Mitochondrial DNA replication: a PrimPol perspective

Abstract

PrimPol, (primase-polymerase), the most recently identified eukaryotic polymerase, has roles in both nuclear and mitochondrial DNA maintenance. PrimPol is capable of acting as a DNA polymerase, with the ability to extend primers and also bypass a variety of oxidative and photolesions. In addition, PrimPol also functions as a primase, catalysing the preferential formation of DNA primers in a zinc finger-dependent manner. Although PrimPol's catalytic activities have been uncovered in vitro, we still know little about how and why it is targeted to the mitochondrion and what its key roles are in the maintenance of this multicopy DNA molecule. Unlike nuclear DNA, the mammalian mitochondrial genome is circular and the organelle has many unique proteins essential for its maintenance, presenting a differing environment within which PrimPol must function. Here, we discuss what is currently known about the mechanisms of DNA replication in the mitochondrion, the proteins that carry out these processes and how PrimPol is likely to be involved in assisting this vital cellular process.

Keywords: DNA replication and recombination; PrimPol; mitochondria; polymerase; primase; repriming.

Figure 1.. The diversity of mitochondrial genomes.

This table presents the wide variety of mtDNA sizes found across different kingdoms and how their organisation and replication mechanisms differ. Also highlighted are primases and polymerases shown to be, or speculated to be (?), involved in these processes. Black strands represent parental DNA, with newly synthesised DNA shown in blue and RNA in red.

Figure 2.. Potential functions of PrimPol during mtDNA replication.

Highlighted here are ways in which PrimPol may play key roles in allowing the maintenance of mtDNA replication in many fork-stalling situations. (A) After Pol γ is stalled by a lesion (yellow star), PrimPol is capable of repriming synthesis downstream to allow replication to proceed while the slower process of replication across the lesion is dealt with by Pol γ itself or another specialised TLS polymerase. Alternatively, PrimPol may act as a TLS polymerase that directly bypasses the lesion. (B) PrimPol may play a role in repriming synthesis when the replication fork is stalled by DNA secondary structures, e.g. G4s. By priming after the structure, it allows replication to continue downstream, while specialised helicases are recruited to facilitate synthesis through the structure. (C) Nucleoside analogues (cyan star), incorporated into the newly synthesised strand, prevent further elongation and must be removed by the exonuclease of Pol γ, which is a slow process. PrimPol may reprime downstream from CTNAs to allow replication to continue in a timely fashion, while the process of removing these is completed. In each case, the second DNA strand is shown as dsDNA for simplicity; however, this could be coated with mtSSB or RNA transcripts, depending on the mode of replication.