The evolution of an alternatively Spliced Exon in the alpha A-Crystallin gene

The evolutionary aspects of alternative splicing, as a mechanism to increas e the diversity of gene products, are poorly understood. Here we analyse th e evolution of a 69-bp exon that is alternatively spliced in the primary tr anscript of the gene for the mammalian eye lens protein alphaA-crpstallin. In rodents, the skipping of this exon 2 is attributed to the presence of a nonconsensus 5' splice site GC, and results in the expression of 10-20% of alphaA(ins)-crystallin, with an insert of 23 residues, as compared with nor mal alphaA-crystallin. alphaA(ins)-crystallin is also expressed in some non -rodent mammals, including kangaroo, while Lacking in others. We now demons trate that the alternatively spliced exon 2 is present in mammals from diff erent orders that do not express alphaA(ins)-crystallin. The expression of this exon has thus been silenced independently in various lineages. Sequenc e comparison in 16 species reveals that-whether or not alphaA(ins)-crystall in is expressed-exon 2 is always flanked by the non-consensus donor splice site GC, while a consensus branch point sequence and 3' pyrimidine-rich reg ion are hardly detectable in the downstream intron. Increased numbers of am ino acid replacements in the peptide encoded by exon 2 indicate that it is subject to much lower selective constraints than the exons that code for no rmal alphaA-crystallin. The absence of any apparent advantage at the protei n level may suggest that exon 2 DNA sequences are conserved as cis-acting f actors for proper splicing of the alphaA-crystallin transcript.