[Studies on regulation of RNA-synthesis by phytochrome in the mustard seedling (Sinapis alba L.)]

P730, the active phytochrome, increases the rate of RNA synthesis (Table) and the RNA contents in the cotyledons of the mustard seedling (Sinapis alba L.) (Fig. 1) whereas the RNA contents in the hypocotyl is decreased under the influence of P730 (Fig. 2).-It takes about 6 hours until changes in the RNA contents-which must be attributed to the formation of P730-can be measured after the onset of light (continuous far-red). Since the lag-phases of "positive" photoresponses in the cotyledons and "negative" photoresponses in the hypocotyl (MOHR, 1966) are in general much shorter than 6 hours, the changes of the RNA contents of the organs cannot be regarded as being directly connected with the formation of characteristic "positive" photoresponses such as anthocyanin synthesis, induced enzyme synthesis, ascorbic acid synthesis, etc., or "negative" photoresponses such as inhibition of hypocotyl lengthening.We have rather to conclude that the changes of RNA contents are secondary adaptations of the organs which lead to an increase (cotyledons) or decrease (hypocotyl) of protein synthesizing capacity of the cells and tissues. The P730-dependent increase of bulk RNA in the cotyledons is probably due to a differential gene activation and the P730-dependent decrease of bulk RNA in the hypocotyl is due to a differential gene repression. The causalities of these processes are possibly complex.The hypothesis of differential gene activation or repression by P730 (MOHR, 1966; SCHOPFER, 1967a, b) is not disproved by these results. We have rather to reach a conclusion which has already been suggested by other data (e.g. KAROW and MOHR, 1966), namely, that positive as well as negative photoresponses are due to changes in the activity of a limited (possibly small) number of enzymes. Correspondingly changes in only a minute amount of the total RNA are directly involved in the formation of photoresponses. These changes cannot be detected by following RNA contents.-It seems to be of great interest, however, that P730 eventually brings about strong tissue specific changes in the RNA contents per cell as described in the present paper.