Reductive methylation of MxiCNΔ73 yielded crystals that diffracted to higher resolution: 2.85 Å and 2.5 Å in space groups P212121 and P222, respectively (Table 1). MxiC is an elongated rod-shaped molecule with a long axis of 86 Å (Fig. 2a). It is composed of three domains, each possessing a four-helix X-bundle fold (Fig. 2b).19 The first and last domains consist only of the X-bundle motif while the central domain also possesses a bent helix (α5) that is packed against domain 1. It is the first two domains of MxiC that are equivalent to YopN while the third domain of MxiC, equivalent to TyeA, is connected to domain 2 via a ten-residue linker. This linker acts only to tether the domains and does not have any major structural role, allowing the equivalent regions of MxiC and YopN–TyeA to adopt similar folds. There are a total of seven independent MxiC molecules in the crystallographic asymmetric units of the three refined crystal forms (see Table 1). These structures reveal that, although the fold of each domain is maintained in all structures (rmsd over Cα atoms of domains 1, 2 and 3 are 0.5 Å, 0.9 Å and 0.8 Å, respectively), there is some flexibility at the interfaces between domains resulting in a “wobble” of the terminal domains about the central domain (rmsd over all Cα atoms of 1.4 Å; Fig. 2c). The elongated shape of MxiC means that the most distal regions undergo the greatest displacement while the more central interdomain interfaces undergo minimal change.