Comparative structural analysis of HipBA systems. (a) Structure-guided sequence alignment of H. influenzae HipAN with the N-terminal parts of other HipAs. The secondary structural organization of H. influenzae HipAN is displayed on the alignment. Conserved residues are highlighted in red and yellow. (b) Structural comparison of H. influenzae HipAN and HipBA systems from other organisms. Residues showing high conservation are marked in the overlay. HipBA binding interfaces of E. coli and S. oneidensis are identified in circles and enlarged in squares. (c) Structural comparison of HipB antitoxins from E. coli O127:H6 (PDB code: 7ab3) and S. oneidensis (PDB code: 4pu3) using E. coli HipB antitoxin bound to the DNA (PDB code: 5k98) structure as model.