Abstract
To cope with toxic levels of H2S, the plant pathogens Xylella fastidiosa and Agrobacterium tumefaciens employ the bigR operon to oxidize H2S into sulfite. The bigR operon is regulated by the transcriptional repressor BigR and it encodes a bifunctional sulfur transferase (ST) and sulfur dioxygenase (SDO) enzyme, Blh, required for H2S oxidation and bacterial growth under hypoxia. However, how Blh operates to enhance bacterial survival under hypoxia and how BigR is deactivated to derepress operon transcription is unknown. Here, we show that the ST and SDO activities of Blh are in vitro coupled and necessary to oxidize sulfide into sulfite, and that Blh is critical to maintain the oxygen flux during A. tumefaciens respiration when oxygen becomes limited to cells. We also show that H2S and polysulfides inactivate BigR leading to operon transcription. Moreover, we show that sulfite, which is produced by Blh in the ST and SDO reactions, is toxic to Citrus sinensis and that X. fastidiosa-infected plants accumulate sulfite and higher transcript levels of sulfite detoxification enzymes, suggesting that they are under sulfite stress. These results indicate that BigR acts as a sulfide sensor in the H2S oxidation mechanism that allows pathogens to colonize plant tissues where oxygen is a limiting factor.
Highlights
Hydrogen sulfide (H2S) is a gaseous molecule that is well known for its strong odor and toxicity
We provided evidence that the ETHE1 domain of Blh would function as an Sulfur Dioxygenase (SDO) like the mitochondrial ETHE1 protein, since the blh-defective mutant of A. tumefaciens accumulated higher levels of H2S13
In agreement with the predictions, we found that O2 is consumed in the presence of GSSH and the Blh ETHE1 domain, and that sulfite is generated as a reaction product (Fig. 1a and b)
Summary
Hydrogen sulfide (H2S) is a gaseous molecule that is well known for its strong odor and toxicity. Despite its beneficial role in preventing oxidative stress, H2S can become toxic to both mitochondria and bacterial cells, primarily because it blocks aerobic respiration through inhibition of the cytochrome c oxidase[7,8,9] To circumvent such problem, bacterial cells, plant and animal mitochondria have evolved common mechanisms to eliminate H2S10–16. Bacterial cells, plant and animal mitochondria have evolved common mechanisms to eliminate H2S10–16 In both plant and animal mitochondria, as well as in some bacterial species, three enzymes, including the Sulfide Quinone Oxidoreductase (SQOR), Thiosulfate Sulfur Transferase (TST) and Sulfur Dioxygenase (SDO) catalyze the oxidization of H2S into sulfite[10,11,12,14,15,16]. Because sulfite is toxic to cells, animal mitochondria further oxidizes it into sulfate, whereas in plant pathogenic bacteria such as Xylella fastidiosa and Agrobacterium tumefaciens, sulfite is secreted to the medium by a sulfite exporter[13,14,18,19]
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