Redundant and Flexible Electron Transfer Pathways Underlie <scp>MsrPQ</scp> ‐Mediated Repair of Oxidized Periplasmic Proteins

variants, which lack the FMN cofactor and exhibit altered quinone-binding capacity, respectively, retain full functionality in vivo, albeit with a delayed growth phenotype on methionine sulfoxide. However, we show that Fre becomes necessary for efficient methionine sulfoxide utilization by the MsrPQ system when MsrQ is plasmid-expressed, and that this dependence is further increased when MsrQ is impaired in ubiquinone binding. These observations indicate that the contribution of Fre is conditionally determined by MsrQ abundance. Collectively, our findings reveal that the MsrPQ system operates through a redundant network of electron transfer pathways, with ubiquinone as a central player. This redundancy likely represents an evolutionary adaptation to ensure robust proteostasis even when specific components of the electron transfer chain are compromised.