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 chloropl asts via at least two regulatory pathways. In the first, the light signal i s transduced as a thiol-mediated signal that modulates translation in paral lel to light intensity. The second light-controlled pathway, termed priming , is a prerequisite to the thiol-mediated regulatory pathway. Light regulat ion is rapid and requires intra-chloroplast photoreceptor(s). To delineate the signaling pathways controlling each of these regulatory events, we assa yed the effect of photosynthetic inhibitors and electron donors on the tran slation 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 reduc tion of plastoquinone. These findings suggest that translation of chloropla st psbA mRNA is controlled by both linear photosynthetic electron transport , exerted by the reduction of the ferredoxin-thioredoxin system, and the re lative activities of photosystems I and II, signaled by the redox state of the plastoquinone pool. These data underscore the function of the light-cap turing reactions of photosynthesis as chloroplast photoreceptors.