(a) Bioinformatic analysis of SpaO. PSIPRED secondary-structure predictions and sequence homology suggest the presence of two putative SPOA domains in SpaO. Probability of helical character is plotted in red, strand in blue and disorder in yellow. The arrow at codon 203 represents a predicted ValGTG internal translation start site, as has been shown for YscQ Met218 in Yersinia pseudotuberculosis5. (b,c) Comparison of homotypic SPOA2–SPOA2 and heterotypic SPOA1–SPOA2 structures from SpaO. (b) Ribbon diagrams show the similar organization of secondary-structural elements in both SPOAs. Asterisks denote the antiparallel beta-sheet ‘floor’, (c) Amino (blue) to carboxy terminus (red) Cα traces of SPOA2 (top) and SPOA1 (bottom) reveal a similar topology in interaction with SPOA2 (grey surface representation, top and bottom).