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.