The addition of 4-amino-4-deoxy-L-arabinose to lipid A decreases the negative charge of LPS, which has been demonstrated to increase the resistance of Salmonella to cationic antimicrobial peptides and also to Fe3+ and Al3+ [17, 18]. Analogously, we consider

that the impact of PP0033 and PP0034 in metal tolerance may rely on their ability to modify LPS. Notably, there is another gene in the ColR regulon, which can putatively decrease the negative charge of cell surface by LPS modification. The ColR-activated PP2579 encodes a protein homologous to CptA phosphotransferase, which catalyzes the phosphoethanolamine addition to the LPS core [57]. Interestingly, genes responsible for the addition of 4-amino-4-deoxy-L-arabinose selleck chemical and phosphoethanolamine to LPS in Salmonella are regulated by the PmrAB two-component system [57]–[59], which, like ColRS, responds to external iron [16]. This suggests that the mechanism how ColRS system impacts the metal tolerance of P. putida partly resembles that of PmrAB, where modification of LPS plays a major role in protecting bacteria from metal toxicity [18, 60, 61]. However, we want to emphasize that the effect of PP0035-PP0033 and PP2579 in metal tolerance is rather low and that the ColR-controlled metal tolerance is actually provided by the joint action of the whole regulon. Several signaling systems which regulate

bacterial response to external metals are induced by the same environmental cue they respond to. For example, expression

of pmrAB in Salmonella is induced by iron, basSR in E. coli is induced by iron and zinc, bqsRS and czcRS in P. aeruginosa are upregulated by iron and cadmium, MK2206 respectively [16, 26, 45, 46]. Differently from these systems, the expression of colRS is not affected by metals and the ColRS-promoted response to metal excess only involves activation of the signal transduction between the system counterparts and the resulting changes in the expression of the ColR regulon genes. This suggests that the basal constitutive expression level Selleckchem Rucaparib of the colRS operon is sufficient to guarantee an appropriate response to metal stress. Mutational analysis of ColS indicates that a conserved ExxE motif of the periplasmic loop of the sensor kinase is required for sensing both iron and zinc, because substitution of either of the conserved glutamic acid residues in this motif abolishes the ability of ColS to respond to both metals and to promote the activation of the ColR regulon (Figure 6). The ExxE motif has been demonstrated to bind iron in several eukaryotic and prokaryotic proteins, including, for instance, the iron transporter FTR1 in Saccharomyces cerevisiae [48], the iron sensor PmrA in Salmonella enterica [16], the iron- and heme-binding HbpS in Streptomyces reticuli [49]. Interestingly, as far as we know, there are no previous reports demonstrating that the iron-binding ExxE motif could also bind zinc.