THE CHICKEN BETA-A4-CRYSTALLIN-ENCODING AND BETA-B1-CRYSTALLIN-ENCODING GENES ARE TIGHTLY LINKED

Analysis of the 5' flanking region of the chicken beta B1-crystallin-e ncoding gene (beta B1-cry) revealed regions of sequence homology with the bovine beta A4-crystallin-encoding gene (beta A4-cry). Subsequentl y, the chicken beta A4-cry cDNA sequence was determined, and it was de monstrated that beta A4- and beta B1-cry are linked head to head in th e chicken chromosome with 2147 nucleotides (nt) of intergenic spacer. Chicken beta A4-cry contains six exons, with the first exon being nonc oding. Chicken beta A4-cry is the smallest beta-cry ever described, du e to the small size of its introns which range in length from 68 to 96 nt. While three polymorphisms were noted between some cDNA clones and the genomic sequence, Southern blot analysis demonstrated that beta A 4-cry exists as a single copy in the chicken genome. Northern blot ana lysis indicated that beta A4-cry is a lens-specific transcript which i s expressed at higher levels in the embryo than in the adult. The beta A4-cry mRNA is present at 400-fold lower levels than the beta B1-cry mRNA in the 14-day embryonic chicken lens, and at 2000-fold lower leve ls than the beta B1-cry mRNA in the adult lens. These results are cons istent with the idea that the beta-cry family was once clustered in th e chromosome as the gamma-cry family is today, and raises the possibil ity that the relatively low expression of beta A4-cry is mechanistical ly linked to the high expression of beta B1-cry in the chicken lens.