Evolution of the single copy alpha A-crystallin gene: differently sized mRNAs of mammals and birds show homology in their 3' non-coding regions.

alpha A-Crystallin, a major structural polypeptide of the vertebrate eye lens, is evolutionarily highly conserved. We have analyzed the corresponding nucleic acid sequences in both genomic DNA digests as well as in lens cytoplasmic RNA preparations from a wide variety of vertebrates by blot hybridization with cloned rat alpha A2-crystallin cDNA probes. The probes are not able to hybridize under any conditions to RNA and DNA derived from fishes and amphibia, but do show substantial homology with the sequences of mammals, birds and reptiles. The alpha A-crystallin gene, which has been isolated from a hamster gene library occurs only once in the haploid genome. Coding and 3'-untranslated regions of alpha A2-crystallin mRNA are conserved among all mammals and birds examined. However, the regions comprising the conserved sequences are differently represented in the ultimate mRNA. The alpha A2-mRNA 3'-non-coding regions of reptiles and birds are 300-550 bases longer than those of mammals. Some rodents produce next to the alpha A2-mRNA another messenger that encodes the alpha AIns-polypeptide possessing an insertion of 22/23 amino acid residues between positions 63 and 64 of the alpha A2-polypeptide chain. alpha A2 and alpha AIns-mRNA are generated from a single gene as major and minor species, respectively, in a proportion which is similar to the ratio of the polypeptides found in vivo and in vitro. The size heterogeneity of the alpha A2-mRNA from most mammals examined is due to the variable size of the poly(A) tail.