Concerted and divergent evolution within the rat gamma-crystallin gene family

The nucleotide sequences of six rat gamma-crystallin genes have been determined. All genes have the same mosaic structure: the first exons contain a relatively short (25 to 44 base-pair) 5' non-coding region and the first nine base-pairs of the coding sequence, the second exons encode protein motifs I and II, while protein motifs III and IV are encoded by the third exons. The third exons also contain a 60 to 67-base-pair long 3' non-coding region. In the gamma 1-2 gene, the splice acceptor site of the third exon has been shifted three base-pairs upstream. Hence, the protein product of this gene is one amino acid residue longer. The first introns, though varying in length from 85 to 100 base-pairs, are conserved in sequence. The second introns vary considerably in length (0.9 X 10(3) to 1.9 X 10(3) base-pairs) and sequence. The second exons of the genes show concerted evolution and have undergone multiple gene conversions. In contrast, the third exons show divergent evolution. From the sequences of the third exons, an evolutionary tree of the gene family was constructed. This tree suggests that three of the present genes derive directly from the genes that originated from a tandem duplication of a two-gene cluster. Two duplications of the last gene of the four-gene cluster then yielded the other three genes. Region a' of the third exon, encoding protein motif III, is variable, while the region encoding protein motif IV (b') is constant. We postulate that this variability in region a' is due to a period of radiation after each gene duplication. A comparison of the rat sequences with those of orthologous sequences from other species shows that the variation in region a' is now preserved. Hence, it might specify the specific functional property of each gamma-crystallin protein within the lens.