Complete genomic sequence of the murine low affinity Fc receptor for IgE: Demonstration of alternative transcripts and conserved sequence elements

The complete sequence of the murine low affinity Fc receptor for IgE (Fc∈RII), including the 5′ and 3′ flanking sequences, is reported. The murine Fc∈RII gene spans 12.9 kb and includes 12 exons surrounding 11 introns. The composite exon sequence is virtually identical to previously reported murine Fc∈RII cDNA sequences. Much of the proximal promoter regions of the mouse and human homologues of Fc∈RII show remarkable homology to each other, including three promoter elements previously identified for MHC class II genes. The reported exon/ intron structure of the human Fc∈RII is similar to the murine homologue, except that the latter has an additional exon coding for a fourth amino acid repetitive sequence (vs three in the human gene). RNase protection studies have identified an additional transcript within intron 2 of murine Fc∈RIIa, similar to the human Fc∈RIIb form but with a different predicted sequence of the first six amino acids. This transcript is present in the mRNA of purified splenic B cells, but not in the mRNA of the Fc∈RII⁺ B lymphoma cell line M12.4.5. The murine Fc∈RII gene contains a large intron (4.2 kb) separating the lectin and nonlectin coding regions, and several repetitive sequences are found clustered within this intron. These results emphasize the importance of the demarcation between these domains and allude to their evolutionary and functional significance.