Comparative analysis of two Nramp loci from rainbow trout

Innate resistance to intracellular parasites is controlled in part by Nramp 1 (Natural resistance-associated macrophage protein 1) in mammals and birds . To isolate Nramp homologs from rainbow trout, a combination of library sc reening and rapid amplification of cDNA ends was performed. Two closely rel ated Nramp loci, designated OmNramp alpha and OmNramp beta, were cloned and characterized. OmNramp alpha and OmNramp beta encode two highly conserved proteins of 585 and 558 amino acids, respectively. Deduced amino acid seqen ces showed that the OmNramp alpha and OmNramp beta proteins share 90% of th eir residues and contain all of the signature features of the Nramp family of proteins: 12 transmembrane domains, two N-linked glycosylation sites, an d a conserved transport motif. Phylogenetic analysis supported a close rela tion to Nramp2 proteins, a related member of the Nramp family. Despite this relation, juvenile trout expressed OmNramp alpha in a manner consistent wi th an Nramp1 homolog and OmNramp beta similar to an Nramp2 locus. Both trou t loci were expressed at relatively high amounts in the ovaries of juvenile s, a finding not reported in the investigations of previously characterized mammalian and avian homologs. These results suggest a role for Nramp loci in the follicular development of teleost fishes, as well as in mammals. Bec ause salmonid fishes are ancestral tetraploids, fragments of OmNramp alpha and OmNramp beta were isolated from smelt, a diploid relative, to determine whether the trout loci represent duplicates of a single gene. Homologous s equences for both loci were found in smelt, supporting the hypothesis that OmNramp alpha and OmNramp beta are indeed independent loci that were presen t before the chromosomal duplication of salmonids. The isolation of Nramp l oci from rainbow trout may eventually produce a genetic tool for the contro l of disease in aquaculture operations. Determining the involvement of trou t homologs in innate immunity may also provide insight regarding the evolut ion of host resistance to pathogens.