We isolated bovine complementary DNA clones for the aA-and aB-crystallin subunhts. The aB cDNA clone was used to isolate an aB-crystallin gene. This gene, derived from hamster, occurs as a single copy in the genome and is 3.2 kilobases long. The coding sequences are spread on three exons with a total length of 709 nucleotides. The exon-intron distribution of the hamster aB-crystallin gene is similar to that of the aA-crystallin gene except for the 69 nucleotides that specify the 23 "insert" residues of the aAI`s chain by means of differential splicing. The 3' noncoding region of the aB mRNA (140 bases), which is short compared with the aA mRNA (520 bases), shows a remarkable homology between calf and hamster. Both a-crystallin cDNA clones have been used to assign the chromosomal location of the corresponding human genes with the aid of somatic cell hybrids. It is shown that the single-copy aA-and aB-crystallin genes are located on different chromosomes.Among the structural proteins of the mammalian lens, acrystallin is a major component with a molecular weight of --800,000, which is composed of acidic and basic polypeptides. Of these subunits, only two are primary gene products, named aA2 and aB2, varying in ratio and in rate of synthesis during differentiation of the lens cell. All other a-crystallin subunits arise from aA2 and aB2 by post-translational modification (1-3). The hamster aA-crystallin gene, whose structure has been elucidated recently (4), consists of four exons. Exon 2, which encodes the so-called insert peptide, characteristic for some rodents (5, 6), arises by differential splicing (4-7).To obtain structural evidence for the different expression patterns of the two a-crystallin genes, we isolated aA-and aB-crystallin cDNA clones from a cDNA bank ofbovine lens mRNAs (8). Previously, aA-crystallin cDNA clones had been isolated for a few other species (9-11, §). Quite surprisingly, however, an aB-specific cDNA clone had never been found in spite of the fact that several research groups pursued studies on molecular cloning of the crystallin genes. We have now also isolated and characterized by sequence analysis the hamster aB-crystallin gene and performed comparative studies with the hamster aA-crystallin gene (4). Moreover, we investigated the chromosomal location of the a-crystallin genes to address the question of whether these genes are linked on the genome, like the 8-and y-crystallin families; such linkage might have implications for the regulation of expression.
MATERIALS AND METHODSIsolation and Identification of Bovine aA-and aB-Crystallin cDNA Clones. Isolation and cloning of bovine lens mRNAs, the screening of the recombinant clones in a hybridization selection assay, and identification of the translational products by one-and two-dimensional gel electrophoresis was done as described (8). This resulted in the isolation of the aAand aB-crystallin clones pBLaA-1 and pBLaB-1.Construction and Screening of a Hamster Genomic Library. A partial gene library from a Syrian gold hamster wa...
