THE CHICKEN BETA(2)-MICROGLOBULIN GENE IS LOCATED ON A NONMAJOR HISTOCOMPATIBILITY COMPLEX MICROCHROMOSOME - A SMALL, G-RICH GENE WITH X-BOX AND Y-BOX IN THE PROMOTER(C)

beta(2)-Microglobulin is an essential subunit of major histocompatibil ity complex (Mhc) class I molecules, which present antigenic peptides to T lymphocytes, We sequenced a number of cDNAs and two genomic clone s corresponding to chicken beta(2)-microglobulin. The chicken beta(2)- microglobulin gene has a similar genomic organization but smaller intr ons and higher G+C content than mammalian beta(2)-microglobulin genes, The promotor region is particularly G+C-rich and contains, in additio n to interferon regulatory elements, potential S/W, X, and Y boxes tha t were originally described for mammalian class II but not class I alp ha or beta(2)-microglobulin genes, There is a single chicken PZ. micro globulin gene that has little polymorphism in the coding region, Restr iction fragment length polymorphisms from Mhc homozygous lines, Mhc co ngenic lines, and backcross families, as well as in situ hybridization , show that the beta(2)-microglobulin gene is located on a microchromo some different from the one that contains the chicken Mhc. We propose that the structural similarities between the beta(2)-microglobulin and Mhc genes in the chicken are due to their presence on microchromosome s and suggest that these features and the microchromosomes appeared by deletion of DNA in the lineage leading to the birds.