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Abstract
Plant mitochondrial genomes are much larger and complex in structure than in other eukaryotes. The high recombinational activity of these genomes produces sub-genomic molecules of different sizes that can be maintained in a variable copy number, a phenomenon that is called substoichiometric shifting and is widespread in plants. The regulation of the copy number has been shown to be under nuclear control, but the mechanism of this regulation is still not well understood. CHM is a nuclear gene that influences the mitochondrial genome by modifying the relative copy number of some sub-genomic molecules in Arabidopsis. Mutations in this gene are characterized by green-white variegation, abnormal growth, distorted leaves and, in some cases, cytoplasmic male sterility. The main objectives of this thesis are to map, clone and characterize the CHM locus in Arabidopsis and its orthologs in other plant species. The CHM mapping was done with F2 populations involving the mutant chm1-1. A candidate gene was identified, designated AtMSH1, which is homologous to the MutS protein of E. coli that is involved in DNA mismatch repair. The identity of this gene was confirmed by T-DNA insertional mutagenesis and by sequencing the mutant alleles for the locus. To better understand the structure and function of MSH1 in plants, the full-length orthologous genes in rice, tomato, maize, soybean and common bean were isolated and sequenced. Multiple alignment and phylogenetic analysis allowed the postulation of the origin of this gene and the identification of specific domains that could be very important for their proper function in plant mitochondria, including an endonuclease domain on the c-terminus of MSH1. This endonuclease is unique to plant MSH1 and may be responsible for some of the exclusive properties of this protein. Also, a model for the function of MSH1 in the regulation of substoichiometric shifting is presented.
