Symbiosis-enhanced gene expression in cnidarian-algal associations: cloning and characterization of a cDNA, sym32, encoding a possible cell adhesion protein.

Mutualistic endosymbioses between two partners are complex associations that are regulated by the genetic interactions of the partners. One important marine symbiosis is that between various cnidarians, such as corals and anemones, and their photosynthetic algal symbionts. We have been interested in characterizing cnidarian host genes that are expressed as a function of the symbiotic state, using the temperate sea anemone Anthopleura elegantissima as a model. In this study, we report on symbiosis-enhanced expression and synthesis of sym32 in anemones. We characterized the full-length sym32 cDNA, obtained by RT-PCR, and demonstrated, by semi-quantitative RT-PCR, that sym32 transcript was much more abundant in symbiotic than in non-symbiotic host anemone RNA. Further, using immunoblots, we determined that an antibody made to a sym32 fusion protein labeled a 32 kD band much more strongly in symbiotic compared to non-symbiotic anemone protein homogenates. Databank searches revealed that the sym32 deduced amino acid sequence shares significant homology with the fasciclin I (Fas I) family of homophilic cell adhesion proteins, present in a variety of organisms ranging from bacteria to humans. This strong homology with the Fas I family suggests that sym32 is involved in regulation of the symbiosis by mediating cell-cell interactions.