Sequence analysis of pigeon δ‐crystallin gene and its deduced primary structure Comparison of avian δ‐crystallins with and without endogenous argininosuccinate lyase activity

Abstract

δ-Crystallin is a major lens protein present in the avian and reptilian lenses. To facilitate the cloning of the δ-crystallin gene, cDNA was constructed from the poly(A)⁺ RNA of pigeon lenses, amplified by the polymerase chain reaction (PCR). The PCR product was then subcloned into pUC19 vector and transformed into E. coli strain JM109. Plasmids purified from the positive clones were prepared for nucleotide sequencing by the dideoxynucleotide chain-termination method. Sequencing two clones, containing 1.4 kb DNA inserts coding for δ-crystallin allowed the construction of a complete, full-length reading frame of 1,417 bp covering a deduced protein sequence of 466 amino acids, including the universal translation-initiating methionine. The pigeon δ-crystallin shows 88, 83 and 69% sequence identity to duck δ2, chicken δ1 crystallins and human argininosuccinate lyase respectively. It is also shown that, in contrast to duck δ2 crystallin which has a high argininosuccinate lyase activity, pigeon δ-crystallin appears to contain very low activity of this enzyme, despite the fact that they share a highly homologous structure. A structural comparison of δ-crystallins with or without enzymatic activity suggested several amino acid replacements which may account for the loss of argininosuccinate lyase activity in the lenses of certain avian species.