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Control by Phytochrome of Cytoplasmic and Plastid rRNA Accumulation in Cotyledons of Mustard Seedlings in the Absence of Photosynthesis ¹

^a Biological Institute II, University of Freiburg, D 78 Freiburg, Germany

Light stimulates the accumulation of cytoplasmic and plastid rRNA in the cotyledons of mustard (Sinapis alba L.) seedlings. This effect is mediated by phytochrome. Continuous far red irradiation and brief red light pulses (the effect of which is reversible by brief far red light pulses) increase the amount of 1.3 x 10⁶ + 0.7 x 10⁶ and 1.1 x 10⁶ + 0.56 x 10⁶ molecular weight rRNA species. Large and small ribosomal subunit RNAs are maintained in a 1:1 molar ratio in both cytoplasm and plastids, irrespective of seedling age and light treatment. Continuous white fluorescent light (which saturates chlorophyll synthesis) is less effective than continuous far red light (essentially no chlorophyll synthesis) in producing the response, indicating that the accumulation of plastid rRNA is independent of the build-up of a functional photosynthetic apparatus. We conclude that the transcription of cytoplasmic and plastid rRNA cistrons is under phytochrome control.