Abstract
Density-labelling with deuterium oxide has been used to distinguish preexisting from newly made ascorbate oxidase (EC 1.10.3.3) molecules in cotyledons of the mustard seedling (Sinapis alba L.). The time course of the change in bandwidth of isopycnically banded enzyme, taken together with the accompanying shifts in density, showed that the enzyme was synthesized de novo, was continuously turning over, and had a halt-life of about 1.25 days in darkness. Phytochrome-mediated increases in enzyme activity were accompanied by (i) a faster rate of labelling and (ii) faster progression of the profile to that of a uniformly labelled population, than in dark controls run in parallel. The conclusion is drawn, that phytochrome regulates the rate of synthesis de novo of the protein moiety of ascorbate oxidase in the mustard seedling.
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