Two Cathelicidin Genes Are Present in both Rainbow Trout ( Oncorhynchus mykiss ) and Atlantic Salmon ( Salmo salar )

Abstract

ABSTRACT Further to the previous finding of the rainbow trout rt CATH_1 gene, this paper describes three more cathelicidin genes found in salmonids: two in Atlantic salmon, named as CATH_1 and as CATH_2, and one in rainbow trout, named rt CATH_2. All the three new salmonid cathelicidin genes share the common characteristics of mammalian cathelicidin genes, such as consisting of four exons and possessing a highly conserved preproregion and four invariant cysteines clustered in the C-terminal region of the cathelin-like domain. The as CATH_1 gene is homologous to the rainbow trout rt CATH_1 gene, in that it possesses three repeat motifs of TGGGGGTGGC in exon IV and two cysteine residues in the predicted mature peptide, while the as CATH_2 gene and rt CATH_2 gene are homologues of each other, with 96% nucleotide identity. Salmonid cathelicidins possess the same elastase-sensitive residue, threonine, as hagfish cathelicidins and the rabbit CAP18 molecule. The cleavage site of the four salmonid cathelicidins is within a conserved amino acid motif of QKIRTRR, which is at the beginning of the sequence encoded by exon IV. Two 36-residue peptides corresponding to the core part of rt CATH_1 and rt CATH_2 were chemically synthesized and shown to exhibit potent antimicrobial activity. rt CATH_2 was expressed constitutively in gill, head kidney, intestine, skin and spleen, while the expression of rt CATH_1 was inducible in gill, head kidney, and spleen after bacterial challenge. Four cathelicidin genes have now been characterized in salmonids and two were identified in hagfish, confirming that cathelicidin genes evolved early and are likely present in all vertebrates.