STRUCTURE AND LENS EXPRESSION OF THE GENE ENCODING CHICKEN BETA-A3/A1-CRYSTALLIN

The beta-A1- and beta-A3-crystallins are major polypeptides in the lenses of vertebrates. We present evidence that a single beta-A3/A1 gene encodes these two proteins in the chicken. The beta-A3/A1 gene has been sequenced and its functional promoter identified in transfection experiments. The chicken beta-A3/A1 gene has the same structure as the human orthologue: six exons with standard splice sites and two alternative start codons from which the protein products are apparently translated. Northern analysis revealed an abundant 0.9-kb transcript in the lenses of 1-2-day-old chickens and no detectable transcripts in the rest of the eye, brain, heart, kidney, liver or skeletal muscle. The 5'-flanking sequence of the chicken beta-A3/A1 gene is very similar to that of the human and mouse genes, suggesting conservation of important putative regulatory sequences in addition to the TATA box. A thymidine-rich element (bp -218 to -163) and a potential AP-1-binding site (bp -264 to -258) are present within the chicken 5'-flanking region. A DNA fragment from -382 to +22 of the chicken beta-A3/A1 gene is sufficient to promote expression of the bacterial cat gene in transfected chicken primary lens epithelial cells, but not in transfected dermal fibroblasts. Moreover, the sequence from positions -382 to -143 of the chicken beta-A3/A1 promoter appears to be critical for proper transcription initiation and expression in the transfected lens cells.