1,720,971 research outputs found
Investigation into the role of the truncated denitrification chain in Rhizobium sullae strain HCNT1
No full textMost denitrifying bacteria reduce nitrate to the inert gases nitrous oxide or nitrogen. A remarkable exception to this is Rhizobium sullae strain HCNT1, which catalyses only a single step in the denitrification pathway, the reduction of nitrite to the reactive molecule nitric oxide. Further study demonstrated that HCNT1 does not encode the genes for NO reductase. Prolonged incubation of HCNT1 under anoxic conditions revealed that the cells had reduced culturability but not viability when nitrite was present. This may indicate an adaptation to anoxic conditions to provide resistance to environmental stresses. A closely related strain of R. sullae, strain CC1335, which is unable to denitrify, was found to lose culturability but not viability irrespective of the presence of nitrite. When the gene for nitrite reductase was mobilized into CC1335, this increased culturability with or without nitrite. These results indicate that the presence of nitrite reductase can influence the long-term survival of R. sullae strains and may provide an explanation as to why HCNT1 possesses this unusual truncation of its denitrification electron transport chain
Nitrite reductase in bacteroids of Rhizobium "hedysari" strain HCNT1.
No full textEx planta, bacteroids of the sulla-symbiont Rhizobium “hedysari” strain HCNT 1 terminated reduction of nitrite at nitrous oxide irrespective of the presence or absence of acetylene. Nitrate was not reduced during the experimental period, but slight nitrate reductase activity occurred if incubation with nitrate was prolonged (up to 15 h). As was observed in free-living cells, exposure of the bacteroids to the metal chelator, diethyldithiocarbamate, prevented reduction of nitrite, indicating the presence of a copper-containing nitrite reductase. Pulses of 10–75 μM nitrite transiently impeded O2 uptake in bacteroids, which resumed consumption of O2 when the nitrite had been reduced. Exposure to >1.0 mM nitrite for 24h greatly inhibited nitrogenase activity (assayed as acetylene reduction activity) of bacteroids in planta. Exposure to the same concentrations of nitrite after 1h of incubation in the presence of acetylene almost completely stopped ongoing ethylene production in bacteroids of strain HCNT 1 extracted from nodules. Free cells of the non-nitrite-reducing R. “hedysari” strain CC 1335 were lacking in nitrogenase (acetylene-reduction) activity, whereas identically cultured (low-oxygen) strain HCNT 1 cells reduced both nitrite and acetylene
Nitrite reduction in Rhizobium "hedysari" strain HCNT1.
No full textRhizobium “hedysari” strain HCNT 1 rapidly reduced nitrite to N2O, only slowly reduced nitrate to nitrite and did not exhibit nitrous oxide reductase activity. Nitrite reduction in this rhizobium strain may be a detoxification mechanism for conversion of nitrite, which inhibits O2 uptake, to non-toxic N2O. Concentrations of nitrite as small as 3 μM diminished O2 uptake in whole cells. The bacterium did not couple energy conservation with nitrate or nitrite reduction. Cells neither grew anaerobically at the expense of these nitrogen oxides nor translocated protons during reduction of nitrite. Induction of nitrite reductase activity was not a response to the presence of nitrate or nitrite, but occurred instead when the O2 concentration in culture atmospheres fell to o, which is synthesized only in cells grown under O2-limited conditions, may account for the toxicity of nitrite in strain HCNT 1
Selenite reducing capacity of the copper-containing nitrite reductase of Rhizobium sullae
No full textRhizobium sullae strain HCNT1 contains a nitric oxide-producing nitrite reductase of unknown function due to the absence of a complementary nitric oxide reductase. HCNT1 had the ability to grow on selenite concentrations as high as 50 mM, and during growth, selenite was reduced to the less toxic elemental selenium. An HCNT1 mutant lacking nitrite reductase grew poorly in the presence of 5 mM selenite, was unable to grow in the presence of 25 or 50 mM selenite and also showed no evidence of selenite reduction. A naturally occurring nitrite reductase-deficient R. sullae strain, CC1335, also showed little growth on the higher concentrations of selenite. Mobilization of a plasmid containing the HCNT1 gene encoding nitrite reductase into CC1335 increased its resistance to selenite. To confirm that this ability to grow in the presence of high concentrations of selenite correlated with nitrite reductase activity, a new nitrite reductase-containing strain was isolated from the same location where HCNT1 was isolated. This strain was also resistant to high concentrations of selenite. Inactivation of the gene encoding nitrite reductase in this strain increased selenite sensitivity. These data suggest that the nitrite reductase of R. sullae provides resistance to selenite and offers an explanation for the radically truncated denitrification found uniquely in this bacterium
Insight the role of nirK in a non-denitrifying rhizobium.
