In vitro incorporation of thiothreonine derivatives into csDHFR at position 10. The samples were analyzed by 15% SDS-PAGE at 100 V for 2 h. Phosphorimager analysis was performed using an Amersham Biosciences Storm 820 equipped with ImageQuant version 5.2 software from Molecular Dynamics. Lane 1, wild-type csDHFR; lane 2, incorporation in the absence of any amino acid; lane 3, incorporation of DMT-thiothreonine; lane 4, incorporation of DMT-allo-thiothreonine; lane 5, incorporation of thiothreonine; lane 6, incorporation of allo-thiothreonine; lane 7, incorporation of CPM-allo-thiothreonine.