Gene sharing by delta-crystallin and argininosuccinate lyase.

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Abstract
The lens structural protein .delta.-crystallin and the metabolic enzyme argininosuccinate lyase (ASL; L-argininosuccinate arginin-lyase, EC 4.3.2.1) have striking sequence similarity. We have demonstrated that duck .delta. crystallin has enormously high ASL activity, while chicken .delta.-crystallin has lower but significant activity. The lenses of these birds had much greater ASL activity than other tissues, suggesting that ASL is being expressed at unusally high levels as a structural component. In Southern blots of human genomic DNA, chicken .delta.1-crystallin cDNA hybridized only to the human ASL gene; moreover, the two-chicken .delta.-crystallin genes accounted for all the sequences in the chicken genome able to cross-hybridize with a human ASL cDNA, with preferential hybridization to the .delta.2 gene. Correlations of enzymatic activity and recent data on mRNA levels in the chicken lens suggest that ASL activity depends on expression of the .delta.2-crystallin gene. The data indicate that the same gene, at least in ducks, encodes two different functions, an enzyme (ASL) and a structural protein (.delta.-crystallin), although in chickens specialization and separation of functions may have occurred.