Abstract
Genome sequences of the reef-building coral, Acropora digitifera, have been decoded. Acropora inhabits an environment with intense ultraviolet exposure and hosts the photosynthetic endosymbiont, Symbiodinium. Acropora homologs of all four genes necessary for biosynthesis of the photoprotective cyanobacterial compound, shinorine, are present. Among metazoans, these genes are found only in anthozoans. To gain further evolutionary insights into biosynthesis of photoprotective compounds and associated coral proteins, we surveyed the Acropora genome for 18 clustered genes involved in cyanobacterial synthesis of the anti-UV compound, scytonemin, even though it had not previously been detected in corals. We identified candidates for only 6 of the 18 genes, including tyrP, scyA, and scyB. Therefore, it does not appear that Acropora digitifera can synthesize scytonemin independently. On the other hand, molecular phylogenetic analysis showed that one tyrosinase gene is an ortholog of vertebrate tyrosinase genes and that the coral homologs, scyA and scyB, are similar to bacterial metabolic genes, phosphonopyruvate (ppyr) decarboxylase and glutamate dehydrogenase (GDH), respectively. Further genomic searches for ppyr gene-related biosynthetic components indicate that the coral possesses a metabolic pathway similar to the bacterial 2-aminoethylphosphonate (AEP) biosynthetic pathway. The results suggest that de novo synthesis of carbon-phosphorus compounds is performed in corals.
Highlights
Reef-building corals (Class Anthozoa) typically inhabit shallow and relatively clear tropical waters; they are constantly exposed to high levels of ultraviolet radiation
In this study, we investigated whether the coral genome contains genes encoding proteins that are homologous to cyanobacterial enzymes involved in scytonemin synthesis
The scytonemin gene cluster in Nostoc punctiforme consists of one subcluster of genes involved in aromatic amino acid biosynthesis, but the functions of many novel genes in another subcluster are unknown [19]
Summary
Reef-building corals (Class Anthozoa) typically inhabit shallow and relatively clear tropical waters; they are constantly exposed to high levels of ultraviolet radiation. UV-absorbing substances potentially act as photoprotective compounds These include mycosporine-like amino acids (MAAs), scytonemin, carotenoids, and other compounds of unknown structure [3,4]. We scanned the Acropora gene models for homologs of the shinorine gene cluster, and found that this four-gene pathway is present in both Acropora and Nematostella, but not in Hydra [15] This strongly suggests that both Acropora and Nematostella can synthesize shinorine, which may be used to produce photoprotective compounds. In this study, we investigated whether the coral genome contains genes encoding proteins that are homologous to cyanobacterial enzymes involved in scytonemin synthesis. Anthozoan genomes encode a gene homologous to aroB, involved in aromatic amino acid metabolism, which is not found in higher metazoans
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