Christiane M.-R. Fauron

The maize mitochondrial genome: dynamic, yet functional

The organization of the mitochondrial genome of higher plants is complex. It has two striking features: a large size that can vary among plant species; and the ability to undergo homologous recombination that results in variation within species. From cosmid clone mapping studies, the total genetic information of the plant mitochondrial genome can be arranged into a single circular molecule that is referred to as the master chromosome. This circular DNA molecule contains repeated sequences that can generate, via intramolecular recombination, either isomeric forms of the master chromosome or smaller subgenomic circular DNA molecules. The maize mitochondrial genome is the most complex and largest mitochondrial genome for which a physical map is presently available. Its organization varies considerably among the different maize cytotypes. In an attempt to understand the numerous different mitochondrial DNA rearrangements encountered among those cytotypes, we have proposed a general model of genome evolution that can explain a multitude of genomic rearrangements, not only for the maize mitochondrial DNA but also for other higher plant mitochondrial genomes as well.

Maize as a model of higher plant mitochondrial genome plasticity

Higher plant mitochondrial genomes exhibit extraordinary plasticity in their organization where each genome is represented by a set of molecules at a stoichiometric equilibrium. This population of mtDNA molecules is generated by recombination between repeated sequences and those recombinogenic rearrangements seem to be the main force producing variability. This review emphasizes the consequences of such genomic rearrangements on higher plant mitochondrial gene organization and expression, and how the underlying evolutionary process might explain the size variability between plant species.

STRUCTURE AND REPLICATION OF MITOCHONDRIAL DNA FROM THE GENUS DROSOPHILA

ABSTRACT Mitochondrial DNA (mtDNA) molecules from different species of the melanogaster group of the genus Drosophila differ in size from 15.7 to 19.5 kilobase pairs (kb). These differences appear to be accounted for by differences in size (1.0 to 5.4 kb) of a single adenine and thymine (A+T)-rich region in each molecule. The sizes of the mtDNA molecule of other Drosophila species are within the narrow range 15.7 to 16.8 kb, and contain an A+T-rich region of approximately 1.0 kb. Restriction enzyme mapping results indicate the A+T-rich region of D. melanogaster, D. simulans, D. mauritiana, D. takahashi, and D. virilis to be at homologous positions on the mtDNA molecules. We have studied the various structural forms of partially replicated mtDNA molecules from the above mentioned species, and concluded that in each species most molecules are replicated by a highly asymmetrical mode in which synthesis on one strand can be between 60% and 100% complete before synthesis on the other strand is initiated. Using the A+T-rich regions and EcoRI cleavage sites as markers, we have determined that in all species studied replication of mtDNA molecules is initiated in the A+T-rich region and proceeds unidirectionally around the molecule towards the nearest EcoRI site common to the mtDNAs of all six species. We have found that the A+T-rich regions of the different species have little or no sequence homologies.

Nucleotide sequence of Rattus norvegiens mitochondrial DNA that includes the genes for tRNAile, tRNAgln and tRNAf-met

The nucleotide sequence of a segment of mtDNA from Rattus norvegicus (rat) which contains the genes for tRNAile, tRNAgln and tRNAf-met has been determined. A detailed comparison has been made between this sequence and the corresponding sequences of mouse, human and bovine mtDNAs with regard to the primary and secondary structure of the tRNA genes, the regions connecting the tRNA genes, and the regions flanking the tRNA genes which code for the carboxyl terminus of URF-1 and the amino terminus of URF-2. No differences were found in the nucleotide sequences of the genes for tRNAile, tRNAgln and tRNAf-met in mtDNAs from three different female lines of rats (SASCO-1, SASCO-2 and Wild-UT) that differ by substitutions of 0.8% to 1.8% of their total nucleotides.

Replication of Drosophila Mitochondrial DNA

Mitochondrial DNA (mtDNA) molecules of Drosophila include a region of exceptionally high adenine + thymine (A+T) content which, though homologously located, varies in size from 1.0 kb to 5.1 kb in different species. In all species examined, replication begins at a unique site within the A+T-rich region and proceeds in the same direction towards the molecule’s ribosomal RNA genes. Most molecules are replicated by a highly asymmetrical mode in which synthesis of one strand can be up to 100% complete before synthesis of the second strand is initiated. In a minority of molecules, a more symmetrical mode of synthesis is employed. Evidence has been obtained that within A+T-rich regions extensive sequence divergences have taken place both between species and within species. The A+T-rich region shares many features with the replication origin-containing region of vertebrate mtDNA molecules, supporting the view that the two regions are derived from a common ancestral sequence.