[The influence of actinomycin D on phytochrome-mediated changes of the RNA contents in the mustard seedling (sinapis alba L.)].

Actinomycin D (10 μg/ml) cancels completely the phytochrome-mediated RNA net synthesis in the cotyledons of the mustard seedling whereas RNA net synthesis in the cotyledons of the dark-grown seedling is only partially inhibited (Fig. 1). - In the hypocotyl Actinomycin D of the same concentration lowers the RNA contents in the light (i.e. far-red)-grown seedling as well as in the dark-grown seedling down to the same level (Fig. 2). In the presence of Actinomycin D phytochrome has no significant influence on the RNA contents neither in the cotyledons nor in the hypocotyl (Fig. 1,2).The data support the view that P730, the active phytochrome, acts through differential gene activation in the cotyledons and predominantly through differential gene repression in the hypocotyl (cf. MOHR, 1966; SCHOPFER, 1967a, b). -The data further support the conception that "active" genes (as defined by MOHR, 1966 and SCHOPFER, 1967a, b) are much less sensitive towards Actinomycin D than "potentially active" and "repressible" genes (cf. SCHOPFER, 1967a; MOHR and BIENGER, 1967).