Proteins related to iron acquisition are extremely important in allowing bacterial pathogens to sustain growth in the iron-limited environment of the host. Taking into account that tat mutants in many

bacteria present growth defects under iron-limiting conditions, Mtat was grown in the presence of the iron-chelating agent 2,2′-dipyridyl (Fig. 1). The presence of the iron-chelating agent (0.04–0.2 mM range) resulted in a significant decrease (c. 30%) in the OD600 nm reached by the Mtat mutant as regarding the wild type (P=0.05). Dipyridyl has been described as an effector of some regulators such as Rob (Rosner et al., 2002). In order to confirm that the 17-AAG mw growth impairment of the tat mutant in the presence of this chelator was due to iron limitation and not due to other cellular defects in iron homoeostasis or oxidative

stress defences, the iron chelator EDDHA was Sirolimus cell line also tested. At 2 mM EDDHA, the tat mutant showed a significant reduction of the OD600 nm reached (c. 35%, see Fig. 1). Among the Tat substrates predicted for D. dadantii 3937 in this work, none was specifically related to iron homoeostasis. In Pseudomonas syringae pv. tomato DC3000 and Pseudomonas aeruginosa, several predicted Tat substrates were involved in iron metabolism; notably, tat mutants from these species were unable to use the siderophore pyoverdine due to its inability to export some Tat-dependent proteins involved in pyoverdine biosynthesis and transport (Ochsner et al., 2002; Bronstein et al., 2005; Caldelari et al., 2006). Dickeya dadantii produces two siderophores, chrysobactin and achromobactin (Franza et Galactosylceramidase al., 2005), but none of the predicted Tat-dependent proteins listed in

Table 1 are apparently related to the synthesis or the transport of these siderophores. Consistent with this, we found no significant effect of the tat mutation on siderophore production, as estimated by the halo size on plates containing chromoazurol (Schwyn & Neilands, 1987; data not shown). It is interesting to note that seven out of 44 substrates identified in Table 1 are periplasmic components of ABC transport systems. ABC systems are known as major components of the iron uptake ability of bacteria (Krewulak et al., 2004), and so a role of some of these periplasmic proteins in iron transport cannot be ruled out. Copper resistance in many bacteria is mediated by a number of periplasmic and outer membrane proteins, in particular, multicopper oxidases. Interestingly, D. dadantii 3937 encodes two proteins with plausible Tat signal sequences homologous to multicopper oxidases: CueO and SufI (Table 1). Therefore, we compared the susceptibility to copper of wild-type and Mtat strains (Fig. 2). Both wild-type and Mtat strains grew equally well in KB media containing up to 1 mM CuCl2.