Unlike other bacteria that use FNR to regulate anaerobic respiration, Shewanella oneidensis MR-1 uses the cyclic AMP receptor protein (CRP) for this purpose. Three putative genes, cyaA, cyaB, and cyaC, predicted to encode class I, class IV, and class III adenylate cyclases, respectively, have been identified in the genome sequence of this bacterium. Functional validation through complementation of an Escherichia coli cya mutant confirmed that these genes encode proteins with adenylate cyclase activities. Chromosomal deletion of either cyaA or cyaB did not affect anaerobic respiration with fumarate, dimethyl sulfoxide (DMSO), or Fe(III), whereas deletion of cyaC caused deficiencies in respiration with DMSO and Fe(III) and, to a lesser extent, with fumarate. A phenotype similar to that of a crp mutant, which lacks the ability to grow anaerobically with DMSO, fumarate, and Fe(III), was obtained when both cyaA and cyaC were deleted. Microarray analysis of gene expression in the crp and cyaC mutants revealed the involvement of both genes in the regulation of key respiratory pathways, such as DMSO, fumarate, and Fe(III) reduction. Additionally, several genes associated with plasmid replication, flagellum biosynthesis, and electron transport were differentially expressed in the cyaC mutant but not in the crp mutant. Our results indicated that CyaC plays a major role in regulating anaerobic respiration and may contribute to additional signaling pathways independent of CRP.Shewanella oneidensis MR-1 is a metal reducer that uses more than 14 terminal electron acceptors for respiration. These electron acceptors include oxygen, nitrate, fumarate, dimethyl sulfoxide (DMSO), Fe(III) oxides, uranium, and chromium (21,25,26). In Escherichia coli and other bacteria, the shift from aerobic respiration to anaerobic respiration requires activation of the global transcriptional regulator FNR (11,34). FNR is an oxygen-sensing protein that is activated under anaerobic conditions by the formation of a [4Fe-4S] cluster (14). The S. oneidensis FNR homolog, EtrA, complements an E. coli FNR mutant (27) but does not appear to have the same role as the E. coli protein in S. oneidensis (17). Our previous findings demonstrated that instead of EtrA, the cyclic AMP (cAMP) receptor protein (CRP) controls anaerobic respiration in S. oneidensis MR-1 (28). crp mutants are deficient in anaerobic respiration of Fe(III), Mn(IV), fumarate, nitrate, and DMSO. Furthermore, fumarate, DMSO, and nitrate reductase activities are either severely decreased or undetectable in the crp mutants, suggesting that CRP regulates the expression of these anaerobic reductases (28).Although genetic and phenotypic data clearly have implicated CRP in the activation of anaerobic reductase systems in S. oneidensis MR-1, the mechanisms of this regulation remain unclear. CRP lacks obvious redox-sensing domains and is not expected to respond to changes in oxygen concentrations like FNR. Complementation of the S. oneidensis crp mutants with E. coli crp indicates that CRP is activa...

The ability of the metal reducer Shewanella oneidensis MR-1 to generate electricity in microbial fuel cells (MFCs) depends on the activity of a predicted type IV prepilin peptidase; PilD. Analysis of an S. oneidensis MR-1 pilD mutant indicated that it was deficient in pili production (Msh and type IV) and type II secretion (T2S). The requirement for T2S in metal reduction has been previously identified, but the role of pili remains largely unexplored. To define the role of type IV or Msh pili in electron transfer, mutants that lack one or both pilus biogenesis systems were generated and analyzed; a mutant that lacked flagella was also constructed and tested. All mutants were able to reduce insoluble Fe(III) and to generate current in MFCs, in contrast to the T2S mutant that is deficient in both processes. Our results show that loss of metal reduction in a PilD mutant is due to a T2S deficiency, and therefore the absence of c cytochromes from the outer surface of MR-1 cells, and not the loss of pili or flagella. Furthermore, MR-1 mutants deficient in type IV pili or flagella generated more current than the wild type, even though extracellular riboflavin levels were similar in all strains. This enhanced current generating ability is in contrast to a mutant that lacks the outer membrane c cytochromes, MtrC and OmcA. This mutant generated significantly less current than the wild type in an MFC and was unable to reduce Fe(III). These results indicated that although nanofilaments and soluble mediators may play a role in electron transfer, surface exposure of outer membrane c cytochromes was the determining factor in extracellular electron transfer in S. oneidensis MR-1.

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