Molecular mechanisms for sensing and responding to electrode potentials in Shewanella oneidensis MR-1
1Tokyo Univ. Pharm. Life Sci.
Shewanella oneidensis MR-1 has been studied as a model organism to elucidate how microbes generate electricity in bioelectrochemical systems (BESs). Previous studies have shown that potentials of working electrodes affect catabolic activities of MR-1. However, it was yet undiscovered how MR-1 senses and responds to electrode potentials at the molecular levels. In the present study, transcriptomic responses of MR-1 were compared under different electrode potentials to identify genes that are affected by electrode potentials. In addition, transcriptomic responses of the wild type (WT) and arcS-deletion mutant (ΔarcS) were compared to verify the involvement of the Arc system (ArcS/HptA/ArcA in MR-1), an intracellular redox-sensing system, in the electrode-potential sensing by MR-1.
WT and ΔarcS were cultivated in three-electrode BESs that was connected to a potentiostat to measure current at a given electrode potential. In transcriptomic analyses, bacterial cells were recovered from the surface of working electrodes, and RNA was extracted from these cells and subjected to DNA microarrays for MR-1. Genes whose expression was significantly different (p was smaller than 0.05, fold change was greater than 2) were selected.
Transcriptomic profiles of WT at +0.3 V and -0.3 V were compared. It was found that the expression of 414 genes is electrode-potential dependent. Among them, potential-dependent transcriptional shifts of 382 genes (including nuo encoding NADH/ubiquinone oxidoreductase) were substantially diminished in ΔarcS. Since the Arc system is known to sense the redox state of membrane quinones, we were interested in the regulation of nuo by the Arc system, and a gel-shift assay confirmed that phosphorylated ArcA binds to an upstream region of the nuo genes. We suggest that Nuo serves as a gate of electrons that is regulated by the electrode potential via membrane quinones and the Arc system.
keywords:bioelectrochemistry,signal transduction,transcriptomics,extracellular electron transfer