Messenger RNA for cataractous lens proteins are also present on normal lens polyribosomes.

We have previously described an experimental model in vivo where cataract is induced by injection of the antimitotic bleomycin in the newborn rat. The first opacity of the lens appears 12–15 days after the injection of the drug concomitantly with a group of precise biochemical modifications among the soluble crystallins. These modifications are mainly the accumulation of two additional low-molecular-weight βLcrystallin subunits and of several smaller α-crystallin polypeptides. Messenger RNA isolated from normal and cataractous lenses was assayed for translation in it cell-free wheat germ extract. Analysis of the translation products by one-dimensional and twodimensional gel electrophoresis indicated that the messenger RNAs coding for the two βL subunits are also present on normal lens polyribosomes. Purification and subsequent analysis by peptide mapping of the cataractous βL subunits suggest that they are precursor polypeptides of the normal low-molecular-weight β-crystalline, which are no longer processed after translation and therefore accumulate in the pathological lens cell. On the other hand, in the translation of the cataractous mRNA in vitro, only the normal a chains are detected. These cataractous α-crystalline are therefore post-translational degradation products of the normal α polypeptides. This phenomenon seems similar to the one observed in the senescent lens. The possible involvement of these modifications in the etiology of this experimental cataract is discussed further.