Control of gene expression by FNR-like proteins in facultatively anaerobic bacteria

HcpR of Porphyromonas gingivalis is required for growth under nitrosative stress and survival within host cells

Although the Gram-negative, anaerobic periodontopathogen Porphyromonas gingivalis must withstand nitrosative stress, which is particularly high in the oral cavity, the mechanisms allowing for protection against such stress are not known in this organism. In this study, microarray analysis of P. gingivalis transcriptional response to nitrite and nitric oxide showed drastic upregulation of the PG0893 gene coding for hybrid cluster protein (Hcp), which is a putative hydroxylamine reductase. Although regulation of hcp has been shown to be OxyR dependent in Escherichia coli, here we show that in P. gingivalis its expression is dependent on the Fnr-like regulator designated HcpR. Growth of the isogenic mutant V2807, containing an ermF-ermAM insertion within the hcpR (PG1053) gene, was significantly reduced in the presence of nitrite (P < 0.002) and nitric oxide-generating nitrosoglutathione (GSNO) (P < 0.001), compared to that of the wild-type W83 strain. Furthermore, the upregulation of PG0893 (hcp) was abrogated in V2807 exposed to nitrosative stress. In addition, recombinant HcpR bound DNA containing the hcp promoter sequence, and the binding was hemin dependent. Finally, V2807 was not able to survive with host cells, demonstrating that HcpR plays an important role in P. gingivalis virulence. This work gives insight into the molecular mechanisms of protection against nitrosative stress in P. gingivalis and shows that the regulatory mechanisms differ from those in E. coli.

Fe/Mn superoxide dismutase-encoding gene in Paracoccus denitrificans is induced by azide and expressed independently of the FNR-type regulators

Paracoccus denitrificans cells undergo changes in protein composition upon exposure to azide, a known activator of the fumarate-nitrate reduction (FNR)-type transcription factor NarR. One of the most prominent protein species inducible by azide is a Fe/Mn-family superoxide dismutase (SOD). Azide induces SOD at protein, mRNA transcript, and enzyme activity levels in the aerobically growing cells. Since SOD expression remains unaffected in the fnrP-, nnr-, and narR-mutant strains, we postulate a mechanism independent of the known FNR-type regulators but involving a redox signal arising from the respiratory chain.

Unraveling an FNR based regulatory circuit in Paracoccus denitrificans using a proteomics-based approach

The switch from aerobic to anaerobic respiration in the bacterium Paracoccus denitrificans is orchestrated by the action of three FNR-type transcription regulators FnrP, NNR and NarR, which are sensors for oxygen, nitric oxide and nitrite, respectively. In this work, we analyzed the protein composition of four strains (wild type, FnrP-, NNR- and NarR-mutant strains) grown aerobically, semiaerobically and semiaerobically in the presence of nitrate to discover the global role of FNR-family transcription regulators using proteomics, with data validation at the transcript and genome levels. Expression profiles were acquired using two-dimensional gel electrophoresis for 737 protein spots, in which 640 proteins were identified using mass spectrometry. The annotated 2-D proteome map provided the most comprehensive coverage of P. denitrificans proteome available to-date and can be accessed on-line at http://www.mpiib-berlin.mpg.de/2D-PAGE/. Our results revealed several types of regulation under the conditions tested: (1) FnrP-controlled regulation of nitrous oxide reductase, UspA and OmpW as confirmed at protein, transcript and DNA level (position of FNR boxes). (2) Proteins regulated via additional regulators, including proteins involved in NNR and NarR regulons: nitrate reductase beta-subunit, TonB-dependent receptors, nitrite reductase, a TenA-type transcription regulator, and an unknown protein with an alpha/beta hydrolase fold. (3) Proteins whose expression was affected mainly by the growth condition. This group contains SSU ribosomal protein S305 / sigma(54) modulation protein, and two short-chain reductase-dehydrogenase proteins.