Blue‐light requirement for the biosynthesis of an NO2− transport system in the Chlamydomonas reinhardtii nitrate transport mutant S10*

The blue-light requirement for the biosynthesis of nitrite reductase and an NO2– transport system was studied in Chlamydomonas reinhardtii mutant S10. The only oxidized nitrogen species that could be taken up by this mutant was NO2–, due to the presence of NO2– transport systems and the absence of high-affinity NO3– transporters. NH4+-grown cells required illumination with blue light to recover the ability to take up NO2– when resuspended in an NO2–-containing NH4+-deprived medium. This blue-light- dependent recovery, which took 1 h, could be suppressed by cycloheximide, indicating that protein biosynthesis was involved. The biosynthesis of nitrite reductase took place in cell suspensions irradiated with red light, even in the absence of NO2–, thus suggesting that the process requiring blue light was the biosynthesis of an NO2– transport system. Nitrite reductase-containing cells (pre-irradiated with red light) took 1 h to start consuming NO2– when they were additionally irradiated with blue light in the presence of this anion, and this process was also cycloheximide-sensitive. The NO2– transport system operated either under red plus blue light or red light only. Thus, in C. reinhardtii mutant S10 cells, blue light was only required for the biosynthesis of an NO2– transport system and not for its activity.