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Background   S. oneidensis  MR-1 is a dissimilatory metal-reducing bacterium. Under anoxic conditions  S. oneidensis  MR-1 attaches to and uses insoluble minerals such as Fe(III) and Mn(IV) oxides as electron acceptors. In the laboratory,  S. oneidensis  MR-1 forms biofilms under hydrodynamic flow conditions on a borosilicate glass surface; formation of biofilms was previously found to be dependent on the  mxd  gene cluster ( mxdABCD ).    Results  This study revealed environmental and genetic factors regulating expression of the  mxd  genes in  S. oneidensis  MR-1. Physiological experiments conducted with a  S. oneidensis  MR-1 strain carrying a transcriptional  lacZ  fusion to the  mxd  promoter identified electron donor starvation as a key factor inducing  mxd  gene expression. Tn 5  mutagenesis identified the ArcS/ArcA two-component signaling system as a repressor of  mxd  expression in  S. oneidensis  MR-1 under planktonic conditions. Biofilms of ∆ arcS  and ∆ arcA  strains carrying a transcriptional  gfp  -reporter fused to the  mxd  promoter revealed a reduced  mxd  expression, suggesting that ArcS/ArcA are necessary for activation of  mxd  expression under biofilm conditions. Biofilms of ∆ arcS  and ∆ arcA  mutants were unable to form a compact three-dimensional structure consistent with a low level of  mxd  expression. In addition, BarA/UvrY was identified as a major regulator of  mxd  expression under planktonic conditions. Interestingly, biofilms of ∆ barA  and ∆ uvrY  mutants were able to form three-dimensional structures that were, however, less compact compared to wild type biofilms.    Conclusions  We have shown here that the  mxd  genes in  S. oneidensis  MR-1 are controlled transcriptionally in response to carbon starvation and by the ArcS/ArcA and the BarA/UvrY signaling system. BarA might function as a sensor to assess the metabolic state of the cell, including carbon starvation, leading to expression of the  mxd  operon and therefore control biofilm formation.

The mxd operon in Shewanella oneidensis MR-1 is induced in response to starvation and regulated by ArcS/ArcA and BarA/UvrY