Cleavage cascade of the sigma regulator FecR orchestrates TonB-dependent signal transduction

Abstract

TonB-dependent signal transduction is a versatile mechanism observed in gram-negative bacteria, integrating energy-dependent substrate transport with signal relay. In Escherichia coli, the TonB-ExbBD motor complex energizes the TonB-dependent transporter FecA, facilitating ferric citrate import. FecA also functions as a sensor, transmitting signals to the cytoplasmic membrane protein FecR. We previously demonstrated that FecR undergoes a three-step cleavage process, culminating in the activation of the cytoplasmic sigma factor FecI, which drives fec gene transcription. Here, we describe the complete mechanism of FecR cleavage-mediated ferric citrate signaling involving FecA and TonB. The cleavage cascade begins with FecR autoproteolysis prior to membrane integration. The soluble C-terminal domain (CTD) fragment of FecR is co-translocated with the N-terminal domain (NTD) fragment through a Tat system-mediated process. In the periplasm, the interaction between the CTD and NTD fragments prevents further cleavage. This inhibition is lifted by TonB-mediated motor function, which releases the CTD, allowing the cleavage cascade to proceed. This process is essential for ferric citrate signal-induced activation of fec gene expression. Our findings reveal that the regulation of FecR cleavage, relying on the TonB-FecA axis, plays a central role in bacterial response to ferric citrate signals. Significance StatementScarcity of iron, an essential element for life, has driven bacteria to evolve intricate acquisition systems, yet the molecular basis of their signal transduction mechanisms remains elusive. Unlike conventional pathways, iron transport systems employ outer membrane receptors that mediate signal transduction across both the outer and cytoplasmic membranes, powered by TonB. Using the Escherichia coli Fec system, we uncovered the mechanism by which FecRs cleavage cascade orchestrates ferric citrate signaling through TonB-mediated energy transfer. These findings shed light on similar mechanisms in gram-negative bacteria and have significant implications for understanding bacterial adaptation and pathogenesis.