Nitrate reductase activity in Paul's scarlet rose suspension cultures and the differential role of nitrate and molybdenum in induction.

Induction of nitrate reductase (EC 1.6.6.1) activity was measured in Paul's Scarlet rose cell suspensions cultured in media containing nitrate (NO 3 (-) ) or urea (U) as nitrogen source, and with (+Mo) or without molybdenum (-Mo). There was a lag of 30 min during induction by NO 3 (-) in +Mo cultures but no lag occurred during induction after adding Mo to NO 3 (-) -Mo or to U-Mo cultures preincubated with NO 3 (-) . Actinomycin D, cycloheximide, and puromycin completely blocked induction by NO 3 (-) , but had no effect on the initial rate of induction by Mo. Cycloheximide and puromycin blocked induction by NO 3 (-) more quickly than actinomycin D. Induction by NO 3 (-) appeared to involve mRNA-dependent synthesis of apoprotein followed by rapid activation with molybdenum in intact cells independently of protein synthesis. Nitrate-induced apoprotein appeared less stable than the holoenzyme. When induced by NO 3 (-) in the absence of Mo, apoprotein concentration was about half the amount of maximally induced nitrate reductase. Cycloheximide stabilised preformed nitrate reductase which disappeared steadily in the presence of puromycin. Apoprotein was not stabilised by either antimetabolite.