Mitochondria contain their own circular genome which encodes a number of essential components of the electron transport chain complexes and all of the tRNA and rRNA components required to produce them. This genome is replicated by the mitochondrial specific polymerase POL γ, along with the helicase Twinkle, mitochondrial specific SSB (Falkenberg et al., 2007). Several mechanisms of replication have now been proposed including Ribonucleotide incorporation throughout the lagging strand, RITOLS, strand asynchronous and strand coupled replication (Yasukawa et al., 2006, 2005 and Brown et al., 2005). The fine details of these processes and all of the proteins involved are still being uncovered and there is still much to learn about these processes.

It was initially thought that the mitochondrion did not carry out DNA repair after they were found to be unable to eliminate pyrimidine dimers (Clayton et al.,1974). However, it has now emerged that this is far from the truth and in fact this organelle uses numerous methods to maintain the integrity of its circular genome. As with nuclear DNA repair, a number of different pathways have been found within the mitochondrion, including base excision and mismatch repair, however nucleotide excision has not been identified (reviewed in Gredilla et al., 2010). A number of proteins have now been identified that along with their nuclear DNA maintenance roles are also targeted to the mitochondrion, where they have been found to be essential for genome integrity including, ligase III, Ku, Rad51, Fen1 and Dna2. These are likely to work alongside the mitochondrial-specific replication machinery including POLγ, SSBP1 and Twinkle as well as other mitochondrial DNA proteins whose disruption has been shown to effect mtDNA stability , such as Opa1. Although mtDNA encodes < 0.1 % of human genes, it represents almost 1 % of DNA by mass in some tissues, thus the integrity of this sequence is essential, as is demonstrated by the numerous mitochondrial-opathies that have now been identified and the connection between the disruption of this organelle and numerous multifactorial diseases and ageing.

We are interested in several proteins which may play an important role in these processes.  we are employing both in vitro and in vivo techniques to elucidate the roles these proteins may play in the maintenance of the mitochondrial genome and cellular survival. We are studying the affect of over-expression and RNAi of these proteins in human cells, using techniques such as 1D and 2D agarose gel electrophoresis and fluorescent imaging. Alongside this we are also able to purify these proteins to study their biochemical activities with the ultimate goal of understandign their molecular role in cells.

Co-workers
Laura Baliey
Julie Bianchi
Sean Rudd
Violeta Soura

References