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Centre for Extremophile Research

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Photorepair of DNA damage in the Archaea

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Staff - Dr Momna Hejmadi, Dr David Hough, Prof. Michael Danson

Among the various DNA repair mechanisms that allow organisms to tolerate high levels of ultraviolet radiation, photoreactivation is the simplest and most efficient. A single monomeric protein, DNA photolyase, directly monomerises uv-induced DNA dimers, using energy from visible light. Interestingly, DNA photolyase activity has been lost in all placental mammals, including humans, suggesting a potential use of this enzyme in prevention of skin cancers.

Halophilic archaea are among the few microorganisms that are naturally exposed to high doses of sunlight in their natural environment and have developed a highly efficient photoreactivation system. Previous studies have demonstrated that photoreactivation is an important mechanism for the repair of UV-induced DNA damage in the extreme halophile Haloferax volcanii. Bioinformatics have revealed two putative photolyase gene (phr) homologues in the Halobacterium NRC-1 genome sequence and recently we have identified phr1 and phr2 gene homologues in Hfx. volcanii. However, the function of these gene products remains to be characterised. Similar phr homologues in the genomes of other halophiles are also thought to be involved in photorepair of UV-induced DNA damage. In Halobacterium NRC-1, a photolyase function has been demonstrated for phr2, although the function of the phr1 gene remains unclear. One possibility is that it encodes a cryptochrome, a protein that is thought to be involved in photoentrainment of circadian rhythms in some bacterial species.

The broad aim of the project is the functional characterisation of the phr1 and phr2 gene products in Hfx. volcanii. This will include:

1) Development of expression systems for production of the Hfx. volcanii phr gene products. These expression systems are currently being used for other Haloferax genes.
2) Purification of these proteins, leading to functional studies using in vitro photolyase activity assays and crystallisation trials aimed at determining the 3-D structure. The stability of these recombinant photolyases will also be assessed
3) Looking for evidence of a circadian function in Haloferax cultures grown under controlled illumination. This could lead to further studies to investigate the role of the phr 1 gene product.


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