Sequence analysis of one major basic β-crystallin (β-Bp) of amphibian lenses: Evolutionary comparison and phylogenetic relatedness between β-crystallin and γ-crystallin

βBp-Crystailin a major basic β-crystallin of vertebrate eye lens, is developmentally regulated during the process of amphibian metamorphosis. In order to facilitate the determination of the primary sequence of this ubiquitous crystallin present in all vertebrate species, cDNA mixture was synthesized from the poly(A)⁺ mRNA isolated from bullfrog eye lenses. A protocol of Rapid Amplification of cDNA Ends (RACE) was used to amplify cDNAs encoding βBp-crystallin by polymerase chain reaction (PCR). PCR-amplified product corresponding to βBp-crystallin was then ligated into pGEM-T vector and then transformed into E. coli strain JM109. One complete full-length reading frame of 615 base pairs, which covers a deduced protein sequence of 205 amino acids, including the universal initiating methionine, was revealed by automatic nucleotide sequencing with a fluorescence-based dideoxynucleotide chain-termination method. It shows 83, 74, 78 and 80 percent sequence similarity to the homologous β₂ crystallins of chicken, rat, bovine, and human species, respectively, revealing the close structural relationship among βBp-crystallins even from remotely related species. In this study phylogenetic trees based on nucleotide and protein sequences of various β- and γ-crystallins from different vertebrate classes are constructed using a combination of distance matrix and approximate parsimony methods. which corroborate the previous supposition that β- and γ-crystallins form a superfamily with a common ancestry.