Genomic organisation and sequence of the extracellular domain exons of the bovine Fc gamma RI receptor, and evidence for restricted binding of ruminant IgG to U937 cells.

Southern blotting of bovine genomic DNA and hybridization with a human Fc gamma RI cDNA probe, p135, has identified a single copy of the bovine Fc gamma RI gene. A bovine genomic lymphocyte library in lambda EMBL3 was screened with probe p135. A positive lambda clone, 15.5.4, containing the three extracellular domain exons of Fc gamma RI, has been cloned, mapped and sequenced. Each extracellular domain is encoded within a single exon. All three domains are assigned to the C-2 set of the Ig superfamily with 58% identity between amino acid residues of bovine, human and mouse Fc gamma RI. Pairs of cysteine residues are conserved in each domain as potential sites for intra-chain disulphide bonding. Human monocytoid U937 cells were used as a model to test binding homology within the Fc gamma RI family. The binding of IgG isotypes to IFN-gamma stimulated U937 cells was determined by FACScan flow cytometry. U937 cell Fc gamma RI receptor does not bind bovine or ovine IgG isotypes. On the basis of these studies and by comparison of the Fc determinant region sequences of IgG, the introduction of species specificity in Fc gamma RI/IgG interaction by evolutionary drift is proposed.