Description of two Mhc-C-related sequences in the New World monkey Saguinus oedipus

Two new Mhc class I partial exon 1, intron 1, exon 2, intron 2 and partial exon 3 DNA sequences from the New World monkey Saguinus oedipus (Saoe) are described. These two sequences show certain Mhc-C sequence-specific changes . The only difference between these two new sequences is a productive subst itution at position 152 [GCG (Ala)-->GAG (Glu)]. This change occurs in a po sition which in Mhc classical class I molecules affects the interaction bet ween the peptide and the T-cell receptor A dendrogram with Mhc sequences fr om different loci and different species was constructed, which clearly show s that these two new sequences cluster closer to Mhc-C sequences than to ot hers. These data suggest that the new sequences may be related to the Mhc-C locus, and they have been named Mhc-Saoe-CR*01 and -CR*02. However, they s hare only a few of the conserved residues (from gorilla to human) of Mhc-C sequences, which suggests that the relationships with an ancestor of the Mh c-C lineage are very distant or that these two sequences are products of co nvergent evolution to perform a C locus related function. Furthermore, in t he fragment of DNA sequenced, there is a loss of two invariant residues con served in antigen-presenting molecules from reptiles to humans; thus, it is unlikely that these two Mhc-C-like sequences have an antigen-presenting fu nction, or even that they are two alleles of a pseudogene; however, the G C percentage (86.1%) at the third base of codons approaches that of an exp ressed gene in Saoe. It is concluded that Mhc molecules with C-locus charac teristics existed in primates 50 million years ago and that this does not s upport a more recent origin of Mhc-C genes.