CLONING AND SEQUENCING OF EXPRESSED DRB GENES OF THE RED DEER (CERVUS-ELAPHUS) MHC

The expressed major histocompatibility complex (Mhc) class II DRB gene s of 50 unrelated deer were examined by reverse transcription polymera se chain reaction, cloning, and sequencing of DRB exon 2. Deer, like o ther mammals, have at least one highly polymorphic Mhc class II DRB ge ne. Thirty-four different sequences were identified. Most of the varia tion in amino acid composition occurred at positions that have been sh own to form the peptide binding site (PBS). Eighteen deer-specific sub stitutions were found, 11 of these occurred in the PBS. Significantly higher rates of replacement substitutions than silent substitutions we re found in the deer sequences, indicating strong positive selection p ressure for diversity in DRB sequences. Between one and four DRB seque nces were found per deer. Inheritance of these sequences in pedigrees showed Mendelian segregation with up to two expressed DRB genes per ha plotype. Sheep are the only other ruminant in which the presence of mo re than one expressed DRB gene has been demonstrated. Phylogenetic tre es were constructed in an attempt to assign the deer DRB sequences to specific loci, but no clear segregation of the DRB sequences for diffe rent loci was found. It would seem likely that sequence exchange betwe en the loci has occurred. As has been shown in other species, the alph a-helix and beta-sheet regions of exon 2 appeared to have different ev olutionary histories.