Different evolution rates within the lens-specificβ-crystallin gene family

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Abstract
We have determined the sequence of a rat βA3/A1-crystallin complementary DNA (cDNA) clone and the (partial) sequence of the human βB3-crystallin gene. Calculation of the ratio of silent to nonsynonymous substitution between orthologous βA3/A1-, βB3-, and other β- and γ-crystallin sequences revealed that the region encoding the two globular domains of the βA3/A1-crystallin sequence is the best conserved during evolution, much better than the corresponding region of the βB1-, βB3-, or the γ-crystallin sequences, and even better (at least in the rodent/frog comparison) that the well-conserved αA-crystallin sequence. Remarkably, the rate of change of the βA3/A1-crystallin coding sequence does not differ in the rodent and primate lineages, in contrast with previous findings concerning the evolution rates of the αA- or γ-crystallin sequences in these two lineages. Comparison of the regions that encode the four motifs of the β-crystallin between orthologous mammalian sequences showed that the extent of nonsynonymous substitution in each of these four homologous motif regions is the same. However, when the orthologous β-crystallin genes of more distantly related species (mammals vs chicken or frog) are compared, the extent of nonsynonymous substitution is higher in the regions encoding the external motifs I and III than in the regions encoding the internal motifs II and IV. This phenomenon is also observed when paralogous members of the β/γ-crystallin supergene family are compared.