[Comparative analysis of mitochondrial and nuclear DNA topoisomerase I from maize]

We conducted a comparative study of the properties of topoisomerase I isolated from maize nuclei and mitochondria. We found that nuclear and mitochondrial enzymes possess different ability to bind single stranded DNA. Study of the enzyme activity dependence on Mg2+ demonstrated an absolute dependence of the mitochondrial topoisomerase activity. Contrary, nuclear enzyme activity was not absolutely dependent but stimulated by the magnesium cation. Mitochondrial topoisomerase formed covalent bond with the 5'-end of the cleaved DNA what is unique property of prokaryotic topoisomerase I. Nuclear enzyme bound covalently to the 3'-end like all eukaryotic topoisomerases I. The search through databases revealed genes which could encode mitochondrial topoisomerase I in the genomes of higher plants. Using both cDNA sequencing and in silico methods we demonstrated an existence of the ortholog gene in the maize genome. This gene shares significant homology with prokaryotic topoisomerase I genes that may explain differences in the properties of the mitochondrial and nuclear enzyme. Data obtained is of a significant interest both from the point of view of plant organelle evolution and mitochondrial genome expression mechanisms study.