages examined. Transcript ends were mapped using circular-RT-PCR and phosphatase treatment at three different developmental stages and revealed two processing sites as well as a single 3' end common to all three transcripts. The 5' ends of the processed rps1 transcripts are heterogeneous and do not always include the start codon, questioning the rps1 transcript functionality.
Gene order varies between plant species due to the high recombination rate in mitochondrial genomes, as is seen for rpl5-Psirps14 in wheat and rice. In both plants, the functional rps14 gene is encoded in the nucleus and the mitochondrial rps14 copy is a pseudogene. In wheat, rpl5-Psirps14 are co-transcribed with cox3 as two RNA species of 3.5 kb and 2.7 kb at 24hr post-imbibition and exhibit developmentally-specific differences in abundance in seedlings. Two promoter regions were mapped in wheat upstream of rpl5 and both transcripts have the same 3' end. In rice 24hr and 6d however, rpl5-Psirps14 are co-transcribed as a 1.4 kb bicistronic mRNA. This presumably reflects the different regulatory signals used in different species. In addition, rpl5 has been subject to several independent gene transfers to the nucleus in the cereal lineages. For example, there is a functional copy of rpl5 in the mitochondria and the nucleus in wheat but it is absent from the mitochondria in rye and maize. In oat mitochondria, rpl5 appears to be a pseudogene and in barley, rearrangements at the 3' end and low transcript levels question its functionality.
The characterization of transcription initiation sites, processing sites and 3' ends for these two loci reflect the relaxed nature and flexibility of signals exploited by plant mitochondria. This research supports the significant role of post-transcriptional events in the regulation of gene expression in plant mitochondria.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/27580 |
| Date | January 2008 |
| Creators | Calixte, Sophie |
| Publisher | University of Ottawa (Canada) |
| Source Sets | Université d’Ottawa |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 141 p. |
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