Stepwise Neofunctionalization of c-Rel during Vertebrate Evolution (RNA-Seq)

Adaptive immunity and the five vertebrate NF-kB/Rel family members first appeared in cartilaginous fish, suggesting that NF-kB family expansion allowed the acquisition of new functions to regulate adaptive immune responses. Transcriptome profiling revealed that, even in macrophages, the NF-kB family member, c-Rel, most potently regulates a cytokine gene linked to adaptive immunity, Il12b, with limiting roles at key regulators of innate immunity. Neofunctionalization of c-Rel to regulate Il12b depends on its unique DNA-binding properties, which we examined using structural, biochemical, functional, and genomic approaches. Among our findings was functional c-Rel homodimer binding to motifs with little resemblance to canonical NF-kB motifs. To determine whether c-Rel’s unique binding properties drove c-Rel-RelA divergence, we compared binding properties in various vertebrate species. c-Rel-RelA binding properties diverged in mammals and amphibians but were comparable in earlier vertebrates, suggesting that divergent DNA binding emerged relatively late during vertebrate evolution to support increasing complexity of adaptive immune regulation.