Characterization of the alternative oxidase of Chlamydomonas reinhardtii in response to oxidative stress and a shift in nitrogen source

Abstract

To study the transcriptional regulation of Aox1, the major alternative oxidase (AOX) gene, we fused 1072 bp of its promoter to the promoterless arylsulfatase reporter gene. We find that the reporter is strongly activated when cells are shifted from a medium containing ammonium to one containing nitrate but is unresponsive to treatments known to induce Aox1 in higher plants -H 2 O 2 , antimycin A and cold stress. However, induction of Aox1 by all these factors was found when changes in gene expression were monitored using RNA blot analysis. Our data suggest that transcriptional upregulation of Aox1 by H 2 O 2 , antimycin A and cold-stress requires one or more enhancers which are not found in the proximal promoter region of the gene. Interestingly, while nitrate, H 2 O 2 and cold stress result in increased AOX abundance and increased alternative pathway respiration, these changes are not seen when cells are treated with antimycin A. We find that H 2 O 2 , antimycin A and cold-stress result in an increase in intracellular reactive oxygen species (ROS) but that a significant change in ROS does not occur when cells are shifted into a medium-containing nitrate. Overall, our data are consistent with there being two distinct pathways regulating Aox1 transcription and AOX abundance in Chlamydomonas reinhardtii: one in response to oxidative stress and a second due to metabolic changes brought about by a shift in nitrogen source from ammonium to nitrate.

IMMUTANS Does Not Act as a Stress-Induced Safety Valve in the Protection of the Photosynthetic Apparatus of Arabidopsis during Steady-State Photosynthesis

“…AOX1a has been previously reported as a stress-induced gene (Vanlerberghe and McIntosh, 1997;Maxwell et al, 1999;Molen et al, 2006). However, we have shown through a direct comparison of IM to AOX1a that there is no correlation in gene expression between these two genes due to stress, including oxidative stress (Fig.…”

Linking toxicity and adaptive responses across the transcriptome, proteome, and phenotype of Chlamydomonas reinhardtii exposed to silver

“…Among the regulated transcripts, of interest are those regulated uniquely, such as the alternative oxidase 1 (Aox1) and carbonic anhydrase, which were up-regulated at 10 nM at 15 min but down-regulated at all other time points and concentrations. Alternative oxidase is a key protein in mitochondrial electron transport and helps to control the excess production of ROS under stress conditions (43). In addition, carbonic anhydrase, involved in carbon fixation, has been shown to play a key role in detoxification processes by activation of the lysosomal system in toxicant exposed organisms (44).…”

The Chlamydomonas alternative oxidase 1 is regulated by cadmium stress: New insights into control of expression

“…Previous studies have indicated that Chlamydomonas AOX1 mRNA levels are increased in cells subjected to exogenous H 2 O 2 in the presence of light (Molen et al, 2006). To assess the overall impact of H 2 O 2 on the AOX1 expression, we quantified the abundance of AOX1 transcripts in H 2 O 2 -exposed cells irradiated with white light versus cells incubated in the darkness (Fig.…”

“…It has also been shown that only the AOX1 gene is significantly expressed (Molen et al, 2006;Ermilova et al, 2010). This gene is found in a gene cluster group involved in the nitrate assimilation pathway (Quesada et al, 1993).…”

“…This gene is found in a gene cluster group involved in the nitrate assimilation pathway (Quesada et al, 1993). It has been evidenced that, two distinct pathways regulate AOX1 transcription in C. reinhardtii: one in response to a metabolic shift brought about by a change in nitrogen source from ammonium to nitrate and a second in response to oxidative stress (Molen et al, 2006).…”