Structural and Functional Evidence for Differential Promoter Activity of the Two Linked Delta-crystallin Genes in the Chicken

There are two delta-crystallin genes (delta 1 and delta 2) in the chicken which are oriented with the same transcriptional polarity (5' delta 1-delta 2-3') and are separated by approximately 4.2 kb of DNA. Existing evidence indicates that delta 1 is very active in the embryonic lens; in contrast, delta 2 appears very inactive, if expressed at all. We have sequenced the 5' regions of delta 1 and delta 2 and tested their ability to promote chloramphenicol acetyltransferase (CAT) activity using the pSVO-CAT expression vector in transfected embryonic lens epithelia. The sequence data establish that the previously reported delta-crystallin cDNAs were derived from mRNAs encoded in delta 1 and not in delta 2. The transfection experiments indicated that the delta 1 promoter is appreciably stronger than the delta 2 promoter. Interestingly, a consensus CCAAT sequence (position - 71) and two consensus viral core-enhancer-like sequences (positions - 308 and +350) are confined to the more active, delta 1 gene. delta 1 and delta 2 have similar TATA boxes (TAAAA) at positions - 28 and - 27, respectively. Exon 1 in delta 1 (35 bp) and in delta 2 (34 bp) are extremely homologous, containing just two mismatches; exon 2 in delta 1 (64 bp) and in delta 2 (51 bp) are only 56% homologous and contain the putative translation initiating codon and three encoded amino acids. Unexpectedly, intron 1 of both genes has numerous pentameric repeats present in the murine immunoglobulin heavy-chain switch regions (GAGCT, GGGGT and GGGCT). Other direct repeats were found in delta 1 and delta 2, and short homologies were noted between the chicken delta-crystallin and murine alpha A-crystallin genes.