Checkpoint control on mitochondrial division in Cyanidioschyzon merolae

It is generally accepted that mitochondria proliferate by division. However, since the apparatus for mitochondrial division was discovered only recently, the basic mechanism of mitochondrial division remains poorly understood. The unicellular red alga Cyanidioschyzon merolae is the only organism in which the existence of the apparatus for mitochondrial division (mitochondrion-dividing ring) has been proved by electron microscopy. Since mitochondrial division, mitosis, and cytokinesis regularly occurred in that order, we can assume that tight linkage exists between mitochondrial division and the mitotic cycle. To examine this assumption, we performed experiments with aphidicolin, a specific inhibitor of DNA polymerase a, using cells that had been synchronized by a 12 h light/12 h dark treatment. The effects of aphidicolin on C. merolae cells were examined by both epifluorescence and electron microscopy. When cells synchronized at the S phase were treated with aphidicolin, neither mitosis nor cytokinesis occurred. Epifluorescence microscopy after staining with 3,3'-dihexyloxacarbocyanine iodide (DiOC(6); a mitochondrion-specific fluorochrome) revealed that mitochondrial division was also completely inhibited. Nevertheless, electronmicroscopic examination of the aphidicolin-treated cells clearly revealed the presence of a mitochondrion-dividing ring in mitochondria in all cells examined, in spite of the absence of mitochondrial division. Microbodies, which might be related to mitochondrial division in C. merolae, also failed to divide and became attached to the mitochondrion-dividing rings. These results imply the presence of a checkpoint control mechanism that inhibits division of mitochondria and microbodies in the absence of the synthesis of cell-nuclear DNA.