K ⁺ -activated pump current with unnatural amino acids at position 780. (A) Mean 10 mM K⁺-induced current in oocytes injected with Y780tag cRNA and tRNAs for tyrosine (NSS-Tyr), phenylalanine (NSS-Phe), and other phenylalanine derivatives. The label pdCpA designates the full-length ligated (but unacylated) tRNA control (see Materials and methods). (B) Mean K_0.5.K for substitutions with large enough pump currents. Grey circles show individual data points in different oocytes. The chemical structure of the amino acids is shown on top of the bars and the error bars represent the SEM.