Discovery of multiple beta-defensin like homologues in teleost fish.

Using a database mining approach, multiple defensin like genes have been discovered for the first time in fish, in species including zebrafish Danio rerio and the pufferfish, Takifugu rubripes and Tetraodon nigroviridis. They share the common features of vertebrate defensins, including small size, net cationic charge, and six conserved cysteines in the mature region. Based on their cysteine arrangement, the identified fish defensin like peptides resemble beta-defensin family members in birds and mammals. Computing modelling detected three beta-strands in all three zebrafish defensins and an extra N-terminal alpha-helix in one of the peptides. The coding regions of the fish genes contain three exons and two introns, the same as avian defensin genes. In zebrafish and tetraodon, two defensin genes identified are located in the same chromosome. An additional locus containing a third defensin gene has also been found in a different chromosome in zebrafish, demonstrating that multiple defensin loci may be present in fish. Comparative studies suggest that beta-defensins may represent the primitive form of the defensin family, which expanded during evolution by gene or genome duplication. In healthy zebrafish, constitutive expression of defensins was detected by RT-PCR in gill, gonad, gut, kidney, muscle, skin and spleen but the levels and patterns varied for individual defensin genes.