Deletion of mt DNA disrupts mitochondrial function and structure, but not biogenesis

The role of nuclear DNA (n DNA)-encoded proteins in the regulation of mitochondrial fission and fusion has been documented, yet the role of mitochondrial DNA ( mt DNA) and encoded proteins in mitochondrial biogenesis remains unknown. Long-term treatment of a lymphoblastoid cell line Molt-4 with ethidium bromide generated mt DNA-deficient rho⁰ mutants. Depletion of mt DNA in rho⁰ cells produced functional and morphological changes in mitochondria without affecting the nuclear genome and encoded proteins. Indeed, the gene encoding subunit II of mitochondrial cytochrome c oxidase (COX II), a prototypical mitochondrial gene, was reduced in rho⁰ mutants blunting the activity of mitochondrial cytochrome c oxidase. Yet, the amount of the nuclear β-actin gene and the activity of citrate synthase, a mitochondrial matrix enzyme encoded by n DNA, remained unaffected in rho⁰ cells. Loss of mt DNA in rho⁰ cells was associated with significant distortion of mitochondrial structure, decreased electron density of the matrix and disorganized inner and outer membranes, resulting in the appearance of 'ghost-like' mitochondria. However, the number of mitochondria-like structures was not significantly different between mt DNA-deficient and parental cells. Thus, we conclude that cells lacking mt DNA still generate mitochondrial scaffolds, albeit with aberrant function.