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. 1999 Jul 6;96(14):8074–8079. doi: 10.1073/pnas.96.14.8074

Figure 2.

Figure 2

Exon duplication in the super αA-crystallin gene. (A) Schematic representations of the super α-crystallin mRNAs as deduced from cDNA sequencing. Exons are numbered below the mRNAs. Exon 2 is retained only in the alternatively spliced αAins-crystallin mRNA. The duplicated exon 3 sequence is indicated as 3*. Numbers above the mRNAs give the amino acid residue positions corresponding to the 3′ end of each exon and the total coding region to indicate the lengths of the mutant proteins and inserted regions. Arrowheads point to the positions of the forward primer (a), used to obtain cDNA, and the forward (b) and reverse (c) primers used for sequencing the mutant gene in the genomic DNA. (B) Nonhomologous recombination between two copies of the transfected hamster αA-crystallin gene (top and middle genes). Introns are numbered above the genes. Recombination has occurred at the position of the vertical dotted line, apparently facilitated by the CCCAT homologies in exon 2 and intron 3 (arrowheads). As a result, the super αA-crystallin gene (bottom gene) contains a 1.7-kilobase duplication resulting in a truncated intron 3 (indicated as 3−) and exon 2 (2−), as well as an additional intron 2 (2*) and exon 3 (3*). Because the intron splice site between pseudointron 3− and pseudoexon 2− is lost, these sequences are combined with intron 2* to form the large intron 3+ between the duplicated exons 3 and 3*. The double arrow beneath the super gene indicates the region that was amplified by using primers b and c in Fig. 2A. The thicker part of this line indicates the position of the sequence presented in Fig. 2C. (C) The sequence around the illegitimate recombination junction in intron 3+ of the super αA-crystallin gene is aligned with the 3′ end of intron 3 and the 5′ end of exon 2 of the normal hamster αA-crystallin gene. The switch from the intron 3 to the exon 2 sequence occurs at the CCCAT homology at positions 596–600. Numbers above the sequence correspond to the position in the hamster αA-crystallin intron 3 and exon 4.