Abstract
The Saccharomyces cerevisiae Orp1–Yap1 sensor and its prokaryotic counterpart OxyR are master regulators of cellular H 2O 2 homeostasis. These sensors share a stereotypical mechanism, involving the reaction of a unique cysteine with H 2O 2 and a physiology coupled to metabolism of H 2O 2 and thiol reduction–oxidation (redox). However they differ fundamentally in their core mechanism. Whereas OxyR carries out both sensing and transcriptional regulatory functions, these functions are dissociated in yeast, where Orp1 senses H 2O 2 and converts this signal into a cysteine-based redox cascade that culminates in oxidation of the Yap1 regulator. We propose a model in which H 2O 2 sensors are viewed as archetypical receptor-initiated redox signaling modules and suggest, on the basis of the high specificity of the H 2O 2-sensing mechanism, that kinetic factors rather than thermodynamics define the cellular pathways of cysteine residue oxidation.
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