The macrophage-specific membrane protein Nramp controlling natural resistance to infections in mice has homologues expressed in the root system of plants

In mice, natural resistance or susceptibility to infection with Mycobacteria, Salmonella, and Leishmania is controlled by a gene named Bcg. Bcg regulates the capacity of macrophages to limit intracellular replication of the ingested parasites, and is believed to regulate a key bactericidal mechanism of this cell. Recently, we have cloned the Bcg gene and shown that it encodes a novel macrophage-specific membrane protein designated Nramp. A routine search of the public databases for sequences homologous to Nramp identified 3 expressed sequence tags (EST) that show strong similarities to the mammalian protein. We report the identification and cloning of a full-length cDNA clone corresponding to a plant homologue (OsNramp1) of mammalian Nramp. Predicted amino acid sequence analysis of the plant protein indicates a remarkable degree of similarity (60% homology) with its mammalian counterpart, including identical number, position, and composition of transmembrane domains, glycosylation signals, and consensus transport motif, suggesting an identical overall secondary structure and membrane organization for the two proteins. This high degree of structural similarity indicates that the two proteins may be functionally related, possibly through a common mechanism of transport. RNA hybridization studies and RT-PCR analyses indicate that OsNramp1 mRNA is expressed primarily in roots and only at very low levels in leaves/stem. DNA hybridization studies indicate that OsNramp1 is not a single gene, but rather forms part of a novel gene family which has several members in all plants tested including cereals such as rice, wheat, and corn, and also in common weed species. The striking degree of conservation between the macrophage-specific mammalian Nramp and its OsNramp1 plant homologue is discussed with respect to possible implications in the metabolism of nitrate in both organisms.