Studies on the specialized transfer RNA population of the lens

Abstract

The transfer RNA populations of bovine lens tissue and bovine brain tissue were compared by reversed-phase chromatography. The differences seen were the same as those reported previously for a comparison of lens and muscle tRNA. This is consistent with the hypothesis that lens tRNA population has become specialized and that these changes may facilitate the rapid synthesis of lens proteins. Comparisons of the tRNA populations obtained from preparations of lens capsule and cortex show that similar tRNA changes can be seen in different regions of the lens. It was shown that there is a specific decrease in a major isoaccepting lysine tRNA (lys-tRNA 2) and a loss of two minor lysine tRNAs (lys-tRNA 1 and lys-tRNA 4). No change was seen, however, in the other major lysine tRNA (lys-tRNA 5). Phenylalanine tRNA from lens cortex had two major isoaccepting species (phe-tRNA 1 and phe-tRNA 2) while capsule tRNA had only one (phe-tRNA 2). These results suggest that specific tRNA modifications occur during the process of cell differentiation in lens tissue. Ribosomal binding experiments with capsule and cortex lysine tRNA showed a two-fold decrease in the ability to bind to the codon AAG with no decrease in the ability to bind to the codon AAA. Similar experiments with lens cortex phe-tRNA 1 and phe-tRNA 2 however, showed no differences in the binding of these tRNAs to either poly (U) or poly(U·C). Both phe-tRNA 1 and phe-tRNA 2 were also equally active in transferring phenylalanine into protein in an in vitro protein-synthesizing system from rat liver. Phe-tRNA 1 therefore, is not an inactive from of phe-tRNA 2 but is a fully functional phe-tRNA which is produced during lens cell differentiation and which accounts for the two-fold increase in phenylalanine acceptance activity seen in lens cortex.