Reactivation by exon shuffling of a conservedHLA-DR3-like pseudogene segment in a New World primate species

The common marmoset (Callithrix jacchus), a New World monkey species with a limited MHC class II repertoire, is highly susceptible to certain bacterial infections. Genomic analysis of exon 2 sequences documented the existence of only oneDRBregion configuration harboring three loci. Two of these loci display moderate levels of allelic polymorphism, whereas the-DRB*W12gene appears to be monomorphic. This study shows that only theCaja-DRB*W16and-DRB*W12loci produce functional transcripts. TheCaja-DRB1*03locus is occupied by a pseudogene, given that most of the transcripts, if detected at all, show imperfections and are present at low levels. Moreover, two hybrid transcripts were identified that feature the evolutionarily conserved peptide-binding motif characteristic for theCaja-DRB1*03gene. Thus, the severely reduced MHC class II repertoire in common marmosets has been expanded by reactivation of a pseudogene segment as a result of exon shuffling.