CHECKPOINT CONTROL ON MITOCHONDRIAL DIVISION IN CYANIDIOSCHYZON MEROLAE

It is generally accepted that mitochondria proliferate by division. Ho wever, since the apparatus for mitochondrial division was discovered o nly recently, the basic mechanism of mitochondrial division remains po orly understood. The unicellular red alga Cyanidioschyzon merolae is t he only organism in which the existence of the apparatus for mitochond rial division (mitochondrion-dividing ring) has been proved by electro n microscopy. Since mitochondrial division, mitosis, and cytokinesis r egularly occurred in that order, we can assume that tight linkage exis ts between mitochondrial division and the mitotic cycle. To examine th is assumption, we performed experiments with aphidicolin, a specific i nhibitor of DNA polymerase a, using cells that had been synchronized b y a 12 h light/12 h dark treatment. The effects of aphidicolin on C. m erolae cells were examined by both epifluorescence and electron micros copy. When cells synchronized at the S phase were treated with aphidic olin, neither mitosis nor cytokinesis occurred. Epifluorescence micros copy after staining with 3,3'-dihexyloxacarbocyanine iodide (DiOC(6); a mitochondrion-specific fluorochrome) revealed that mitochondrial div ision was also completely inhibited. Nevertheless, electronmicroscopic examination of the aphidicolin-treated cells clearly revealed the pre sence of a mitochondrion-dividing ring in mitochondria in all cells ex amined, 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 ri ngs. These results imply the presence of a checkpoint control mechanis m that inhibits division of mitochondria and microbodies in the absenc e of the synthesis of cell-nuclear DNA.