**2.2.4 Weak dimer interface**

The tight dimer units of the *E. coli* DHDPS tetramer associate via two isologous interfaces formed between corresponding monomers (Fig. 3). This interface buries approximately 538 Å2 of surface area. Nine residues from each monomer are involved in contacts at the weak dimer interface (Mirwaldt et al., 1995), situated within the 6, 7 and 9-helices. The interface is stabilised by hydrophobic contacts between Leu167, Thr168 and Leu197 (Dobson et al., 2004a). The importance of Leu197 at the interface has been demonstrated with mutations resulting in a dimeric species, unable to form a tetramer (Griffin et al., 2008, 2010). This interface is not conserved in other DHDPS structures. A greater number of contacts are observed at the weak dimer interface in DHDPS from *B. anthracis* (Blagova et al., 2006; Voss et al, 2010), *M. tuberculosis* (Kefala et al., 2008), and most strikingly, *T. maritima* (Pearce et al., 2006) with 20 residues involved in many interactions.
