The chicken delta-crystallin gene family. Two genes of similar structure in close chromosomal approximation

Characterization and enzyme activity of argininosuccinate lyase/delta-crystallin of the embryonic duck lens

Argininosuccinate lyase (ASL)/delta-crystallin, a major soluble protein of the transparent eye lens of birds and reptiles, is a mixture of tetramers comprising all possible combinations of two similar polypeptides (delta 1 and delta 2). Only the delta 2 polypeptide has ASL activity. In the present investigation we have purified each of the 5 major isoforms (delta A to delta E, pI 5.2 to 5.8) of delta-crystallin tetramers from the embryonic duck lens by isoelectric focussing and established by peptide sequencing that the delta 1 and delta 2 polypeptides are encoded in the previously identified, linked delta 1 and delta 2 genes, respectively. The relative amounts of the different tetramers in the 14-day-old embryonic lens were consistent with equal expression of the 2 delta-crystallin genes and no preference for assembly of the 2 delta polypeptides. The relative amount of ASL activity of the tetramers was a linear function of the relative amount of their delta 2 polypeptides, with delta A (only delta 1) lacking enzymatic activity altogether. delta B (3 delta 1:1 delta 2), delta C (2 delta 1:2 delta 2), delta D (1 delta 1:3 delta 2) and delta E (4 delta 2) all gave normal Michaelis-Menten kinetics for fumarate production from argininosuccinate at 40 degrees C and had a similar Km (average Km for mixture was 0.15 mM). delta E had a Km of 0.187 mM and a Vmax of 9 mumol/min per mg protein. Unlike bovine and like human ASL, both reported previously, embryonic duck ASL/delta-crystallin showed no evidence of cooperativity or activation by GTP. Each isoform had a similar far ultraviolet circular dichroism spectrum and thermal stability between 20 degrees C and 60 degrees C, with denaturation occurring at 65 degrees C. Our data suggest that gene duplication, structural modifications leading to greater thermal stability of the delta 1 and delta 2 polypeptides, and selective loss of ASL activity in the delta 1 polypeptide all occurred during the recruitment of ASL for a refractive role in the duck lens, resulting in the generation of ASL isoenzymes.

The chicken beta A4- and beta B1-crystallin-encoding genes are tightly linked

Analysis of the 5' flanking region of the chicken beta B1-crystallin-encoding gene (beta B1-cry) revealed regions of sequence homology with the bovine beta A4-crystallin-encoding gene (beta A4-cry). Subsequently, the chicken beta A4-cry cDNA sequence was determined, and it was demonstrated that beta A4- and beta B1-cry are linked head to head in the chicken chromosome with 2147 nucleotides (nt) of intergenic spacer. Chicken beta A4-cry contains six exons, with the first exon being noncoding. Chicken beta A4-cry is the smallest beta-cry ever described, due to the small size of its introns which range in length from 68 to 96 nt. While three polymorphisms were noted between some cDNA clones and the genomic sequence, Southern blot analysis demonstrated that beta A4-cry exists as a single copy in the chicken genome. Northern blot analysis indicated that beta A4-cry is a lens-specific transcript which is expressed at higher levels in the embryo than in the adult. The beta A4-cry mRNA is present at 400-fold lower levels than the beta B1-cry mRNA in the 14-day embryonic chicken lens, and at 2000-fold lower levels than the beta B1-cry mRNA in the adult lens. These results are consistent with the idea that the beta-cry family was once clustered in the chromosome as the gamma-cry family is today, and raises the possibility that the relatively low expression of beta A4-cry is mechanistically linked to the high expression of beta B1-cry in the chicken lens.

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.

