Recent duplication of the two human CD8 β-chain genes

Isolation of cDNA clones encoding the β-chain of the human T cell surface glycoprotein CD8 revealed the presence of five distinct forms of cDNA resulting from alternative splicing. In the process of analysis of the gene organization, we found that there exist two recently duplicated genes for CD8β. These genes, designated CD8β1 and β2, consist of nine and seven exons, respectively. The organization of CD8β1 and β2 genes is almost identical except in their 3'-ends. There are nine nucleotide differences between the coding regions of the CD8β1 and β2 genes in spite of the extremely high similarity of these genes which extends over the entire genes including introns. Pulse field gel analysis demonstrated that CD8β1 and β2 genes are located more than 1.5 Mb apart. It was found that the CD8β1 gene is approximately 25 kb upstream from the CD8α gene in the same transcriptional orientation on chromosome 2. Although both CD8β1 and β2 genes appear functional from the nucleotide sequence, the five distinct forms of CD8β cDNAs and corresponding mRNAs found in thymus, PBL, and leukemic cell line HPB-ALL are all derived by alternative splicing from CD8β1 transcripts.