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Background   Shewanella oneidensis  MR-1 exhibits diverse metal ion-reducing capabilities and thus is of potential utility as a bioremediation agent. Knowledge of the molecular components and regulatory mechanisms dictating cellular responses to heavy metal stress, however, remains incomplete. In a previous work, the  S. oneidensis so2426  gene, annotated as a DNA-binding response regulator, was demonstrated to be specifically responsive at both the transcript and protein levels to acute chromate [Cr(VI)] challenge. To delineate the cellular function of SO2426 and its contribution to metal stress response, we integrated genetic and physiological approaches with a genome-wide screen for target gene candidates comprising the SO2426 regulon.    Results  Inactivation of  so2426  by an in-frame deletion resulted in enhanced chromate sensitivity and a reduced capacity to remove extracellular Cr(VI) relative to the parental strain. Time-resolved microarray analysis was used to compare transcriptomic profiles of wild-type and SO2426-deficient mutant  S. oneidensis  under conditions of chromate exposure. In total, 841 genes (18% of the arrayed genome) were up- or downregulated at least twofold in the Δ so2426  mutant for at least one of six time-point conditions. Hierarchical cluster analysis of temporal transcriptional profiles identified a distinct cluster (n = 46) comprised of co-ordinately regulated genes exhibiting significant downregulated expression ( p  < 0.05) over time. Thirteen of these genes encoded proteins associated with transport and binding functions, particularly those involved in Fe transport and homeostasis ( e.g ., siderophore biosynthetic enzymes, TonB-dependent receptors, and the iron-storage protein ferritin). A conserved hypothetical operon ( so1188 - so1189 - so1190 ), previously identified as a potential target of Fur-mediated repression, as well as a putative bicyclomycin resistance gene ( so2280 ) and cation efflux family protein gene ( so2045 ) also were repressed in the  so2426  deletion mutant. Furthermore, the temporal expression profiles of four regulatory genes including a  cpxR  homolog were perturbed in the chromate-challenged mutant.    Conclusion  Our findings suggest a previously unrecognized functional role for the response regulator SO2426 in the activation of genes required for siderophore-mediated Fe acquisition, Fe storage, and other cation transport mechanisms. SO2426 regulatory function is involved at a fundamental molecular level in the linkage between Fe homeostasis and the cellular response to chromate-induced stress in  S. oneidensis .

Transcriptome analysis reveals response regulator SO2426-mediated gene expression in Shewanella oneidensis MR-1 under chromate challenge