Abstract
The spatial relationship between the mitochondrial nucleus (mt-nucleus) containing a large amount of DNA (mtDNA) and the mitochondrial limiting membrane in Physarum polycephalum was investigated. After labeling the mtDNA with [3H]-thymidine, significant labels appeared over the space of the mitochondrial matrix as well as the mt-nucleus; this suggests that an appreciable amount of the mtDNA exists in the matrix. When the cristae and inner limiting membrane are removed selectively by Triton X-100 lysis, DNA-like fibers appear to extend from the mt-nucleus to the matrix space. One end of these fibers is attached to a knob-like structure that adheres to the outer limiting membrane of the mitochondrion. A portion of the mtDNA in the isolated mt-nucleus also extends from the mt-nucleus and is associated with mitochondrial membrane-like patches through the knob-like coupling. These results suggest that the mtDNA extends from the mt-nucleus throughout the mitochondrial matrix and that it is associated with the mitochondrial limiting membrane complex in the mitochondrion in situ.
