A unique family of ribonucleases was identified by exhaustive screening of genomic and cDNA libraries using a probe derived from a gene encoding a ribonuclease stored in the mouse eosinophil secondary granule. This family contains at least 13 genes, which encode ribonucleases, and two potential pseudogenes. The conserved sequence identity among these genes (∼70%), as well as the isolation/purification of these ribonucleases from eosinophil secondary granules, has led us to conclude that these genes form a unique clade in the mouse that we have identified as the Ear (Eosinophil-associated ribonuclease) gene family. Analyses of the nucleotide substitutions that have occurred among these ribonuclease genes reveal that duplication events within this family have been episodic, occurring within three unique periods during the past 18 × 106 years. Moreover, comparisons of nonsynonymous (Ka) vs. synonymous (Ks) rates of nucleotide substitution show that although these genes conserve residues necessary for RNase activity, selective evolutionary pressure(s) exist such that acquired amino acid changes appear to be advantageous. The selective advantage of these amino acid changes is currently unclear, but the occurrence of this phenomenon in both the mouse and the human highlights the importance of these changes for Ear and, therefore, eosinophil effector function(s).
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