In addition to its central role in cellular stress signaling, the tumor suppressor p53 modulates mitochondrial
respiration through its nuclear transcription factor activity and localizes to mitochondria, where it enhances
apoptosis and suppresses mitochondrial DNA (mtDNA) mutagenesis. Here we demonstrate a new conserved
role for p53 in mtDNA copy number maintenance and mitochondrial reactive oxygen species (ROS)
homeostasis. In mammals, mtDNA is present at thousands of copies per cell and is essential for normal
development and cell function. We show that p53 null mouse and p53 knockdown human primary
fibroblasts exhibit mtDNA depletion and decreased mitochondrial mass under normal culture growth
conditions. This is accompanied by a reduction of the p53R2 subunit of ribonucleotide reductase mRNA and
protein and of mitochondrial transcription factor A (mtTFA) at the protein level only. Finally, p53-depleted
cells exhibit significant disruption of cellular ROS homeostasis, characterized by reduced mitochondrial and
cellular superoxide levels and increased cellular hydrogen peroxide. Altogether, these results elucidate
additional mitochondria-related functions for p53 and implicate mtDNA depletion and ROS alterations as
potentially relevant to cellular transformation, cancer cell phenotypes, and the Warburg Effect.