The oxygen sensor MgFnr controls magnetite biomineralization by regulation of denitrification in Magnetospirillum gryphiswaldense

Abstract

BackgroundMagnetotactic bacteria are capable of synthesizing magnetosomes only under oxygen-limited conditions. However, the mechanism of the aerobic repression on magnetite biomineralization has remained unknown. In Escherichia coli and other bacteria, Fnr (fumarate and nitrate reduction regulator) proteins are known to be involved in controlling the switch between microaerobic and aerobic metabolism. Here, we report on an Fnr-like protein (MgFnr) and its role in growth metabolism and magnetite biomineralization in the alphaproteobacterium Magnetospirillum gryphiswaldense.ResultsDeletion of Mgfnr not only resulted in decreased N2 production due to reduced N2O reductase activity, but also impaired magnetite biomineralization under microaerobic conditions in the presence of nitrate. Overexpression of MgFnr in the WT also caused the synthesis of smaller magnetite particles under anaerobic and microaerobic conditions in the presence of nitrate. These data suggest that proper expression of MgFnr is required for WT-like magnetosome synthesis, which is regulated by oxygen. Analyses of transcriptional gusA reporter fusions revealed that besides showing similar properties to Fnr proteins reported in other bacteria, MgFnr is involved in the repression of the expression of denitrification genes nor and nosZ under aerobic conditions, possibly owing to several unique amino acid residues specific to MTB-Fnr.ConclusionsWe have identified and thoroughly characterized the first regulatory protein mediating denitrification growth and magnetite biomineralization in response to different oxygen conditions in a magnetotactic bacterium. Our findings reveal that the global oxygen regulator MgFnr is a genuine O2 sensor. It is involved in controlling expression of denitrification genes and thereby plays an indirect role in maintaining proper redox conditions required for magnetite biomineralization.

Effects of dissolved oxygen concentration and iron addition on immediate-early gene expression of Magnetospirillum gryphiswaldense MSR-1

“…This suggests that the expression of MgFnr was neither affected by iron addition nor by different DO levels studied, indicating that the magnetite biomineralization may also be regulated by other unknown oxygen sensors. Our finding is in agreement with Li et al 2014, where MgFnr was only involved in the expression of denitrification genes but not genes encoding oxygen respiration enzyme. The authors therefore concluded that MgFnr plays an indirect role in maintaining proper redox conditions required for magnetite biomineralization.…”

“…MgFnr was reported as an oxygen sensor with the role of controlling magnetite biomineralization by regulation of denitrification in MSR-1 cells (Li et al 2014). In the current study, however, there is no significant (p < 0.05) difference in expression level of MgFnr between the three oxygen conditions throughout fermentations.…”

“…The magnetosomes have attracted much attention for potential biomedical applications such as magnetic resonance imaging, magnetic hyperthermia and drug targeting (Alphandéry 2014), as well as the development of immunoassay and magnetic markers (Arakaki et al 2008). To understand the formation of magnetosomes, Magnetospirillum gryphiswaldense MSR-1 has been used as a model microorganism (Schüler and Baeuerlein 1996;Heyen and Schüler 2003;Sun et al 2008;Liu et al 2010;Zhang et al 2011;Li et al 2014;Loehr et al 2016), most likely as it is not a strict anaerobe, thus simplifying its cultivation.…”

“…A thorough review of the biosynthesis process was recently published (Uebe & Schüler, 2016). Key genes are involved in metal ion activation and substrate recognition (Deng et al 2015), iron utilization (Rong et al 2008;Uebe et al 2010;Rong et al 2012;Qi et al 2012), oxidative stress protection (Rong et al 2012;Qi et al 2012), as well as regulation of denitrification (Li et al 2014). Expression profiles of 13 such genes were previously examined during high-iron (20 μM ferric citrate) and low-iron (without ferric citrate) cultivations of MSR-1 cells (Wang et al 2013).…”

Generation of Multifunctional Magnetic Nanoparticles with Amplified Catalytic Activities by Genetic Expression of Enzyme Arrays on Bacterial Magnetosomes

“…[28,37,38] We demonstrate that arrays genetically stitched together of up to five GusA monomers plus an additional mEGFP can be fused as a large hybrid protein to a single MamC anchor protein. Purified magnetosome particles exhibited mEGFP fluorescence and a stable, up to 2.8-fold increased specific activity compared to monomeric GusA protein expressed as single-copy magnetosome fusions.…”