Sequences of the primers used in this study.

<div><p>Competitive bacteria like the human pathogen <i>Pseudomonas aeruginosa</i> can acquire iron from different iron carriers, which are usually internalized via outer membrane TonB-dependent receptors (TBDRs). Production of TBDRs is promoted by the presence of the substrate. This regulation often entails a signal transfer pathway known as cell-surface signaling (CSS) that involves the TBDR itself that also functions as transducer (and is thus referred to as TBDT), a cytoplasmic membrane-bound anti-σ factor, and an extracytoplasmic function σ (σ<sup>ECF</sup>) factor. TBDTs contain an extra N-terminal domain known as signaling domain (SD) required for the signal transfer activity of these receptors. In the current CSS model, presence of the signal allows the interaction between the TBDT and the anti-σ factor in the periplasm, promoting the proteolysis of the anti-σ factor and in turn the σ<sup>ECF</sup>-dependent transcription of response genes, including the TBDT gene. However, recent evidence shows that σ<sup>ECF</sup> activity does not depend on this interaction, suggesting that the contact between these 2 proteins fulfills a different role. Using the <i>P</i>. <i>aeruginosa</i> Fox CSS system as model, we show here that the SD of the FoxA TBDT already interacts with the C-terminal domain of the FoxR anti-σ factor in absence of the signal. This interaction protects FoxR from proteolysis in turn preventing transcription of σ<sup>FoxI</sup>-dependent genes. By structural modeling of the FoxR/FoxA<sup>SD</sup> interaction, we have identified the interaction sites between these 2 proteins and provide the molecular details of this interaction. We furthermore show that to exert this protective role, FoxA undergoes proteolytic cleavage, denoting a change in the paradigm of the current CSS model.</p></div>