THB1, a truncated hemoglobin, modulates nitric oxide levels and nitrate reductase activity.

Hemoglobins are ubiquitous proteins that sense, store and transport oxygen, but the physiological processes in which they are implicated is currently expanding. Recent examples of previously unknown hemoglobin functions, which include scavenging of the signaling molecule nitric oxide (NO), illustrate how the implication of hemoglobins in different cell signaling processes is only starting to be unraveled. The extent and diversity of the hemoglobin protein family suggest that hemoglobins have diverged and have potentially evolved specialized functions in certain organisms. A unique model organism to study this functional diversity at the cellular level is the green alga Chlamydomonas reinhardtii because, among other reasons, it contains an unusually high number of a particular type of hemoglobins known as truncated hemoglobins (THB1-THB12). Here, we reveal a cell signaling function for a truncated hemoglobin of Chlamydomonas that affects the nitrogen assimilation pathway by simultaneously modulating NO levels and nitrate reductase (NR) activity. First, we found that THB1 and THB2 expression is modulated by the nitrogen source and depends on NIT2, a transcription factor required for nitrate assimilation genes expression. Furthermore, THB1 is highly expressed in the presence of NO and is able to convert NO into nitrate in vitro. Finally, THB1 is maintained on its active and reduced form by NR, and in vivo lower expression of THB1 results in increased NR activity. Thus, THB1 plays a dual role in NO detoxification and in the modulation of NR activity. This mechanism can partly explain how NO inhibits NR post-translationally.