Gene conversion and splice-site slippage in the argininosuccinate lyases/delta-crystallins of the duck lens: members of an enzyme superfamily

Argininosuccinate lyase(ASL)/delta-crystallin is a prominent example of an enzyme-crystallin with roles as both a catalyst and a major structural component of the eye lens in birds and reptiles. In chicken it appears that gene duplication and separation of function may have occurred with one gene product acting primarily as a crystallin and one primarily as an enzyme. However, two delta-crystallin-encoding genes are abundantly expressed in the lens of the embryonic duck (Anas platyrhynchos) which has extremely high ASL activity. Here the isolation and sequence analysis of full length cDNA clones for both duck delta-crystallins are described. The two delta-crystallins are highly similar (94% identical in predicted aa sequence), probably as a result of gene conversion. However, the cDNA for duck delta 2-crystallin contains an in-frame insertion of two codons, probably the result of a recent intron boundary slippage. ASL/delta-crystallin belongs to a superfamily of lyases, including fumarases, aspartases and adenylosuccinate lyase which possess some highly conserved blocks of aa sequence. There may be some clues to the tertiary structures of these conserved motifs in otherwise unrelated proteins for which three-dimensional structures are known.

Evidence for positive and negative regulation in the promoter of the chicken delta 1-crystallin gene

We investigated the role of sequences flanking the transcription initiation site of the delta 1-crystallin gene in transient transfection assays of primary embryonic chicken lens epithelial cells or fibroblasts. Varying lengths of the 5' flanking sequence of the delta 1-crystallin gene (containing some untranslated sequence from exon 1) were fused to the bacterial chloramphenicol acetyltransferase (CAT) gene in the pSVOCAT plasmid. A plasmid carrying the bacterial beta-galactosidase gene driven by the Rous sarcoma virus (RSV) promoter was used as an internal control. Standardized results showed that the sequence located between -120 to -43 exhibited strong promoter activity; however, the promoter activity was markedly reduced (20-fold) when the upstream sequence between -603 and -120 was included in the construct. The delta 1-crystallin promoter displayed little lens preference. This upstream sequence did not reduce the activity of the Simian virus 40 (SV40) early promoter (with or without its enhancer) or the Herpes thymidine kinase promoter in transfection tests, indicating some specificity in its effect. Evidence for a delta 1-crystallin negative trans-acting factor was provided by competition experiments. Our data raise the possibility that expression of the delta 1-crystallin gene involves a negative cis-acting transcription element, a speculation which may deserve further attention in view of the gradual decrease in delta-crystallin synthesis in the developing lens.

Expression of the delta-crystallin genes in the embryonic chicken lens

The amounts of lens mRNA derived from the delta 1- and delta 2-crystallin genes of the chicken were determined by primer extension experiments using gene-specific synthetic oligonucleotides. The primer extended products were sequenced to establish the identity of the resulting cDNAs. The results indicated that most of the delta-crystallin mRNA in the 14-day-old embryonic lens contained transcripts derived from the delta 1 gene. Importantly, however, about 1-2% of the extended products were derived from delta 2 mRNA. Although not quantitative, the primer extension experiments suggested that the delta 1/delta 2 mRNA ratio may differ in the lens fiber cells during development between 6 days of embryogenesis and 3 weeks after hatching. These data provide the first demonstration for the presence of delta 2-crystallin mRNA in the chicken lens and raise the possibility that the two linked, extremely similar delta-crystallin genes are differentially regulated during development.

Conservation of delta-crystallin gene structure between ducks and chickens

A cloned chicken delta-crystallin cDNA was used to identify two putative delta-crystallin genes in the duck by Southern blot hybridization. A DNA fragment containing most of one of these genes was isolated from a library made in bacteriophage lambda Charon 28A containing genomic DNA from 14-day-old embryonic ducks. Electron microscopy, partial gene sequencing, primer extension analysis using duck mRNA, and comparison with the well-characterized chicken delta-crystallin genes suggest that our cloned duck delta-crystallin gene, like the chicken delta-crystallin genes, is 8-10 kb long and contains 17 exons. Hybridization and sequencing data show great similarity between the homologous 5' untranslated and coding exons of the duck and chicken delta-crystallin genes. Overall, the homologous introns also appear to have approximately 30% sequence similarity, and have been subject to deletion/insertion events. Our partial characterization of duck delta-crystallin gene sequences suggests that this avian and reptilian crystallin family has been conserved during evolution, as have the other crystallin gene families that are expressed in the eye lens.

