Recent experiments with bacteria have shown that light and oxygen can control gene expression through effects on oxidation-reduction potential. The term "redox sensor" is proposed as a general term for electron carriers that initiate control of gene expression upon oxidation or reduction. The term "redox response regulator" is proposed for DNA-binding proteins that modify gene expression as a result of the action of redox sensors. Redox sensors and redox response regulators may function together in feedback control of redox potential in photosynthesis and respiration, protecting the cell from damage caused by electrochemistry operating on inappropriate electron donors and acceptors. Chloroplast and mitochondrial redox sensors and redox response regulators, themselves encoded in the nucleus, may place expression of chloroplast and mitochondrial genes under redox regulatory control. This hypothesis offers an explanation for the persistence, in evolution, of chloroplast and mitochondrial genomes, and for the constancy of the subset of chloroplast and mitochondrial proteins encoded and synthesized within the organelle.