Statement of the Problem: Sulfur (S) is an essential element among catalysts and intermediates of primary metabolism. S can be limiting in the environment and strongly influence ecosystem composition. During S deprivation, metabolism of Chlamydomonas cells is refocused on both scavenging the nutrient and remodeling primary metabolism. Although some signaling proteins and regulators of S-specific responses have been identified, the mechanisms triggering the coordinated responses in different cellular compartments are not absolutely clear. Previously, we have reported that nitric oxide is generated upon S deprivation. Purpose: The purpose of this study is to elucidate the role of nitric oxide (NO) and truncated hemoglobin 1 (THB1) in modulating early responses to S deficiency in different compartment of Chlamydomonas cells. Methodology & Theoretical Orientation: To examine transcriptional regulation of a subset of S limitation-responsive genes and role of NO and THB1 in signaling pathway associated with S deprivation, real-time PCR analysis and artificial microRNA method were employed. A comparative analysis of gene expression and NO generation in wild type, nitrate reductase mutants and THB1 knock-down transformant was utilized to understand the functional consequences of NO production. Findings: In S-free medium, Chlamydomonas cells produced NO apparently via nitrate reductase. We found that in S-limited cells NO is important to upregulate some S deficiency-inducible genes (THB1) and repress the others (FDX5, ARS1, ARS2, SULTR2, SLT1 and SLT2). THB1 is involved in this NO-dependent process. Conclusion & Significance: Together, the results demonstrated that THB1 has implicated to function as NO regulator (via conversion of NO into nitrate) and thereby initiate NO-based signaling cascades in S-depleted cells. Moreover, NO generation may be regarded as an early trigger, which contributes to Chlamydomonas adaptability to S starvation. Recent Publications 1. Aksoy M, Pootakham W, Pollock S V, Moseley J L, Gonzalez-Ballester D and Grossman A R (2013) Tiered regulation of sulfur deprivation responses in Chlamydomonas reinhardtii and identification of an associated regulatory factor. Plant Physiology 162:195-211. 2. Minaeva E, Zalutskaya Z, Filina V and Ermilova E (2017) Truncated hemoglobin 1 is a new player in Chlamydomonas reinhardtii acclimation to sulfur deprivation. PLOS One 12(10): e0186851. 3. Zalutskaya Z, Minaeva E, Filina V, Ostroukhova M and Ermilova E (2018) Regulation of sulfur deprivation-induced expression of the ferredoxin-encoding FDX5 gene Chlamydomonas reinhardtii in aerobic conditions. Plant Physiology and Biochemistry 123:18-23. 4. Zalutskaya Z, Filina V, Ostroukhova M and Ermilova E (2018) Regulation of alternative oxidase 1 in Chlamydomonas reinhardtii during sulfur starvation. European Journal of Protistology 63:26�??33. 5. Zhang Z, Shrager J, Jain M, Chang C W, Vallon O and Grossman A R (2004) Insights into the survival of Chlamydomonas reinhardtii during sulfur starvation based on microarray analysis of gene expression. Eukaryotic Cell 3(5):1331-1348.