Stimulation of precursor rRNA synthesis in the cotyledons of mustard seedlings by phytochrome.

Cotyledons of mustard (Sinapis alba L.) seedlings irradiated with far-red light were shown to synthesize high molecular weight precursor rRNA (2.4×10(6)-2.9×10(6) dalton) at a considerably enhanced rate compared to dark-grown cotyledons. This result was obtained by pulse-labelling cotyledons isolated from 48 h old seedlings (which were either irradiated for 12 h with standard far-red light before dissection, or kept in darkness) with [(3)H]uridine for 15 min. The data verify the conclusion that phytochrome is able to activate the transcription of rRNA cistrons.