Control over activation or synthesis of phenylalanine ammonia-lyase by phytochrome in mustard (Sinapis alba L.)? A contribution to eliminate some misconceptions.

1. Density labelling with 80 atom% of 2H2O has been used to examine the mode of action of phytochrome (continuous far-red light) in increasing levels of L-phenylalanine ammonia-lyase (EC 4.3.1.5) activity in cotyledons of developing mustard seedlings (Sinapis alba L.). 2. Bandwidths and density shifts of isopycnically banded enzyme show that in darkness the enzyme was synthesized de novo, continuously turning over (half-life approx. 3 h) and that maximum labelling achievable was reached at 12 h. 3. 2-fold (6h), 5-fold (12h) and 10-fold (24 h) light-mediated increases in enzyme activity were accompanied by a similar pattern of labelling as observed in darkness. 4. Experimental evidence and theoretical arguments are presented which make it unlikely that phytochrome increases enzyme activity by slowing down the rate of degradation or by activating preformed enzyme molecules. 5. The conclusion is drawn that the rate of turnover of phenylalanine ammonia-lyase in dark-grown mustard cotyledons is too rapid compared to the measured rise in enzyme activity for density labelling to reveal directly control over the rate of synthesis de novo by phytochrome. However, the elimination of other control mechanisms leads us to the conclusion that phytochrome most probably does control synthesis of this enzyme in mustard, which agrees with the previous findings for parsley cells.