Linkage and expression of the argininosuccinate lyase/δ-crystallin genes of the duck: insertion of a CR1 element in the intergenic spacer☆

Abstract δ-Crystallin is the major component of the lenses of most birds and reptiles. In the chicken there are two closely linked, tandemly oriented genes. Almost all of the δ-crystallin of the embryonic chicken lens is produced by the 5′ δ1 gene. This high lens activity has been attributed to an enhancer in intron 3. The 3′ δ2 gene encodes the enzyme argininosuccinate lyase (ASL) which is expressed at a low level in the chicken lens. Both chicken 8-crystallin genes are also expressed slightly in heart and brain, with ASL/δ2 predominating over δ1. In the duck ( Anas platyrhynchos ) , ASL/δ2-crystallin serves as both enzyme and crystallin, resulting in very high levels of ASL activity in the lens. Here we show by genomic cloning that the ASL/δ-crystallin locus is highly conserved between duck and chicken, with the two duck δ-crystallin genes closely linked in tandem. 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 by CR1 element, highly reiterated in the duck genome, 1.8 kbp upstream of the duck ASL/δ2 gene. The CR1 sequence is a truncated LINE element containing the 3′ half of an open reading frame for a retroviral pol -like reverse transcriptase. Sequence analysis revealed (i) that intron 3 of the duck ASL/δ2 gene is very similar (80%) to intron 3 of the chicken δ1 and ASL/δ2 genes, especially in the region of the chicken δ1 enhancer core (93% identical) and (ii) that the 3′ boundary of exon 2 of the duck ASL/δ2 gene has undergone a recent splice-site slippage event, resulting in a two amino acid insertion in the encoded polypeptide. Finally, reverse transcription/polymerase chain reaction experiments established that both δ-crystallin genes are equally expressed to a high level in the embryonic duck lens; by contrast, both δ-crystallin genes produce a low amount of mRNA in the heart and brain of the embryonic duck, with the enzymatically active ASL/δ2 being preferentially expressed.