engleski

Chemical Genetics of UV Protection in Coral - Algal Symbioses

In tropical marine environments where nutrient concentrations are often poor, sessile invertebrates compensate for a lack of nutrients by adopting a symbiotic relationship with microorganisms, especially endosymbiotic dinoflagellates and unicellular algae. In these photoautotrophic symbioses, organic carbon is released to the animal host for nutrition by the microorganism that benefits from recycling of waste nutrients and carbon dioxide from the host animal. The requirements of algal symbiosis necessitate exposure to high levels of solar radiation that can lead to potential photo-oxidative toxicity, exacerbated by the release of photosynthetic oxygen to cause hyperoxic saturation of host tissues. One mechanism symbioses have evolved in defence to chronic UV exposure is the synthesis of protective sunscreen compounds that absorb environmentally relevant wavelengths of UV radiation. These compounds, termed mycosporine-like amino acids (MAAs), are a family of some 20+ related natural products possessing a cyclohexenone or cyclohexenimine chromophore conjugated with the nitrogen constituent of an amino acid having UV– absorption maxima in the range 310-360 nm with high molar absorptivity (e = 20, 000-50, 000). Specific inhibitors of shikimic acid biosynthesis block the accumulation of MAAs in coral tissues suggesting that MAAs biosynthesis is derived from a branch point of the shikimate pathway. In aposymbiotic animals and some symbiotic examples, accumulation of MAAs is via a dietary route since animals cannot synthesise de novo essential products of the shikimate pathway. In all other assemblages the MAAs are, therefore, believed to be derived from the symbiotic phototrophic partner. Enzymes forming the putative branch point in the shikimate pathway leading to the formation of MAAs have never been isolated and the genes encoding these enzymes have not been described. We report on our progress to identify the genes responsible for MAA biosynthesis that may enable us in the future, using recombinant DNA technology, to biosynthesize new classes of sunscreens for the prevention of UV-associated diseases such as skin cancer. We anticipate that genetic modification could also be used to incorporate the MAA shikimate branch-point pathway in higher plants to produce new strains of UV-tolerant plants for enhanced crop production in the third-world tropics.

Tropical marine environment; corals; dinoflagellate symbionts; shikimic acid pathway; mycosporine-like amino acids

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