The strains and plasmids used in this study are listed in Additional file 4 : Table S3. E. coli strains were grown in Luria–Bertani medium at 37°C with shaking (180 rpm) or on solid growth medium, which contained 1.6% (w/v) agar. R. sphaeroides strains were cultivated at 32°C in 50-ml Erlenmeyer flasks containing 40 ml malate minimal medium (Additional file 4 : Table S4) with continuous shaking at 140 rpm, resulting in a constant dissolved oxygen concentration of approximately 25–30 μM during the exponential phase. These growth conditions are designated as oxic growth. To achieve anoxic conditions, we used completely filled screw-cap Meplat bottles for liquid cultures, which were sealed with Parafilm and cultivated in the dark. The remaining oxygen was used up by the cultures within 60 seconds, as confirmed using an oxygen sensor. To allow anaerobic respiration, dimethyl sulphoxide (DMSO) was added as electron acceptor at a final concentration of 60 mM. Anoxic incubation over several days resulted in a final OD660 of approximately 0.5. Conditions of iron limitation were achieved by transferring R. sphaeroides into iron-limited malate minimal medium containing the iron chelator 2,2′-dipyridyl (30 μM; Merck KGaA) three times. Inductively coupled plasma mass spectrometry (ICP-MS) using an Agilent 7500ce spectrometer confirmed that the iron content was drastically reduced in iron-limited medium (from 140 mg l-1 to 16 mg l-1) [22 (link)]. When required, antibiotics were added to liquid or solid growth media at the following concentrations: spectinomycin (10 μg ml-1); kanamycin (25 μg ml-1); tetracycline (2 μg ml-1) (for R. sphaeroides); kanamycin (25 μg ml-1); and tetracycline (20 μg ml-1) (for E. coli).
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