Autoradiogram of a 15% SDS-polyacrylamide gel (100 V, 2 h) illustrating the incorporation of tryptophan analogues into positions 22 (upper panel), 30 (middle panel) and 49 (lower panel) of DHFR. Lane 1, wild-type DHFR expression; lane 2, modified DHFR DNA in the presence of abbreviated suppressor tRNACUA-COH; lane 3, incorporation of amino acid A; lane 4, incorporation of amino acid B; lane 5, incorporation of amino acid C; lane 6, incorporation of amino acid D; lane 7, incorporation of amino acid E; lane 8, incorporation of amino acid F. Phosphorimager analysis was performed using an Amersham Biosciences Storm 820 equipped with ImageQuant version 5.2 software from Molecular Dynamics.