[New evidence in favour of the hypothesis of differential gene activation by phytochrome 730].

Phytochrome-mediated anthocyanin synthesis of the mustard seedling (Sinapis alba L.) can be regarded as a prototype of a "positive" photoresponse. "Positive" photoresponses are those which are characterized by an initiation or an increase of biosynthetic or growth processes. The initial lag-phase of this type of photoresponse is relatively long. In the case of anthocyanin synthesis of the mustard seedling it takes about 3 hours after the initial formation of P730, the active phytochrome, until anthocyanin synthesis can be measured (Fig.). This period of time has been called initial (or primary) lag-phase.In earlier papers (e.g. LANGE and MOHR, 1965; MOHR, 1966a, b; SCHOPFER, 1967) we have advanced and supported the hypothesis that the positive photoresponses (in the above sense) of the mustard seedling can be explained by a differential gene activation through P730. Those genes of which the activity can be started or increased by P730 have been called potentially active genes (P730).In order to verify this hypothesis further we have been trying to characterize the processes which occur during the initial lag-phase after the onset of continuous far-red light. There is good evidence (HARTMANN, 1966; CLARKSON and HILLMAN, 1967) that under continuous far-red a low but virtually stationary concentration of the active P730 can be maintained in the seedling's tissues over a period of many hours. Consequently we are dealing with steady state conditions of P730 when we investigate the process of anthocyanin accumulation under continuous far-red.In the present paper we present data which indicate that P730 exerts two functions during the initial lag-phase. First, it eliminates a barrier before the potentially active genes insolfar as it makes these genes accessible for the activating action of P730. Secondly, P730 starts the activity of potentially active genes. To maintain gene activity the continuous presence of P730 is required. On the other hand, it a gene has once been "opened" to the action of P730 it remains easily accessible for the activating action of P730 even in the case when P730 disappeared and gene activity ceased for an extended period of time. After the re-appearance of P730 the gene can be activated almost immediately. These conclusions have been derived from the following facts. 1. The initial lag-phase after the onset of far-red is always in the order of 3 hours (Fig.). If, however, a seedling which was preirradiated with 12 hours of far-red is kept in darkness for an extended period and is then re-irradiated with far-red no lag-phase for the action of the second irradiation can be found even when anthocyanin synthesis had already ceased during the preceding dark period (Fig.).