THB1 regulates nitrate reductase activity and THB1 and THB2 transcription differentially respond to NO and the nitrate/ammonium balance in Chlamydomonas.

Nitric oxide (NO) has emerged as an important regulator of the nitrogen assimilation pathway in plants. Nevertheless, this free radical is a double-edged sword for cells due to its high reactivity and toxicity. Hemoglobins, which belong to a vast and ancestral family of proteins present in all kingdoms of life, have arisen as important NO scavengers, through their NO dioxygenase (NOD) activity. The green alga Chlamydomonas reinhardtii has 12 hemoglobins (THB1–12) belonging to the truncated hemoglobins family. THB1 and THB2 are regulated by the nitrogen source and respond differentially to NO and the nitrate/ammonium balance. THB1 expression is upregulated by NO in contrast to THB2, which is downregulated. THB1 has NOD activity and thus a role in nitrate assimilation. In fact, THB1 is upregulated by nitrate and is under the control of NIT2, the major transcription factor in nitrate assimilation. In Chlamydomonas, it has been reported that nitrate reductase (NR) has a redox regulation and is inhibited by NO through an unknown mechanism. Now, a model in which THB1 interacts with NR is proposed for its regulation. THB1 takes electrons from NR redirecting them to NO dioxygenation. Thus, when cells are assimilating nitrate and NO appears (i.e. as a consequence of nitrite accumulation), THB1 has a double role: 1) to scavenge NO avoiding its toxic effects and 2) to control the nitrate reduction activity.