Evidence against gamma-crystallin DNA or RNA sequences in the chicken

γN-crystallin and the evolution of the βγ-crystallin superfamily in vertebrates

The beta and gamma crystallins are evolutionarily related families of proteins that make up a large part of the refractive structure of the vertebrate eye lens. Each family has a distinctive gene structure that reflects a history of successive gene duplications. A survey of gamma-crystallins expressed in mammal, reptile, bird and fish species (particularly in the zebrafish, Danio rerio) has led to the discovery of gammaN-crystallin, an evolutionary bridge between the beta and gamma families. In all species examined, gammaN-crystallins have a hybrid gene structure, half beta and half gamma, and thus appear to be the 'missing link' between the beta and gamma crystallin lineages. Overall, there are four major classes of gamma-crystallin: the terrestrial group (including mammalian gammaA-F); the aquatic group (the fish gammaM-crystallins); the gammaS group; and the novel gammaN group. Like the evolutionarily ancient beta-crystallins (but unlike the terrestrial gammaA-F and aquatic gammaM groups), both the gammaS and gammaN crystallins form distinct clades with members in fish, reptiles, birds and mammals. In rodents, gammaN is expressed in nuclear fibers of the lens and, perhaps hinting at an ancestral role for the gamma-crystallins, also in the retina. Although well conserved throughout vertebrate evolution, gammaN in primates has apparently undergone major changes and possible loss of functional expression.

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.

The enzyme lactate dehydrogenase as a structural protein in avian and crocodilian lenses

The major components of mammalian lenses are tissue-specific, soluble proteins, the alpha-, beta- and gamma-crystallins. The lenses of other vertebrate classes often contain other major proteins, notably delta-crystallin in birds and reptiles. A fourth distinct type, described as epsilon-crystallin, is prominent in many bird and crocodile lenses. Here we show that epsilon-crystallin is an active glycolytic enzyme, lactate dehydrogenase (LDH) (EC 1.1.1.27) and that duck epsilon-crystallin appears to be identical to duck LDH-B4. LDH is a normal metabolic component in other lenses, but in duck is present in amounts far exceeding the requirements of any likely catalytic role. It appears that an active enzyme has been recruited, unchanged, to an extra role as a structural protein in the lens without gene duplication and sequence divergence. This surprising discovery raises the possibility that other crystallins may similarly be enzymes expressed at high levels in lens as structural proteins.

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.

Differential regulation of gamma-crystallin genes during mouse lens development

Using gene-specific probes derived from four mouse gamma-crystallin cDNAs, we have examined the regulation of different members of the mouse gamma-crystallin gene family during lens development. Our analysis revealed that, while the different gamma-crystallin genes appear to be coordinately activated during embryogenesis, the steady-state levels of their corresponding transcripts are differentially regulated, resulting in variations in the relative abundance of individual species at different stages of development. This complex pattern of gene regulation presumably accounts for one of the mechanisms determining the spatial distribution of different gamma-crystallins within the lens.

Lens-specific promoter activity of a mouse gamma-crystallin gene

Crystallins are the major water-soluble proteins in vertebrate eye lenses. These lens-specific proteins are encoded by several gene families, and their expression is differentially regulated during lens cell differentiation. Here we show that a cloned mouse gamma-crystallin promoter is active in lens explants derived from 14-day-old chicken embryos but inactive in a variety of cells of non-lens origin. We also show that sequences required for proper utilization of this promoter are contained between nucleotide positions -392 and +47 relative to the transcription initiation site; deletion of sequences from positions -392 to -171 completely abolishes promoter activity. Since chickens do not have gamma-crystallin genes, the expression of a mouse gamma-crystallin promoter in chicken lens cells suggests that different classes of crystallin genes may be regulated by common lens tissue-specific mechanism(s) independent of species.

Lens-specific promoter activity of a mouse gamma-crystallin gene

Crystallins are the major water-soluble proteins in vertebrate eye lenses. These lens-specific proteins are encoded by several gene families, and their expression is differentially regulated during lens cell differentiation. Here we show that a cloned mouse gamma-crystallin promoter is active in lens explants derived from 14-day-old chicken embryos but inactive in a variety of cells of non-lens origin. We also show that sequences required for proper utilization of this promoter are contained between nucleotide positions -392 and +47 relative to the transcription initiation site; deletion of sequences from positions -392 to -171 completely abolishes promoter activity. Since chickens do not have gamma-crystallin genes, the expression of a mouse gamma-crystallin promoter in chicken lens cells suggests that different classes of crystallin genes may be regulated by common lens tissue-specific mechanism(s) independent of species.

tau-Crystallin from the turtle lens: purification and partial characterization

A lens protein with a molecular mass near 46 000 daltons has been purified from the turtle lens. It is a monomeric protein which differs from other lens crystallins in its antigenicity, near-UV circular dichroism spectrum and amino acid composition. Like delta-crystallin, it has appreciable secondary structure (52% alpha-helix). This lens protein has been named tau-crystallin. tau-Crystallin cross-reacts with the novel 48 000 dalton protein identified recently in lenses of the sea lamprey. Although distinct, the possibility that tau-crystallin shares an ancestral relationship with delta-crystallin is discussed.