[The control by phytochrome of phenylalanine ammonia-lyase activity in mustard seedlings (Sinapis alba L.)].

The present paper is a contribution to the "molecular" analysis of photomorphogenesis. L-phenylalanine ammonia-lyase (=PAL) (EC 4.3.1.5) has been used as a model system to demonstrate that enzyme synthesis, enzyme inactivation and gene repression are important in determining the response of a particular enzyme to phytochrome.The level of PAL in the mustard seedling is controlled by Pfr (the active form of phytochrome) in a characteristic manner which is illustrated in Fig. 1. The seedlings were irradiated with continuous standard far-red light. Long time irradiation with far-red will maintain a low but virtually constant level of the effector molecule Pfr in the seedling over an extended period of time. At the moment when the far-red light is turned off the action of Pfr will instantly decrease and will eventually cease probably within the order of an hour (cf. KAROW and MOHR, 1969). The approach followed in the present paper has been to turn off the far-red light after varying periods and follow the enzyme kinetics in darkness (Fig. 2). The main results can be summarized as follows: The far-red kinetics of PAL (Fig. 1) can be explained as the result of three processes, namely, Pfr-mediated enzyme synthesis, inactivation of PAL by an "inactivator", and eventual repression of enzyme synthesis.-During the period 1.5-12 hrs after the onset of far-red only enzyme synthesis occurs. Then enzyme inactivation comes into play while enzyme synthesis continues at a constant rate (Fig. 3). This antagonism of synthesis and inactivation leads to a true steady state which is observed between about 24 and 27 hrs after the onset of far-red. After this period the rate of enzyme synthesis decreases and as a consequence, inactivation dominates. 36 hours after the onset of far-red the Pfr-mediated PAL synthesis is hardly dtectable. The results of "secondary irradiations" with far-red (Fig.4) indicate that the "inactivator" of PAL does not have any direct influence on PAL synthesis. The kinetics in darkness (Fig.1,2) can best be understood by assuming that a certain enzyme level represented by the plateau cannot be overcome in the dark. The "overshoot" response which is obvious in the enzyme kinetics immediately after the cessation of far-red (Fig. 2) cannot be explained readily in molecular terms.