Primate DRB6 gene expression and evolution: a study in Macaca mulatta and Cercopithecus aethiops.

DRB6 has been found to be transcribed in human and apes. Promoter region and exon 1 come from a 5' LTR from a mammary tumour retrovirus. However, the putative protein structure would be very different to other DR molecules and it is doubtful that it may function as an antigen presenting molecule. Primate DRB6 alleles previously published together with the two new macaque sequences reported here support the existence of a strong selective pressure working on exon 2 to generate stop codons at the end of the exon (between codons 74 and 94) during at least 23 million years. The topology of dendrograms constructed with different primate DRB6 alleles supports the "trans-species" evolution proposed for MHC class I, class II and possibly C4 genes. Finally, DRB6, which is one of the oldest DRB genes, has been lost in the HLA-DRB3 (or DR52) group of haplotypes (DR3, DR5, DR6 and DR8) and a small DRB6 sequence is present at the exon 2 first hypervariable region of DRB4 (or DR53) gene, which is present in DR4, DR7 and DR9 haplotypes.