Interruption of the denitrification pathway influences cell growth and magnetosome formation in Magnetospirillum magneticum AMB-1

Abstract

Aims: Intracellular magnetosome synthesis in magnetotactic bacteria has been proposed to be a process involving functions of a variety of proteins. To learn more about the genetic control that is involved in magnetosome formation, nonmagnetic mutants are screened and characterized. Methods and Results: Conjugation‐mediated transposon mutagenesis was applied to screen for nonmagnetic mutants of Magnetospirillum magneticum AMB‐1 that were unable to respond to the magnetic field. A mutant strain with disruption of a gene locus encoding nitric oxide reductase was obtained. Growth and magnetosome formation under different conditions were further characterized. Conclusions: Interruption of denitrification by inactivating nitric oxide reductase was responsible for the compromised growth and magnetosome formation in the mutant with shorter intracellular chains of magnetite crystals than those of wild‐type cells under anaerobic conditions. Nevertheless, the mutant displayed apparently normal growth in aerobic culture. Significance and Impact of the Study: Efficient denitrification in the absence of oxygen is not only necessary for maintaining cell growth but may also be required to derive sufficient energy to mediate the formation of magnetosome vesicles necessary for the initiation or activation of magnetite formation.

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

“…In contrast to Nap, NirS is not absolutely required for anaerobic respiration, which substantiates the previous finding that nitrate reduction catalyzed by Nap is the primary energy-generating process in denitrification of MSR-1 (13). In M. magneticum, a similar phenotype (poor an-aerobic growth, impaired magnetite synthesis) was observed upon concomitant interruption of norB and nirS (amb1395) (37). Since an additional nirS homolog (amb4165) is present in the M. magneticum genome, the results of Wang et al (37) imply that Amb1395 might be the main nitrite reductase, while Amb4165 has no significant function in denitrification and magnetite synthesis.…”

“…In M. magneticum, a similar phenotype (poor an-aerobic growth, impaired magnetite synthesis) was observed upon concomitant interruption of norB and nirS (amb1395) (37). Since an additional nirS homolog (amb4165) is present in the M. magneticum genome, the results of Wang et al (37) imply that Amb1395 might be the main nitrite reductase, while Amb4165 has no significant function in denitrification and magnetite synthesis. However, in our experiments, both amb1395 and amb4165 from M. magneticum when placed under the control of the nirS promoter region from MSR-1 were able to fully restore WT-like growth and magnetosome formation in the MSR-1 ⌬nirS mutant, indicating that the speculated lack of amb4165 function might be due to transcriptional inactivity under the test conditions.…”

The Periplasmic Nitrate Reductase Nap Is Required for Anaerobic Growth and Involved in Redox Control of Magnetite Biomineralization in Magnetospirillum gryphiswaldense

“…Whereas deletion of nosZ only had a weak effect on anaerobic growth and biomineralization, we were unable to detect any growth in our ⌬norCB mutant of MSR-1 under anaerobic and microaerobic conditions in the presence of nitrate, probably due to the toxicity of the accumulated intermediate nitric oxide, since growth could be rescued by additional deletion of nirS in the ⌬norCB background (Y. Li and D. Schüler, unpublished data). In contrast, in a previous study Wang and colleagues reported some growth for an AMB-1 ⌬norB mutant under anaerobic conditions (61). The discrepancy between nor mutants of MSR-1 (no growth) and AMB-1 (poor growth) might be due to different genotypes, since in the AMB-1 mutant only norB (located downstream of norC) was interrupted by transposon insertion, thereby possibly retaining some residual activity of the nitric oxide reductase enzyme.…”

“…Later, a soluble periplasmic nitrate reductase implicated in magnetite synthesis was purified from MS-1 (56). Wang et al recently interrupted a gene (norB) for nitric oxide reductase in AMB-1 by transposon insertion and found that shorter magnetosome chains were produced under anaerobic conditions (61). However, except for this single study, no genetic evidence has been available for these possible functions in vivo so far, and the exact interrelation of these two pathways as well as the redox process governing magnetite biomineralization has largely remained unclear.…”

Chemical signature of magnetotactic bacteria

“…It is interesting to note that these latter elements are known to be cofactors of metalloenzymes (32) that are essential for bacterial growth and metabolism. Molybdenum is indeed required for nitrate reductase enzymatic activity, which has been reported in AMB-1 and other MTB strains (33,34). The nitrate reductase reduces nitrate for cell respiration and is also necessary for the magnetite biomineralization in strains MSR-1 and AMB-1 (33, 34).…”