REDOX CONTROL OF GENE-EXPRESSION AND THE FUNCTION OF CHLOROPLAST GENOMES - AN HYPOTHESIS

Two-component regulatory systems that respond to changes in redox pote ntial have recently been discovered in bacteria. 'Redox sensors' are d efined as electron carriers which initiate control of gene expression upon oxidation or reduction. 'Redox response regulators' are defined a s DNA-binding proteins which modify gene expression as a result of the action of redox sensors. Redox sensors and redox response regulators may comprise a mechanism for feedback control of redox potential in ph otosynthetic electron transport chains, thereby protecting plants, alg ae and photosynthetic bacteria from damage caused by electrochemistry operating on inappropriate electron donors and acceptors. Chloroplast redox sensors and redox response regulators, themselves encoded in the nucleus, may place chloroplast gene expression under redox regulatory control. This may account for the persistence, in evolution, of chlor oplast genomes, and for the constancy of the sub-set of chloroplast pr oteins encoded and synthesised in situ. These and other predictions ar e discussed.