Translation of chloroplast psbA mRNA is regulated by signals initiated by both photosystems II and I.

Light controls the translation of several mRNAs in fully developed chloroplasts via at least two regulatory pathways. In the first, the light signal is transduced as a thiol-mediated signal that modulates translation in parallel to light intensity. The second light-controlled pathway, termed priming, is a prerequisite to the thiol-mediated regulatory pathway. Light regulation is rapid and requires intrachloroplast photoreceptor(s). To delineate the signaling pathways controlling each of these regulatory events, we assayed the effect of photosynthetic inhibitors and electron donors on the translation of chloroplastic psbA mRNA. We show that the thiol-mediated signal is generated by photosystem I and transduced by vicinal dithiol-containing proteins. We also found that the priming signal probably initiates on reduction of plastoquinone. These findings suggest that translation of chloroplast psbA mRNA is controlled by both linear photosynthetic electron transport, exerted by the reduction of the ferredoxin-thioredoxin system, and the relative activities of photosystems I and II, signaled by the redox state of the plastoquinone pool. These data underscore the function of the light-capturing reactions of photosynthesis as chloroplast photoreceptors.