Disulfide cross-linking between T311C, S302C, S287C, S253C, or S232C in DrrA and S23C in DrrB. Cysteine scan of the C terminus of DrrA (residues 232–311) was performed by introducing cysteines at the indicated positions, followed by a disulfide cross-linking assay for each of these cysteine substitution mutants. A and B, lanes 1 and 2, A(Y89C)B(S23C); lanes 3–5, A(S319C)B(S23C); lanes 6–8, A(T311C)B(S23C); lanes 9–11, A(S302C)B(S23C); lanes 12–14, A(S287C)B(S23C); lanes 15–17, A(S253C)B(S23C); lanes 18–20, A(S232C)B(S23C). This experiment was repeated three times.