Sequence alignment of colicin Ia, Ib, E1, and K hydrophobic segments. Charged residues are shown in boldface. The α-helices from the crystal structures of colicins Ia (Wiener et al., 1997) and E1 (Elkins et al., 1997) are underlined. Residue numbers for colicin Ia are shown. (For the convenience of the reader, these may be converted to colicin E1, K, or Ib numbers by subtracting 104, 78, or 0, respectively.) Plus signs (+) mark the positions in the colicin Ia hydrophobic segment at which one of the other colicins has a charged residue; those residues in colicin Ia are T573, I574, G577, N578, and A582. Position 609, at which colicin E1 has a cysteine residue, is marked by a triangle. The CLUSTAL W algorithm (Thompson et al., 1994) was used to make the alignment; these four colicins are well aligned throughout their C domains, with no gaps required, starting from colicin Ia residue K450. Sequences are from the designated references: E1 (Yamada et al., 1982), Ib (Varley and Boulnois, 1984), Ia (Mankovich et al., 1986), and K (Pilsl and Braun, 1995).