Specificity of lens initiator tRNA for N-terminal recognition.

The optimal magnesium ion concentration for chain initiation in a cell-free system derived from bovine eye lens which synthesizes 4 classes of crystallins appears to be 5 mM. In the synthesis of α-crystallin polypeptides which contain one internal methionine residue and the second one in N-terminal position, Met-RNA(fMet) functions exclusively as initiator. On the other hand at 5 mM Mg(2+) Met-tRNA(Met) inserts its methionine into the internal position. However, at higher magnesium ion concentrations the initiator tRNA also donates methionine for chain elongation while at the same time the cell-free system loses its capacity to initiate new polypeptides.