Unraveling the enigmatic dance of electrons: extracellular electron transfer and sensing via periplasmic cytochromes

Abstract

Geobacter species are a remarkable group of bacteria known for their unique ability to transfer electrons outside of the cell through a process referred as extracellular electron transfer. This pathway involves the transfer of electrons from cells to external electron acceptors, such as metals or electrodes. What enables this process is a series of electron transfer events via multi-heme cytochromes connecting the inner membrane, periplasm, outer membrane, and the surface of cells to a final electron acceptor. Significant progress has been achieved in understanding the structural, functional, and regulatory aspects of the Geobacter extracellular electron transfer system. However, there are still some mysteries unresolved.This dissertation focuses on periplasmic electron transfer, especially through the Ppc-family cytochromes which consist of five paralogous triheme c-type cytochromes. Through genetics, we show that Geobacter periplasmic electron transfer lacks specificity and can utilize any Ppc-family cytochrome when reducing Fe(III). Further, we identify PgcA, a previously described extracellular cytochrome, as being able to participate in periplasmic electron transfer. Interestingly, when reducing electrodes as an electron acceptor, a strain lacking all five paralogs and pgcA only shows a 10% defect. This suggests the possibility that unidentified periplasmic cytochromes could be used in electrode settings. This dissertation also reveals the role of the BccRS two-component regulatory network in activating transcription of cbcAB, needed for reducing Fe(III) below -0.21 V vs. SHE (Standard Hydrogen Electrode). According to this model, BccS is sensor kinase that senses its environmental signal(s) and phosphorylates BccR, a DNA-binding response regulator, causing a cascade of a gene expression. Further, cbcAB promoter fusion data and transcriptomes of mutants confirm that cbcAB expression requires both BccS and BccR, supporting this model. Downstream of the cbcAB operon is cbcD, a homolog of the BccS sensor that lacks a kinase domain. We find that cbcD, is required for full reduction of Fe(III) citrate. However, cbcD is not involved in the regulation of cbcAB operon. Together, this work expands our understanding of how Geobacter perform extracellular electron transfer, through periplasmic electron transfer and utilize periplasmic sensing for gene expression to maximize utilization of available electron acceptors in the environment.