B-G cDNA clones have multiple small repeats and hybridize to both chicken MHC regions.

We used rabbit antisera to the chicken MHC erythrocyte molecule B-G and to the class I alpha chain (B-F) to screen lambda gt11 cDNA expression libraries made with RNA selected by oligo-dT from bone marrow cells of anemic B19 homozygous chickens. Eight clones were found to encode B-G molecules which hybridize with sequences in the chicken MHC as defined by congenic strains; the fusion proteins react with multiple immune but not preimmune sera, they select antibodies from the antisera to B-G, which then react with distinct erythrocyte B-G protein patterns, and they elicit antibodies from mice which in turn react with authentic B-G proteins. None of the clones represent a complete message, some--if not all--bear introns, and none of them match with any sequences presently stored in the data banks. The following new information did, however, emerge. At least two homologous transcripts are present in this homozygous chicken, thereby formally proving the existence of an expressed multigene family. The 3' ends (3'UT) are simple sequences with 80% nucleotide identity between clones, while the 5' ends (either coding or noncoding) are composed of multiple short repeats which are far less similar. These repeats could explain the bewildering variation in size of B-G proteins within and between haplotypes. Southern blots of genomic chicken DNA gave complex patterns for most probes, with many bands in common using different probes, but few bands in common between haplotypes. The sequences detected are all present in the MHC, based on the congenic lines CB and CC. Most of these sequences map into the B-G region, but some map into the B-F/B-L region as defined by the haplotypes B15, B21, and their apparently reciprocal recombinants B21r3 and B15r1.