Cytochrome cd1 Nitrite Reductase NirS Is Involved in Anaerobic Magnetite Biomineralization in Magnetospirillum gryphiswaldense and Requires NirN for Proper d1 Heme Assembly

Abstract

bThe alphaproteobacterium Magnetospirillum gryphiswaldense synthesizes magnetosomes, which are membrane-enveloped crystals of magnetite. Here we show that nitrite reduction is involved in redox control during anaerobic biomineralization of the mixed-valence iron oxide magnetite. The cytochrome cd 1 -type nitrite reductase NirS shares conspicuous sequence similarity with NirN, which is also encoded within a larger nir cluster. Deletion of any one of these two nir genes resulted in impaired growth and smaller, fewer, and aberrantly shaped magnetite crystals during nitrate reduction. However, whereas nitrite reduction was completely abolished in the ⌬nirS mutant, attenuated but significant nitrite reduction occurred in the ⌬nirN mutant, indicating that only NirS is a nitrite reductase in M. gryphiswaldense. However, the ⌬nirN mutant produced a different form of periplasmic d 1 heme that was not noncovalently bound to NirS, indicating that NirN is required for full reductase activity by maintaining a proper form of d 1 heme for holo-cytochrome cd 1 assembly. In conclusion, we assign for the first time a physiological function to NirN and demonstrate that effective nitrite reduction is required for biomineralization of wild-type crystals, probably by contributing to oxidation of ferrous iron under oxygen-limited conditions.

The Terminal Oxidase cbb 3 Functions in Redox Control of Magnetite Biomineralization in Magnetospirillum gryphiswaldense

“…Deletion of nap genes encoding a periplasmic nitrate reductase not only abolished anaerobic growth and delayed aerobic growth but also severely affected magnetite synthesis and led to the formation of fewer, smaller, and irregular magnetosomes during denitrification and microaerobic oxygen respiration (6). Genetic inactivation of the nitrite reductase NirS resulted in defective growth and biosynthesis of smaller and irregular particles during nitrate reduction (7). In addition to denitrification, which occurs only under suboxic conditions, MSR-1 and related MTB are also capable of aerobic respiration using O 2 as the terminal electron acceptor.…”

“…In the alphaproteobacterium Magnetospirillum gryphiswaldense (here referred to as MSR-1) and related MTB, magnetosomes comprise membrane-enveloped magnetite crystals and are aligned in chains (1). Previous studies revealed that magnetosome biosynthesis is largely controlled by a set of about 30 specific genes localized within the genomic magnetosome island (MAI) (2)(3)(4)(5), whereas determinants encoded elsewhere play accessory roles in magnetite biomineralization (6,7). The synthesis of the mixed-valence iron oxide magnetite [FeII(FeIII) 2 O 4 ] was previously proposed to proceed by coprecipitation of balanced amounts of ferrous and ferric iron, which thus requires a precise biological regulation of intracellular redox conditions (8)(9)(10).…”

“…One of the major redox pathways in microaerophilic MSR-1 was found to be denitrification, which is a respiratory process to stepwise reduce nitrate to N 2 (12). Our previous work showed that in MSR-1, denitrification plays an important role in poising redox conditions for magnetite biomineralization (6,7). Deletion of nap genes encoding a periplasmic nitrate reductase not only abolished anaerobic growth and delayed aerobic growth but also severely affected magnetite synthesis and led to the formation of fewer, smaller, and irregular magnetosomes during denitrification and microaerobic oxygen respiration (6).…”

NirN Protein from Pseudomonas aeruginosa is a Novel Electron-bifurcating Dehydrogenase Catalyzing the Last Step of Heme d1 Biosynthesis

“…However, in that study it was also observed by UV-visible absorption spectroscopy of periplasmic protein fractions that the cofactor content of NirS in the P. aeruginosa ⌬nirN strain was different from that in the P. aeruginosa WT strain (15). Later, the same observation was reported for a ⌬nirN strain of Magnetospirillum gryphiswaldense (17).…”

Synthesis, Functionalization, and Design of Magnetic Nanoparticles for Theranostic Applications

“…[280] As it was mentioned, MTB exploit the alignment of magnetosomes to migrate into zones where oxygen and other redox active compounds are horizontally stratified. Thus, controlling the level of oxygen to provide microaerobic and anaerobic conditions [281] as well as controlling the redox potential [282] are of two most important factors which are absolutely required for magnetite biomineralization in MTB. Utilizing an oxygen-controlled fermenter provides a precise control over the culture of MTB, especially the most popular investigated model, Magnetospirillium species.…”