Regulation of sulfur deprivation-induced expression of the ferredoxin-encoding FDX5 gene Chlamydomonas reinhardtii in aerobic conditions.

The unicellular green alga Chlamydomonas reinhardtii reacts to sulfur (S) starvation with the increased expression of numerous genes. One gene which is induced in illuminated anaerobic S-deprived cells is the ferredoxin-5 gene (FDX5). To test FDX5 transcriptional regulation in aerobic cultures, we used a real-time PCR analysis and an artificial microRNA approach. We demonstrated that FDX5 gene is controlled by S deprivation independently of anoxia-treatment. The Ser/Thr kinase SNRK2.1 is necessary for expression of FDX5 during deprivation to S. Copper response regulator 1 (CRR1) is not involved in FDX5 up-regulation in S-deficient cells under aerobic conditions. Furthermore, expression of FDX5 is negatively regulated by nitric oxide (NO). Moreover, truncated hemoglobin 1 (THB1) underexpression resulted in the decrease in FDX5 transcript abundance in S-deficient cells under aerobic conditions. Together, our results imply that the FDX5 gene is controlled by NO in THB1-dependent pathway under conditions of depleted S supply.