Fig. S1. UV-visible absorption spectrum of the SmGrx1 holoform. The spectrum, recorded after an aerobic purification in 30 mM Tris-HCl pH 8.0, is very similar to those recorded for other Fe-S cluster-containing Grxs, with two absorption bands in the 300–500 nm region characteristic of a [2Fe-2S] centre (Rouhier et al., 2007).

Fig. S2. Analysis of the capacity of SmGRXs to form covalent oligomers. Five micrograms of the SmGRX1 (lanes 2–5), SmGRX2 (lanes 1–4) and SmGRX3 (lanes 3–6) recombinant proteins were separated by SDS-PAGE (17% polyacrylamide gel) in reducing (lanes 1–3, presence of 140 mM β-mercaptoethanol in the Laemmli buffer) and non-reducing (lanes 4–6, absence of β-mercaptoethanol) conditions.

Fig. S3. Detection of GRXs in S. meliloti. The presence of Grxs was evaluated, by Western blot analysis, in total protein extracts from the wild-type and mutant strains. The recombinant (r) Grx were used as controls. Note that for SmGRX3, a truncated version corresponding to the Grx domain (10 kDa) was compared with the full-length protein of 28 kDa.

Fig. S4. Analysis of the size of Smgrx mutant nodules. The size of 3-week-old nodules was analysed by microscopy. The data shown are the means for 15 nodules from three experiments ± SEM.

Table S1. Bacterial strains and plasmids used in this study.

Table S2. Primers used for RT-qPCR analysis, mutant construction and cloning of the recombinant proteins.

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