Translation of key proteins in the chloroplast is regulated by light. Genetic and biochemical studies in the unicellular alga Chlamydomonas reinhardtii suggest that light may regulate translation by modulating the binding of activator proteins to the 5’ untranslated region of chloroplast messenger RNAs. in vitro binding of the activator proteins to psbA messenger RNA and in vivo translation of psbA messenger RNA is regulated by the redox state of these proteins, suggesting that the light stimulus is transduced by the photosynthesis-generated redox potential.