The crystallin gene families.

Recent work from our laboratory on the structure and the genetic organization of the lens beta- and gamma-crystallin gene families is reviewed briefly. In the rat six different gamma-crystallin genes are present which all have an identical distribution of exons and introns, namely a small intron after the third translation codon and a larger one within the coding region for the connecting peptide which links the two domains of the gamma-crystallins. We find five rat genes physically linked and located on a DNA segment of only 50 kilobases, whereas the sixth gene is more distant. The polypeptide sequences, as deduced from DNA sequence analysis, of these six rat and two human gamma-crystallin genes are compared and discussed in terms of structural and evolutionary aspects. The gene coding for rat beta B1a-crystallin appears to be a single-copy gene of much larger size than the gamma-crystallin genes. The beta B1 gene is not physically linked to the other beta-crystallin genes, even though the various beta genes are evolutionarily related and in that sense constitute a gene family. In contrast to the gamma-crystallin genes, the beta B1 gene has an intron not only between the domain sequence but also between the motif sequences. In addition, the exon coding for the N-terminal extension of the protein is separated by an intron from the first protein motif sequence. We anticipate that structural and genetic investigations on lens crystallin genes and their expression might provide a framework for revealing the basis of (some) hereditary disorders in the visual system.