Identification and functional analysis of Relish homologs in the silkworm, Bombyx mori

Two cDNAs designated BmRelish1 and 2, that encode Relish homologs, were cloned from the silkworm, Bombyxmori. BmRelish1 had an I kappa B-like domain with 5 ankyrin repeats in addition to Rel homology domain (RHD), nuclear localization signal (NLS), and acidic and hydrophobic amino acids (AHAA) rich regions. On the other hand, BmRelish2 lacked the AHAA and ankyrin repeats (ANK). Knockdown of the BmRelish gene in transgenic silkworms resulted in failure of the activation of antimicrobial peptide genes by Escherichia coli, suggesting that BmRelish plays an important role in antimicrobial peptide gene expression. Functional analysis of BmRelish1 and 2 in mbn-2 cells showed that both Relish homologs do not activate promoters of B. mori antimicrobial peptide genes encoding cecropin B1, attacin, lebocin 3 and lebocin 4. However, a gene construct BmRelish1-d2 lacking the ANK strongly activated promoters of these genes. Another gene construct lacking AHAA and ANK failed to activate these genes, suggesting that BmRelish becomes active by removal of the ANK and that the AHAA-rich region is a transactivation domain. BmRelish2 was shown to repress activation of Cecropin B1 gene expression by BmRelish1-d2, suggesting that BmRelish2 plays a role as a dominant negative factor against the BmRelish1 active form. Necessity of kappa B sites of Cecropin B1, Attacin and Lebocin 4 genes for the full activation of these genes by BmRelish1-d2 was confirmed. The requirement of the mandatory kappa B sites for Lebocin 4 gene expression was different between BmRelish1 active form and BmRe1A, suggesting differential roles for kappa B sites in antimicrobial peptide gene activation by different transcription factors. The binding of the RHD portion of BmRelish I fusion protein to the kappa B sites of Cecropin B1 and Attacin genes was also confirmed. (c) 2007 Elsevier B.V All rights reserved.