Elucidating the function of an ancient NF-κB p100 homologue, CrRelish, in antibacterial defense

The family of NF-kappa B transcription factors essentially regulates immune-related gene expression. Recently, we isolated and characterized the classical NF-kappa B/inhibitor kappa B (I kappa B) homologues from a living fossil, the horseshoe crab, Carcinoscorpius rotundicauda. Interestingly, this ancient species also harbors another class I NF-kappa B p100 homologue, C rotundicauda Relish (CrRelish). Similar to Drosophila Relish and the mammalian p100, CrRelish contains both the Rel-homology domains (RHD) and the I kappa B-like domain. In this study, we found that the RHD of CrRelish can recognize horseshoe crab and human kappa B response elements and activate the downstream reporter in vitro, thereby suggesting the evolutionary conservation of this molecule. Pseudomonas aeraginosa infection transcriptionally upregulates CrRelish, which exhibits a dynamic protein profile over the time course of infection. Surprisingly, secondary infection reinduced an upsurge in CrRelish protein expression to a level which overrode the protein degradation at 12 h postinfection. These observations strongly suggest the involvement of CrRelish in antibacterial defense. Secondary infection causes (i) the maintenance of a favorable expression-competent sequence context of the CrRelish gene and/or (ii) the derepression or stabilization of the CrRelish transcript resulting from the primary infection to enable the more rapid expression and accumulation of the CrRelish protein, reflecting apparent signal/immune priming in a repeated infection.