A structural model of anti‐anti‐σ inhibition by a two‐component receiver domain: the PhyR stress response regulator

PhyR is a hybrid stress regulator conserved in α‐proteobacteria that contains an N‐terminal σ‐like (SL) domain and a C‐terminal receiver domain. Phosphorylation of the receiver domain is known to promote binding of the SL domain to an anti‐σ factor. PhyR thus functions as an anti‐anti‐σ factor in its phosphorylated state. We present genetic evidence that Caulobacter crescentus PhyR is a phosphorylation‐dependent stress regulator that functions in the same pathway as σ^T and its anti‐σ factor, NepR. Additionally, we report the X‐ray crystal structure of PhyR at 1.25 Å resolution, which provides insight into the mechanism of anti‐anti‐σ regulation. Direct intramolecular contact between the PhyR receiver and SL domains spans regions σ₂ and σ₄, likely serving to stabilize the SL domain in a closed conformation. The molecular surface of the receiver domain contacting the SL domain is the structural equivalent of α4‐β5‐α5, which is known to undergo dynamic conformational change upon phosphorylation in a diverse range of receiver proteins. We propose a structural model of PhyR regulation in which receiver phosphorylation destabilizes the intramolecular interaction between SL and receiver domains, thereby permitting regions σ₂ and σ₄ in the SL domain to open about a flexible connector loop and bind anti‐σ factor.