Cg‐Rel, the first Rel/NF‐κB homolog characterized in a mollusk, the Pacific oyster Crassostrea gigas

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Abstract
We report here the identification and functional characterization of Cg -Rel, a gene encoding the Crassostrea gigas homolog of Rel/NF-κB transcription factors found in insects and mammals. Sequence and phylogenetic analysis showed that Cg -Rel shares the structural organization of Rel/NF-κB transcription factors of class II. It includes a Rel homology domain as well as a C-terminal transactivation domain (TD). Overexpression of Cg -Rel in the Drosophila S2 cell line activated the expression of a NF-κB-dependent reporter gene, whereas transfection with a Cg -Rel construct containing a C-terminal deletion of the TD or using a reporter gene with mutated κB binding sites failed to activate expression. These results suggest that Cg -Rel is a functional member of the Rel family of transcription factors, making this the sixth structurally homologous component of the Rel/NF-κB pathway characterized in C. gigas . Based on homology to other invertebrates’ Rel/NF-κB cascade, the function of the oyster pathway may serve to regulate genes involved in innate defense and/or development. These findings serve to highlight a potentially important regulatory pathway to the study of oyster immunology, hence allowing comparison of the immune system in vertebrates and invertebrates, an important key issue to understand its evolution.