The light-dependent accumulation of the P700 chlorophyll a protein of the photosystem I reaction center in barley. Evidence for translational control.

The light-dependent accumulation of the P700 chlorophyll a protein of the photosystem I reaction center has been studied in greening barley (Hordeum vulgare L.) seedlings. Immunoblot analysis of total cellular protein fractions and immunogold labelling of the P700 chlorophyll a protein in ultrathin sections of Lowicryl-embedded leaf tissue revealed that the concentration of this chlorophyll-binding protein in plastids of dark-grown barley seedlings is below the limit of detection. Upon illumination with white light, a rapid accumulation of this protein is induced. This light effect seems not to be regulated at the level of transcription. The gene for the P700 chlorophyll a protein has been mapped within the large single-copy region of the plastid DNA of barley. High levels of transcripts of this gene are present already in dark-grown seedlings and remain fairly constant throughout an extended illumination period. Polysomes were isolated from etioplasts and chloroplasts. The same high relative concentration of mRNA encoding the P700 chlorophyll a protein was present in both polysome fractions. This result suggests that the light-dependent accumulation of the P700 chlorophyll a protein during chloroplast formation in barley seedlings is regulated at the translational, or posttranslational, level.