Cytochromes c Constitute a Layer of Protection against Nitric Oxide but Not Nitrite

Abstract

Nitric oxide (NO) is a radical gas that reacts with various biological molecules in complex ways to inhibit growth as a bacteriostatic agent. NO is nearly ubiquitous because it can be generated both biotically and abiotically. To protect the cell from NO damage, bacteria have evolved many strategies, with the production of detoxifying enzymatic systems being the most efficient. Here, we report that -type cytochromes (cytochromes) constitute a primary NO protection system in , a Gram-negative environmental bacterium renowned for respiratory versatility due to its high cytochrome content. By using mutants producing cytochromes at varying levels, we found that the content of these proteins is inversely correlated with the growth inhibition imposed by NO, whereas the effect of each individual cytochrome is negligible. This NO-protecting system has no effect on nitrite inhibition. In the absence of cytochromes , other NO targets and protective proteins, such as NnrS, emerge to show physiological influences during the NO stress. We further demonstrate that cytochromes also play a similar role in , albeit only modestly. Our data thus identify the function of an important group of proteins in alleviating NO stress. It is widely accepted that the antibacterial effects of nitrite are attributable to nitric oxide (NO) formation, suggesting a correlation of bacterial susceptibilities to these two chemicals. However, compared to , is highly sensitive to nitrite but resistant to NO, implying the presence of robust NO-protective systems. Here, we show that -type cytochromes (cytochromes) play a main role in protecting against damages from NO but not from nitrite. In their absence, impacts of proteins that promote NO tolerance and that are targets of NO inhibition become evident. Our data thus reveal the specific activity of cytochromes in alleviating the stress caused by NO but not nitrite.

Nitrite modulates aminoglycoside tolerance by inhibiting cytochrome heme-copper oxidase in bacteria

“…By this logic, we focus on the respiratory chain, especially terminal oxidases. This coincides with the findings of systematic investigations into nitrite and NO physiology in Shewanella oneidensis, a facultative Gram-negative γ-proteobacterium renowned for respiratory versatility [15][16][17][18][19][20] . In this bacterium, the primary target of nitrite is heme-copper oxidase (HCO) cbb 3 (hereafter referred to as cbb 3 ), whereas NO indistinguishably interacts with all cytochromes c 15,16,19,20 .…”

“…These data, all together, manifest that nitrite modulates aminoglycoside susceptibility by compromising cbb 3based respiration, implying that cbb 3 , the bc 1 complex, or both could be targets of nitrite. This study focused on cbb 3 , because it has been shown to be susceptible to nitrite [15][16][17][18][19][20] .…”

“…The correlation between the restoration of ΔΨ by bo 3 and aa 3 in the cbb 3 -less mutant and lowered tolerance to aminoglycosides supports that PMF is critical for uptake of aminoglycosides. Interestingly, with respect to respiration, non-HCO oxidases almost suffice, based on the observation that the HCO-deficient mutants grow only marginally slower than the wild type 15,16,20 . In contrast, non-HCO oxidases are inferior to HCOs in generating PMF, because they could not pump protons 21,22 .…”

Complex Oxidation of Apocytochromes c during Bacterial Cytochrome c Maturation

“…comprise a group of facultative bacteria renowned for their respiratory versatility, a feature attributable to a large repertoire of iron-containing proteins, particularly cyts c (24). In Shewanella oneidensis, the extensively studied genus representative, there are up to 42 cyts c (in comparison, E. coli and R. capsulatus contain only 5 or so cyts c [25]), which also account for many other physiological activities, such as combating various stresses (26)(27)(28)(29)(30). This high cyt c content confers on the colony and cell pellet of S. oneidensis a reddish-brown color (31)(32)(33), a characteristic greatly facilitating studies of CCM because the abundance of cyts c correlates well with the color intensity (27).…”

Distinct Roles of Shewanella oneidensis Thioredoxin in Regulation of Cellular Responses to Hydrogen and Organic Peroxides

“…This is likely to be the case in S. oneidensis given that a dsbD mutant was also deficient in cyt c biosynthesis ( Fig. 1C) (28). It was worth noting that the absence of cyts c was not a major factor for the growth defect of the ΔtrxA strain, because both the ΔccmF and ΔdsbD strains grew much faster (generation time, ϳ48 min) than the ΔtrxA strain ( Fig.…”