Sequence comparison of SsTroA with TpTroA, S. pneumonia PsaA and Synechocytis MntC revealed amino acid sequence identities of 37%, 26% and 24%, respectively (Fig. 5) . Although the A-1 SBPs have a similar topological structure, subtle differences in the metal-coordinating residues are observed. For instance, His 205 in SsTroA is replaced by Glu 205 at the homologous position in PsaA and Glu 220 in MntC (Fig. 1B) .