LINKAGE AND EXPRESSION OF THE ARGININOSUCCINATE LYASE DELTA-CRYSTALLIN GENES OF THE DUCK - INSERTION OF A CR-1 ELEMENT IN THE INTERGENIC SPACER

delta-Crystallin is the major component of the lenses of most birds an d reptiles. In the chicken there are two closely linked, tandemly orie nted genes. Almost all of the delta-crystallin of the embryonic chicke n lens is produced by the 5' delta 1 gene. This high lens activity has been attributed to an enhancer in intron 3. The 3' delta 2 gene encod es the enzyme argininosuccinate lyase (ASL) which is expressed at a lo w level in the chicken lens. Both chicken delta-crystallin genes are a lso expressed slightly in heart and brain, with ASL/delta 2 predominat ing over delta 1. In the duck (Anas platyrhynchos), ASL/delta 2-crysta llin serves as both enzyme and crystallin, resulting in very high leve ls of ASL activity in the lens. Here we show by genomic cloning that t he ASL/delta-crystallin locus is highly conserved between duck and chi cken, with the two duck delta-crystallin genes closely linked in tande m. The 4.6 kbp intergenic spacer in the duck locus is 79% identical to the 4 kbp chicken spacer, except for the existence of a 615 bp CR1 el ement, highly reiterated in the duck genome, 1.8 kbp upstream of the d uck ASL/delta 2 gene. The CRI sequence is a truncated LINE element con taining the 3' half of an open reading frame for a retroviral pol-reve rse transcriptase. Sequence analysis revealed (i) that intron 3 of the duck ASL/delta 2 gene is very similar (80%) to intron 3 of the chicke n delta 1 and ASL/delta 2 gene, especially in the region of the chicke n delta 1 enhancer core (93% identical) and (ii) that the 3' boundary of exon 2 of the duck ASL/delta 2 gene has undergone a recent splice-s ite slippage event, resulting in a two amino acid insertion in the enc oded polypeptide. Finally, reverse transcription/polymerase chain reac tion experiments established that both delta-crystallin genes are equa lly expressed to a high level in the embryonic duck lens; by contrast, both delta-crystallin genes produce a low amount of mRNA in the heart and brain of the embryonic duck, with the enzymatically active ASL/de lta 2 being preferentially expressed.