Since MgFnr only affects expression of denitrification genes but not genes encoding O2 respiration enzymes, magnetite biomineralization is also probably regulated by other unknown O2 sensors. Therefore, further research on respiratory pathways in MTB is likely to gain more insights into the mechanism of oxygen-dependent regulation of biomineralization. Methods Bacterial strains and growth conditions

Bacteria strains and plasmids used in this study are shown in Additional file 5. If not specified otherwise, E. coli strains were grown in lysogeny broth (LB) at 37°C, and MSR-1 strains were cultivated at 30°C in nitrate medium as described before [5]. In ammonium medium, nitrate was substituted by 4 mM ammonium chloride. When necessary, antibiotics were used at the following concentrations: E. coli: tetracycline (Tc), 12 μg/ml, kanamycin (Km), 25 μg/ml, and gentamicin (Gm), 15 μg/ml; MSR-1: Tc, 5 μg/ml, PD0332991 Km, 5 μg/ml,

LY2109761 manufacturer and Gm, 30 μg/ml. When E. coli strain BW29427 was used as donor in conjugation, 300 μM diaminopimelic acid (DAP) was added. Experiments for growth and MK-4827 magnetic response (Cmag) were monitored under microaerobic and anaerobic conditions in 250 ml flasks containing 100 ml media. For microaerobic conditions, flasks were sealed with butyl-rubber stoppers under a microaerobic gas mixture containing 2% O2 and 98% N2 before autoclaving. Anaerobic conditions were achieved

by removing oxygen from gas mixture. For aerobic conditions, strains were cultured in free gas exchange with air in 300 ml flasks containing 20 ml medium agitated at 200 rpm. Optical density (OD) and magnetic response (Cmag) were measured photometrically at 565 nm as previously described [40]. For gas production assay, cells were inoculated and mixed with nitrate medium with 0.3% agar in oxygen gradient tubes and exposed to the air. Genetic and molecular biology techniques Standard molecular and genetic techniques were carried Amoxicillin out for DNA isolation, digestion, ligation, and transformation [41]. All DNA products were sequenced using BigDye Terminator version 3.1 chemistry on an ABI 3700 capillary sequencer (Applied Biosystems, Darmstadt, Germany), and sequence data were analyzed with the software Vector NTI Advance® 11.5.1 (Invitrogen, Darmstadt, Germany). All oligonucleotide sequences used in this work are available if required. Construction of a MSR-1 ΔMgfnr deletion mutant All PCRs were performed using Phusion polymerase (NEB). Enzymes, including restriction enzymes and T4 DNA ligase, were purchased from Fermentas. To generate the unmarked ΔMgfnr deletion mutant, a modified cre-lox method was used as previously described [29]. An about 2-kb downstream PCR fragment of Mgfnr was generated and cloned into NotI/EcoRI-digested pAL01 to obtain pLYJ106.