Nitric oxide (NO), synthesized in eukaryotes by the NO synthases, has multiple roles in signalling pathways and in protection against pathogens. Pathogenic microorganisms have apparently evolved defence mechanisms that counteract the effects of NO and related reactive nitrogen species. Regulatory proteins that sense NO mediate the primary response to NO and nitrosative stress. The only regulatory protein in enteric bacteria known to serve exclusively as an NO-responsive transcription factor is the enhancer binding protein NorR (refs 9, 10-11). In Escherichia coli, NorR activates the transcription of the norVW genes encoding a flavorubredoxin (FlRd) and an associated flavoprotein, respectively, which together have NADH-dependent NO reductase activity. The NO-responsive activity of NorR raises important questions concerning the mechanism of NO sensing. Here we show that the regulatory domain of NorR contains a mononuclear non-haem iron centre, which reversibly binds NO. Binding of NO stimulates the ATPase activity of NorR, enabling the activation of transcription by RNA polymerase. The mechanism of NorR reveals an unprecedented biological role for a non-haem mononitrosyl-iron complex in NO sensing.

The International Pseudomonas aeruginosa Consortium is sequencing over 1000 genomes and building an analysis pipeline for the study of Pseudomonas genome evolution, antibiotic resistance and virulence genes. Metadata, including genomic and phenotypic data for each isolate of the collection, are available through the International Pseudomonas Consortium Database (http://ipcd.ibis.ulaval.ca/). Here, we present our strategy and the results that emerged from the analysis of the first 389 genomes. With as yet unmatched resolution, our results confirm that P. aeruginosa strains can be divided into three major groups that are further divided into subgroups, some not previously reported in the literature. We also provide the first snapshot of P. aeruginosa strain diversity with respect to antibiotic resistance. Our approach will allow us to draw potential links between environmental strains and those implicated in human and animal infections, understand how patients become infected and how the infection evolves over time as well as identify prognostic markers for better evidence-based decisions on patient care.

The regulatory protein NsrR, a member of the Rrf2 family of transcription repressors, is specifically dedicated to sensing nitric oxide (NO) in a variety of pathogenic and non-pathogenic bacteria. It has been proposed that NO directly modulates NsrR activity by interacting with a predicted [Fe-S] cluster in the NsrR protein, but no experimental evidence has been published to support this hypothesis. Here we report the purification of NsrR from the obligate aerobe Streptomyces coelicolor. We demonstrate using UV-visible, near UV CD and EPR spectroscopy that the protein contains an NO-sensitive [2Fe-2S] cluster when purified from E. coli. Upon exposure of NsrR to NO, the cluster is nitrosylated, which results in the loss of DNA binding activity as detected by bandshift assays. Removal of the [2Fe-2S] cluster to generate apo-NsrR also resulted in loss of DNA binding activity. This is the first demonstration that NsrR contains an NO-sensitive [2Fe-2S] cluster that is required for DNA binding activity.

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