Antagonistic small proteins enable magnesium-dependent tuning of two-component signaling

Small proteins are emerging regulators of bacterial virulence via signaling pathways. However, how multiple small proteins coordinate to control a single pathway remains unclear. Here we show that two antagonistic small proteins, SafA and MgrB, forming a magnesium-dependent control module, enhance the signaling sensitivity of the PhoQ/PhoP two-component system in E. coli. Leveraging conditions that enable sequential expression of SafA and MgrB, we dissect and characterize their regulatory dynamics. Our results reveal magnesium as the central control parameter modulating the abundance and affinities of the small proteins for the sensor kinase PhoQ. This coupling together with the competitive binding of the small proteins enables self-adjustment of the signaling network and enhances its sensitivity to environmental magnesium changes. Functionally, loss of either small proteins influences the ability of enteropathogenic E. coli to evade macrophage phagocytosis, linking this regulatory scheme to host interaction. Our findings establish a framework for achieving a high level of input sensitivity through antagonistic regulations in a signaling network.