REGULATION OF NITRATE REDUCTASE IN ACETABULARIA-MEDITERRANEA

Nitrate reductase (NR) activity has been characterized on cell-free extracts of the marine unicellular green alga, Acetabularia mediterranea. Reduced methyl viologen and bromophenol blue as well as NAD(P)H were effective electron donors for the enzyme. NADH-NR activity increased during cell development reaching its maximum level when the cells approached their maximum length. A substantial increase was also seen in enucleated cells. The enzyme activity was only present in the green parts of the cell and became diminished as the stalks bleached in ageing cells. Under a 12 h light:12 h darkness photoperiod, NADH-NR activity exhibited pronounced daily fluctuations reaching its maximum on the first hour of illumination (this increase was impaired by cycloheximide) and a minimum in the middle of the dark period. In N-starved algal cultures, NR was constitutively present at appreciable levels and new synthesis took place in the presence of either NO3- or low concentrations of NH4+. However, in the presence of this cation the enzyme remained mostly in its inactive form, and could be reactivated in vitro with ferrycyanide.