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Derepression and Repression of Lysine-sensitive Aspartokinase during in Vitro Culture of Carrot Root Tissue ¹

^a Department of Botany, Faculty of Science, University of Tokyo, Hongo, Tokyo 113, Japan

The increment of lysine-sensitive aspartokinase (EC 2.7.2.4) activity during in vitro culture of carrot (Daucus carota, cv. Oogata sanzun) root tissue was explained in terms of derepression caused by an earlier decrease in the endogenous level of lysine, a possible end product repressor. Tissue content of free lysine decreased to about one-third of the initial level after 1 day of culture and no lysine was detected in the 2nd day. Inclusion of lysine (0.1 to 1.0 millimolar) in the culture medium resulted in a specific suppression of increase in lysine-sensitive aspartokinase activity without affecting the increase in threonine-sensitive aspartokinase activity.