Abstract
Truncated hemoglobins constitute a large family, present in bacteria, in archaea and in eukaryotes. However, a majority of physiological functions of these proteins remains to be elucidated. Identification and characterization of a novel role of truncated hemoglobins in the model alga provides a framework for a more complete understanding of their biological functions. Here, we use quantitative RT-PCR to show that three truncated hemoglobins of Chlamydomonas reinhardtii, THB1, THB2 and THB12, are induced under conditions of depleted sulfur (S) supply. THB1 underexpression results in the decrease in cell size, as well in levels of proteins, chlorophylls and mRNA of several S-responsive genes under S starvation. We provide evidence that knock-down of THB1 enhances NO production under S deprivation. In S-deprived cells, a subset of S limitation-responsive genes is controlled by NO in THB1-dependent pathway. Moreover, we demonstrate that deficiency for S represses the nitrate reduction and that THB1 is involved in this control. Thus, our data support the idea that in S-deprived cells THB1 plays a dual role in NO detoxification and in coordinating sulfate limitation with nitrate assimilation. This study uncovers a new function for the Chlamydomonas reinhardtii THB1 in the control of proper response to S deprivation.
Highlights
Hemoglobins (Hbs) constitute a large superfamily of the globular heme-containing metalloproteins, present in archaea, in bacteria, and in eukaryotes
THB1 was recently shown to be linked to nitrogen metabolism [16] while THB8 was vital for anoxic growth [14]
Our results suggest that THB1 is involved in the control of several genes activation during C. reinhardtii acclimation to S deprivation
Summary
Hemoglobins (Hbs) constitute a large superfamily of the globular heme-containing metalloproteins, present in archaea, in bacteria, and in eukaryotes. There are three main types of Hbs: symbiotic, non-symbiotic and truncated (TrHb) [2,3,4]. THB1 was recently shown to be linked to nitrogen metabolism [16] while THB8 was vital for anoxic growth [14] Both of these THBs were proposed to participate in NO-dependent signaling pathways. During S limitation, C. reinhardtii cells demonstrate increased transcription of numerous genes encoding proteins associated with sulfate uptake and assimilation, internal S recycling and changes in metabolism [28]. The responses of C. reinhardtii THB1-12 genes to S limitation conditions have not been analyzed. This may serve to gain additional information on the function of these proteins. Our data demonstrate that nitrate reduction is regulated by S starvation and that THB1 plays a role in this regulation
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