Background: Multidomain sensory kinases are involved in numerous receptive processes in all kingdoms of life. Results: The multidomain sensory kinase MA4561 covalently binds a redox-active heme cofactor, which triggers kinase activity. Conclusion: Covalently bound heme is utilized to detect redox changes. Significance: Learning how Archaea perceive environmental stimuli will enhance our understanding of the evolution of prokaryotic signal transduction.