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Construction and calibration of the pheS* counterselection marker. (A) Amino acid multiple-sequence alignment of PheS protein C termini, achieved with Clustal Omega. The sequences of the indicated species were aligned, and the conserved alanine corresponding to A294 of E. coli was identified (arrow); on the right is the number of amino acids in the protein. (B) Growth of the indicated strains on BHI agar plates with or without 18 mM p-Cl-phe.