Complete nucleotide sequence of the chicken alpha A-crystallin gene and its 5' flanking region

The chicken alpha A-crystallin gene and 2.6 kb of its 5' flanking sequence have been isolated and characterized by electron microscopy and sequencing. The structural gene is 4.5 kb long and contains two introns, each approx. 1 kb in length. The first intron divides codons 63 and 64, and the second intron divides codons 104 and 105, as in rodents. There is little indication that the insert exon of rodents (an alternatively spliced sequence) is present in complete form in the chicken alpha A-crystallin gene; small stretches of similarity to this sequence were found throughout the gene. The 5' flanking sequence of the chicken alpha A-crystallin gene shows considerable sequence similarity with other mammalian alpha B-crystallin genes. In addition, one consensus sequence (GCAGCATGCCCTCCTAG) present in the 5' flanking region of the chicken alpha A-crystallin gene was found in the 5' flanking region of most reported crystallin genes.

Expression of the delta 1-crystallin gene: salt-dependent alteration in the cell-free synthesis of delta-crystallin polypeptides

We have investigated the origins of the two major size classes of delta-crystallin polypeptides (48 kDa and 50 kDa on sodium dodecyl sulfate-urea-polyacrylamide gels) in the chicken lens. Both the 48-kDa and 50-kDa polypeptides were produced by cell-free translation in a salt-dependent (Na+, K+, acetate and Cl-) pattern from mRNA derived from a cloned delta 1-crystallin cDNA. The salt-dependent alteration in the ratio of cell-free synthesis of the 48-kDa to 50-kDa delta-crystallin polypeptides was greatly enhanced by capping of the delta 1 mRNA. Translation of the delta 1 mRNA containing a premature termination codon suggested that the larger delta-crystallin band contains two polypeptides which differ in their N-terminal one-third. In vitro transcription/translation analysis of several mutant delta 1 cDNA clones and immunoblot analysis of authentic delta-crystallin with antisera raised to peptides contained in delta-crystallin showed that neither alternative initiation at two in-phase AUG codons nor alternative termination at sites on the delta 1 mRNA are responsible for generating the two sizes of the delta-crystallin polypeptides. Taken together our data suggest (but do not prove) that delta-crystallin heterogeneity is generated by co-translational modification of the primary polypeptide encoded in the delta 1-crystallin gene.

Alterations in crystallin gene expression during subculture of chick lens cells

We report here on the changes in crystallin gene expression during serial subculture of lens epithelial cells derived from day-old post-hatch chicks. Total cellular RNA from mass cultures were analysed by in vitro cell-free translation and by RNA blot (Northern) hybridization using a cloned delta-crystallin cDNA. Our results indicate that following subculture, lens epithelial cells which still retain the capacity for lens fibre differentiation (lentoid body formation) show a selective loss of delta-crystallin synthesis, and that this is related to the loss of delta-crystallin mRNA. The data suggest that older epithelial-cell populations give rise to lentoid bodies which in terms of crystallin gene expression closely resemble the later-formed cortical fibres of the adult chick lens. Tertiary cultures had an accelerated growth rate, formed no lentoids, contained no translatable alpha- or delta-crystallin mRNAs but still contained translatable beta-crystallin mRNAs.

Two delta-crystallin polypeptides are derived from a cloned delta 1-crystallin cDNA

Previous studies have shown that there are 2 similar delta-crystallin genes (delta 1 and delta 2) and at least 2 delta-crystallin polypeptides in the chicken eye lens. We show here that both delta-crystallin polypeptides can be synthesized from mRNA transcribed in vitro from a cloned delta 1-crystallin cDNA. Both polypeptides co-migrate in SDS-urea-polyacrylamide electrophoresis with their authentic counterparts isolated from 15-day-old embryonic chicken lenses, and both react with sheep anti-chicken delta-crystallin serum. Screening nearly 900 delta-crystallin cDNA clones from a 15-day-old embryonic lens library with an oligonucleotide probe specific for exon 2 of the delta 2-crystallin gene failed to detect any delta 2 cDNA clones, indicating that the delta 2 gene produces little or no mRNA in the lens at this stage of development. Our results suggest that both of the observed delta-crystallin polypeptides are derived from mRNA transcribed from the delta 1 gene, with heterogeneity arising at the translational or co-translational level.