Summary.-Evidence is presented which indicates that S-(1,2,3,4-tetrahydro-2-hydroxy-l-naphthyl)-L-cysteine (THN-cysteine), formed by the reaction of 1,2-epoxy-THN with cysteine, can be incorporated into protein. The position of incorporation of THN-cysteine into protein would depend on whether the epoxide of THN reacts with cysteinyl-tRNACYs or with cysteine. In both cases, the mechanism of incorporation of THN-cysteine into protein is the same as for the natural amino acids. For example, the incorporation of THN-cysteinyl-tRNACYs is stimulated by Poly-UG, the code for tRNACys, and would be expected to be substituted for cysteine in protein being synthesized, whereas THN-cysteine not previously esterified to tRNA is activated by the isoleucyl-and valyl-RNA synthetases, and its incorporation is stimulated by Poly-AU and Poly-UG, respectively. Consequently, in this case, THN-cysteine would substitute for isoleucine and valine during protein synthesis.

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