The accessory subunit of mitochondrial DNA polymerase γ determines the DNA content of mitochondrial nucleoids in human cultured cells

The accessory subunit of mitochondrial DNA polymerase gamma, POLG beta, functions as a processivity factor in vitro. Here we show POLG beta has additional roles in mitochondrial DNA metabolism. Mitochondrial DNA is arranged in nucleoprotein complexes, or nucleoids, which often contain multiple copies of the mitochondrial genome. Gene-silencing of POLG beta increased nucleoid numbers, whereas overexpression of POLG beta reduced the number and increased the size of mitochondrial nucleoids. Both increased and decreased expression of POLG beta altered nucleoid structure and precipitated a marked decrease in 7S DNA molecules, which form short displacement-loops on mitochondrial DNA. Recombinant POLG beta preferentially bound to plasmids with a short displacement-loop, in contrast to POLG alpha. These findings support the view that the mitochondrial D-loop acts as a protein recruitment centre, and suggest POLG beta is a key factor in the organization of mitochondrial DNA in multigenomic nucleoprotein complexes.