Glutathione Is Required by Rhizobium etli for Glutamine Utilization and Symbiotic Effectiveness

Abstract

Here, we provide genetic and biochemical evidence indicating that the ability of Rhizobium etli bacteria to efficiently catabolize glutamine depends on its ability to produce reduced glutathione (l-γ-glutamyl-l-cysteinylglycine [GSH]). We find that GSH-deficient strains, namely a gshB (GSH synthetase) and a gor (GSH reductase) mutant, can use different amino acids, including histidine, alanine, and asparagine but not glutamine, as sole source of carbon, energy, and nitrogen. Moreover, l-buthionine(S,R)-sulfoximine, a GSH synthesis inhibitor, or diamide that oxidizes GSH, induced the same phenotype in the wild-type strain. Among the steps required for its utilization, glutamine uptake, occurring through the two well-characterized carriers (Aap and Bra systems) but not glutamine degradation or respiration, was largely reduced in GSH-deficient strains. Furthermore, GSH-deficient mutants of R. etli showed a reduced symbiotic efficiency. Exogenous GSH was sufficient to rescue glutamine uptake or degradation ability, as well as the symbiotic effectiveness of GSH mutants. Our results suggest a previously unknown GSH-glutamine metabolic relationship in bacteria.

Glutathione affects the transport activity of Rhizobium leguminosarum 3841 and is essential for efficient nodulation

“…2012). In GSH-deficient strains of this organism, glutamine transport was reported to be the defective step in glutamine utilisation (Taté et al.…”

“…Interestingly, the converse was observed in R. etli , where the essential role of GSH in glutamine uptake was through regulation of the Aap transport system and the Bra system was found to be dispensable (Taté et al. 2012). Thus, lack of GSH reduces transcription of some, but not all, R. leguminosarum transport operons suggesting that GSH does not globally alter transcription of transport genes by means of a single mechanism.…”

Thiol-based redox signaling in the nitrogen-fixing symbiosis

“…Both gshA and gshB mutants exhibited higher catalase activity than the wild-type, suggesting that the two mutants were experiencing oxidative stress (Harrison et al, 2005). Furthermore, gshB mutants of Rhizobium tropici and Rhizobium etli were affected in their ability to compete during nodulation of common bean, and nodules induced by gshB mutants displayed early senescence (Riccillo et al, 2000; Tate et al, 2012). A deficiency in GSH was associated with increased levels of ${\mathrm{O}}_{\mathrm{2}}^{\mathrm{•}}$ radicals in nodules infected with the gshB mutant of R. tropici , and thus antioxidant mechanisms dependent on bacterial GSH might be impaired (Muglia et al, 2008).…”

“…A deficiency in GSH was associated with increased levels of ${\mathrm{O}}_{\mathrm{2}}^{\mathrm{•}}$ radicals in nodules infected with the gshB mutant of R. tropici , and thus antioxidant mechanisms dependent on bacterial GSH might be impaired (Muglia et al, 2008). In R. etli , GSH deficiency was linked to a reduction of glutamine uptake in growing cultures, suggesting a complex GSH-glutamine metabolic relationship that may be important for symbiotic efficiency (Tate et al, 2012). Finally, the mutation in the gshA gene of Bradyrhizobium sp.…”

Glutathione Deficiency in Sinorhizobium meliloti Does Not Impair Bacteroid Differentiation But Induces Early Senescence in the Interaction With Medicago truncatula

“…On the bacterial side, the use of mutants deficient in the GSHS-encoding gene (gshB) impairs the BNF efficiency during the symbiosis between Medicago sativa/S. meliloti strain 1021, Phaseolus vulgaris/Rhizobium tropici, P. vulgaris/Rhizobium etli, and Pisum sativum/Rhizobium leguminosarum (Harrison et al, 2005;Muglia et al, 2008;Taté et al, 2012;Cheng et al, 2017). The reduced BNF observed with the gshB mutant strain was correlated with nodule and bacteroid early senescence in M. sativa and P. vulgaris (Harrison et al, 2005;Muglia et al, 2008;Taté et al, 2012).…”