Integration host factor (IHF) is a widely distributed small heterodimeric protein member of the bacterial Nucleoid-Associated Proteins (NAPs), implicated in multiple DNA regulatory processes. IHF recognizes a specific DNA sequence and induces a large bend of the nucleic acid. IHF function has been mainly linked with the regulation of RpoN-dependent promoters, where IHF commonly recognizes a DNA sequence between the enhancer-binding region and the promoter, facilitating a close contact between the upstream bound activator and the promoter bound, RNA polymerase. In most proteobacteria, the genes encoding IHF subunits (ihfA and ihfB) are found in a single copy. However, in some Deltaproteobacteria, like Geobacter sulfurreducens, those genes are duplicated. To date, the functionality of IHF reiterated encoding genes is unknown. In this work, we achieved the functional characterization of the ihfA-1, ihfA-2, ihfB-1, and ihfB-2 from G. sulfurreducens. Unlike the ΔihfA-2 or ΔihfB-1 strains, single gene deletion in ihfA-1 or ihfB-2, provokes an impairment in fumarate and Fe(III) citrate reduction. Accordingly, sqRT-PCR experiments showed that ihfA-1 and ihfB-2 were expressed at higher levels than ihfA-2 and ihfB-1. In addition, RNA-Seq analysis of the ΔihfA-1 and ΔihfB-2 strains revealed a total of 89 and 122 differentially expressed genes, respectively. Furthermore, transcriptional changes in 25 genes were shared in both mutant strains. Among these genes, we confirmed the upregulation of the pilA-repressor, GSU1771, and downregulation of the triheme-cytochrome (pgcA) and the aconitate hydratase (acnA) genes by RT-qPCR. EMSA experiments also demonstrated the direct binding of IHF to the upstream promoter regions of GSU1771, pgcA and acnA. PilA changes in ΔihfA-1 and ΔihfB-2 strains were also verified by immunoblotting. Additionally, heme-staining of subcellular fractions in ΔihfA-1 and ΔihfB-2 strains revealed a remarkable deficit of c-type cytochromes. Overall, our data indicate that at least during fumarate and Fe(III) citrate reduction, the functional IHF regulator is likely assembled by the products of ihfA-1 and ihfB-2. Also, a role of IHF controlling expression of multiple genes (other than RpoN-dependent) affects G. sulfurreducens physiology and extracellular electron transfer.

BACKGROUND: Microbial fuel cell (MFC) power production is limited by the cathodic oxygen reduction reaction (ORR). The quest for platinum-free materials for improving the cathodic ORR in MFCs is a challenging task. Birnessite-type MnO 2 /carbon cathodes in MFCs have been rarely reported so far. In this work, a birnessite/C cathode was tested in a MFC. RESULTS: A birnessite/C cathode was synthesized (using a rapid, facile and low-cost method) and its structural and morphological characteristics were assessed. The ORR on such a cathode was investigated using linear sweep voltammetry in a catholyte of 0.8 mol L-1 Na 2 SO 4 , pH 2 (same conditions as those of experimental MFCs). The current density values were higher than those for a plain C cathode. The volumetric power density improved from 224 to 6201 mW m −3 on replacing the plain C cathode with the birnessite/C cathode in a MFC. Deltaproteobacteria were found at the C anode and they were associated with the high power densities obtained. CONCLUSIONS: Birnessite/C is a promising cathodic electrode because its synthesis is straightforward, it is not expensive and it could promote the ORR and improve the power density in MFCs.