title: Gene expression in the scleractinian Acropora microphthalma exposed to high solar irradiance reveals elements of photoprotection and coral bleaching.
abstract: BACKGROUND
  The success of tropical reef-building corals depends on the metabolic co-operation between the animal host and the photosynthetic performance of endosymbiotic algae residing within its cells. To examine the molecular response of the coral Acropora microphthalma to high levels of solar irradiance, a cDNA library was constructed by PCR-based suppression subtractive hybridisation (PCR-SSH) from mRNA obtained by transplantation of a colony from a depth of 12.7 m to near-surface solar irradiance, during which the coral became noticeably paler from loss of endosymbionts in sun-exposed tissues.
  
  
  RESULTS
  A novel approach to sequence annotation of the cDNA library gave genetic evidence for a hypothetical biosynthetic pathway branching from the shikimic acid pathway that leads to the formation of 4-deoxygadusol. This metabolite is a potent antioxidant and expected precursor of the UV-protective mycosporine-like amino acids (MAAs), which serve as sunscreens in coral phototrophic symbiosis. Empirical PCR based evidence further upholds the contention that the biosynthesis of these MAA sunscreens is a 'shared metabolic adaptation' between the symbiotic partners. Additionally, gene expression induced by enhanced solar irradiance reveals a cellular mechanism of light-induced coral bleaching that invokes a Ca(2+)-binding synaptotagmin-like regulator of SNARE protein assembly of phagosomal exocytosis, whereby algal partners are lost from the symbiosis.
  
  
  CONCLUSIONS
  Bioinformatics analyses of DNA sequences obtained by differential gene expression of a coral exposed to high solar irradiance has revealed the identification of putative genes encoding key steps of the MAA biosynthetic pathway. Revealed also by this treatment are genes that implicate exocytosis as a cellular process contributing to a breakdown in the metabolically essential partnership between the coral host and endosymbiotic algae, which manifests as coral bleaching.
authors.affiliation: Section for Bioinformatics, Department of Biochemical Engineering, Faculty of Food Technology and Biotechnology, University of Zagreb, Zagreb, Croatia.
authors.forename: Antonio
authors.initials: A
authors.lastname: Starcevic
authors.forename: Walter C
authors.initials: WC
authors.lastname: Dunlap
authors.forename: John
authors.initials: J
authors.lastname: Cullum
authors.forename: J Malcolm
authors.initials: JM
authors.lastname: Shick
authors.forename: Daslav
authors.initials: D
authors.lastname: Hranueli
authors.forename: Paul F
authors.initials: PF
authors.lastname: Long
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D037421:Anthozoa / Q000235:genetics* / Q000378:metabolism / Q000382:microbiology; D001483:Base Sequence; D053898:Biosynthetic Pathways; D000458:Cyanobacteria / Q000235:genetics* / Q000502:physiology; D003511:Cyclohexanols / Q000737:chemistry / Q000378:metabolism; D018076:DNA, Complementary / Q000737:chemistry / Q000235:genetics; D015870:Gene Expression / Q000528:radiation effects; D020869:Gene Expression Profiling; D015723:Gene Library; D008969:Molecular Sequence Data; D015394:Molecular Structure; D009693:Nucleic Acid Hybridization / Q000379:methods; D017422:Sequence Analysis, DNA; D013472:Sunlight; D013559:Symbiosis / Q000235:genetics* / Q000528:radiation effects
chemical_list: D003511:Cyclohexanols; D018076:DNA, Complementary; C034120:gadusol