Linkage and expression of the argininosuccinate lyase/delta-crystallin genes of the duck: insertion of a CR1 element in the intergenic spacer.
delta-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 delta-crystallin of the embryonic chicken 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 encodes the enzyme argininosuccinate lyase (ASL) which is expressed at a low level in the chicken lens. Both chicken delta-crystallin genes are also expressed slightly in heart and brain, with ASL/delta 2 predominating over delta 1. In the duck (Anas platyrhynchos), ASL/delta 2-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/delta- crystallin locus is highly conserved between duck and chicken, with the two duck delta-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 bp CR1 element, highly reiterated in the duck genome, 1.8 kbp upstream of the duck ASL/delta 2 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/delta 2 gene is very similar (80%) to intron 3 of the chicken delta 1 and ASL/delta 2 genes, especially in the region of the chicken 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-site slippage event, resulting in a two amino acid insertion in the encoded polypeptide. Finally, reverse transcription/polymerase chain reaction experiments established that both delta-crystallin genes are equally 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/delta 2 being preferentially expressed.
Li
X
X
Section on Molecular Genetics, National Eye Institute, National Institutes of Health, Bethesda, MD 20892.
Wistow
G J
GJ
Piatigorsky
J
J
D000818:Animals; D001123:Argininosuccinate Lyase / Q000096:biosynthesis* / Q000235:genetics; D001483:Base Sequence; D002645:Chickens / Q000235:genetics; D003459:Crystallins / Q000096:biosynthesis* / Q000235:genetics; D004372:Ducks / Q000235:genetics*; D005136:Eye Proteins / Q000096:biosynthesis* / Q000235:genetics; D005786:Gene Expression Regulation; D015751:Genes, pol; D008040:Genetic Linkage; D007438:Introns / Q000235:genetics; D008969:Molecular Sequence Data; D016254:Mutagenesis, Insertional; D016366:Open Reading Frames; D012091:Repetitive Sequences, Nucleic Acid; D016415:Sequence Alignment; D012689:Sequence Homology, Nucleic Acid; D013045:Species Specificity
D003459:Crystallins; D005136:Eye Proteins; D001123:Argininosuccinate Lyase