Complete nucleotide sequence of a functional class I HLA gene, HLA-A3: implications for the evolution of HLA genes.

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Abstract
The complete nucleotide sequence of an active class I HLA gene, HLA‐A3, has been determined. This sequence, together with that obtained for the HLA‐CW3 gene, represents the first complete nucleotide sequence to be determined for functional class I HLA genes. The gene organisation of HLA‐A3 closely resembles that of class I H‐2 genes in mouse: it shows a signal exon, three exons encoding the three extracellular domains, one exon encoding the transmembrane region and three exons encoding the cytoplasmic domain. The complete nucleotide sequences of the active HLA genes, HLA‐A3 and HLA‐CW3, now permit a meaningful comparison of the nucleotide sequences of class I HLA genes by alignment with the sequence established for a HLA‐B7‐specific cDNA clone and the sequences of two HLA class I pseudogenes HLA 12.4 and LN‐ 11A . The comparisons show that there is a non‐random pattern of nucleotide differences in both exonic and intronic regions featuring segmental homologies over short regions, which is indicative of a gene conversion mechanism. In addition, analysis of the frequency of nucleotide substitution at the three base positions within the codons of the functional genes HLA‐A3, HLA‐B7 and HLA‐CW3 shows that the pattern of nucleotide substitution in the exon coding for the 3rd extracellular domain is consistent with strong selection pressure to conserve the sequence. The distribution of nucleotide variation in the other exons specifying the mature protein is nearly random with respect to the frequencies of substitution at the three nucleotide positions of their codons. The evolutionary implications of these findings are discussed.