No evidence for the putative nitric oxide sensor NsrR as a key regulator of magnetosome formation in <i>Magnetospirillum gryphiswaldense</i>

Magnetotactic bacteria produce membrane-bound organelles known as magnetosomes, which consist of chains of magnetite crystals and function as sensors for orientation within the Earth's magnetic field. Magnetosome biosynthesis is a complex, multistep process that depends on iron availability and suboxic conditions. However, the expression of the ca. 30 core magnetosome biosynthetic genes has previously been described as constitutive and largely unaffected by environmental conditions. A recent study by Pang et al., published in this journal, reported the identification of a transcriptional regulator, NsrRMg, which was proposed to activate magnetosome biosynthesis in Magnetospirillum gryphiswaldense in response to the endogenous signaling molecule nitric oxide (NO). Furthermore, the study suggested that NO also regulates a putative, previously unrecognized nitrification pathway that presumably supports endogenous NO production via denitrification, even in the absence of extracellular nitrate. Here, we present results of genetic and transcriptional analyses demonstrating that, contrary to the findings of Pang et al., NsrRMg is not required for magnetosome biosynthesis. We also refute the existence of the proposed nitrification pathway and conclude that there is no compelling evidence supporting a role of NsrRMg as a key regulator of magnetosome formation in M. gryphiswaldense.