BarA/UvrY functions as an activator of the mxd genes under planktonic growth conditions and has a role in the regulation of biofilm formation We showed here that BarA/UvrY activates mxd expression under organic rich medium conditions when planktonic cells entered stationary phase (Figure 7). BarA/UvrY is highly conserved in Gram-negative bacteria, and controls a variety XAV-939 of physiological functions including carbon storage [26–30]. In carbon storage regulation (Csr) BarA/UvrY

regulates small RNAs controlling elements of this pathway, which are major posttranscriptional regulators of biofilm formation in E. coli[31]. The stimuli for the BarA sensor histidine kinase in E. coli are aliphatic carboxylic acids, such as formate, acetate, propionate and others, Kinase Inhibitor Library ic50 providing a physiological signal reflecting the metabolic state of cells and thereby linking posttranscriptional

control by the Csr system with central metabolism [30]. Interestingly, S. oneidensis MR-1 biofilms of both ∆barA and ∆uvrY mutants formed less compact biofilms when grown under hydrodynamic flow conditions. Based on these data and the above discussed findings that low carbon concentration induces mxd expression, we hypothesize that BarA might function as a sensor for carbon starvation, e.g., at high cell density when nutrients become growth limiting in planktonic culture. We hypothesize that under these conditions starvation-sensing BarA signals to UvrY, Urease which, in return, directly or indirectly activates mxd expression and, by this cascade, controls biofilm formation. Homologous of BarA/UvrY have been shown to control secondary metabolism, including the excretion of biofilm exopolysaccacharides in other γ-proteobacteria [32–36]. In the closely related bacterium Pseudomonas fluorescens production of several antibiotic-like secondary metabolites is regulated by the orthologs GacA/GacS and via the small RNAs RsmXYZ [37]. In P. fluorescens expression of these small RNAs was found to be positively controlled by GacS/GacA at high cell

density and intermediates of central metabolism such as 2-oxoglutarate, succinate and fumarate which may be present at elevated intracellular concentration under conditions when cells are electron acceptor-limited [37]. It is conceivable that S. oneidensis MR-1, similar to P. fluorescens, senses its metabolic state at the level of primary metabolites, and uses the level to control aspects of secondary metabolism including biofilm formation. The BarA/UvrY system and its components have been studied to some extent in S. oneidensis MR-1 [23]. It was found to contain all major components of the BarA/UvrY/Csr pathway. UvrY in S. oneidensis MR-1 positively regulates the two small RNAs, csrB1 and csrB2 and a corresponding CsrA ortholog was also identified.