Testing the functional importance of the CydX C-terminal amino acids. (A) Alignment of the E. coli CydX protein sequence along with six mutant sequences containing mutated C-terminal amino acid sequences. (B) Assay of CydX function was conducted using a zone assay testing the sensitivity to β-mercaptoethanol. Sensitivity was measured using zones of inhibition, and the diameter of the zone after addition of 10 μL of 12 M β-mercaptoethanol to a plate of bacteria is shown. The average and standard deviation of zone sizes was calculated from at least three replicate plates. Alignments were generated using the program MUSCLE .