EVIDENCE FOR THE INVOLVEMENT OF PLASMA-MEMBRANE-BOUND NITRATE REDUCTASE IN SIGNAL-TRANSDUCTION DURING BLUE-LIGHT STIMULATION OF NITRATE UPTAKE IN CHLORELLA-SACCHAROPHILA

Nitrate uptake in Chlorella saccharophila (Kruger) Nadson was found to be stimulated by blue light, leading to a doubling of the rate. In the presence of background red light (300 mu mol photons . m(-2). s(-1)), only 15-20 mu mol photons . m(-2). s(-1) of blue light was sufficient to saturate this increased uptake rate. Incubation of Chlorella cells with anti-nitrate-reductase immuno-globulin-G fragments inhibited blue-light stimulation. However, ferricyanide (10 mu M) doubled and dithiothreitol (100 mu M) inhibited the stimulatory effect of blue light. Among the protein-kinase inhibitors used, only staurosporine (10 mu M) prevented the blue-light stimulation. Phosphatase inhibitors were without effect and sodium vanadate totally inhibited nitrate uptake, pointing to an involvement of the plasma-membrane ATPase. Preincubation of the cells with calmodulin antagonists or calcium ionophores did not significantly reduce blue-light stimulation of nitrate uptake. The data are discussed with regard to transduction of the signal for blue-light stimulation of nitrate uptake and the possibility that the plasma-membrane-bound nitrate reductase is the blue-light receptor.