No full textDuring the last few years an attempt to determine the role of the gene encoding nitrite reductase, nirK, in some Rhizobium sullae strains was pursued. These strains show the peculiarity of possessing only a nitrite reductase (Nir) without any of the other enzymes needed for the complete reduction of nitrite and nitrate to molecular nitrogen. Once it was verified that the reduction of nitrite to nitric oxide (NO) is not apparently linked to energy conservation and that this Cu-containing enzyme only requires low oxygen for its expression, physiological roles other than nitrite reduction were investigated. More recently an active involvement of Nir of R. sullae in the reduction of other oxyanions was suggested. Particularly, this protein seems to be able to reduce selenite to elemental Se. Further investigation revealed that the selenite reductase does not need low oxygen concentration for its induction, unlike when Nir is serving as a nitrite reductase. Several experiments concerning the introduction of nirK in different rhizobial strains unable to reduce both nitrite and selenite suggested that, although the two activities occur in quite different conditions (aerobic and anaerobic), the enzyme could be the same. Attempts to purify the R. sullae Nir are in progress. This presentation would like to summarize the work made during the last five years of COST 856 research activity
Analysis of the Role of the nnrR gene Product in the Response of Rhodobacter sphaeroides 2.4.1 to Exogenous Nitric Oxide
No full textRhodobacter sphaeroides 2.4.1, which is incapable of denitrification, has been found to carry nnrR, the nor operon, and nnrS, which are utilized for denitrification in R. sphaeroides 2.4.3. The gene encoding nitrite reductase was not found in 2.4.1. Expression of β-galactosidase activity from a norB-lacZ fusion was activated when cells of 2.4.1 were incubated with NO-producing bacteria. This result indicates that the products of nnrR and the genes flanking it are utilized when 2.4.1 is growing in an environment where denitrification occurs
Electrocatalytic reduction of nitric oxide at electrodes modified with electropolymerized films of Cr (v-tpy)2 3+ and their application to cellular NO determinations
No full textNitric oxide can be electrocatalytically reduced at electrodes modified with electropolymerized films of [Cr(v-tpy)2]3+. Upon further modification with a thin film of Nafion (to prevent interferences from anions, especially nitrite), these electrodes can be employed as NO sensors in solution with submicromolar detection limits and fast response. We have carried out preliminary studies of cellular NO release from Rhodobacter sphaeroides bacterial cells with excellent results
Nitrogen oxide reduction in rhizobia
No full textRhizobia are soil bacteria typically able to symbiotically interact with legume plants to produce nitrogen fixing root nodules. While all rhizobia were once placed under the same genus, Rhizobium, nowadays the classification of rhizobia using molecular analysis has revealed a more complex relationship among strains. Several genera have been described and accepted (i.e. Rhizobium, Allorhizobium, Azorhizobium, Mesorhizobium, Sinorhizobium, Bradyrhizobium) with many species in each genus. However, within the same species, it is common to observe different strains with quite different physiological and biochemical profiles. While some traits are common among all rhizobia selected traits such as denitrification seem to be randomly distributed among genera and species. Moreover, many species are true denitrifiers (strains of Bradyrhizobium japonicum) while some are only partial NOx-reducers (strains of Rhizobium sullae) (Toffanin et al., 1996; Squartini et al. 2002). While the evolutionary advantage deriving from the complete denitrification pathways may be easily explained, the advantage gained from expressing only a fragment of such a metabolic property has not been completely clarified (Toffanin et al., 2000). A comparison will be made among rhizobia showing these traits, taking into account both free and symbiotic form
The involvement of nitrite reductase of R. sullae in the reduction of different oxyanions
No full textSeveral strains belonging to the symbiotic, nitrogen fixing species of Rhizobium, Bradyrhizobium and Sinorhizobium have been shown to encode one or more terminal reductases required for denitrification. Some isolates belonging to Rhizobium sullae have been shown to express only a copper-containing nitrite reductase, encoded by nirK, which is closely related to nitrite reductases in true denitrifiers, generating the toxic end product nitric oxide. This enzyme cannot support bacterial growth under anoxic conditions and does not require the presence of a nitrogen oxide for its expression, which depends only upon a decrease in oxygen concentration. This behaviour was previously connected with the low oxygen concentration present within the root nodule, but investigations performed so far on its host legume Hedysarum coronarium have not revealed any significant differences between wild type and nitrite reductase-deficient strains for nodulation efficiency, plant growth and nitrogen fixation. The role of this Cu-containing nitrite reductase was also investigated as a detoxification strategy or as a means to reduce the energy content in the bacterial cell in order to induce the VBNC status, so prolonging cell viability under certain conditions.
More recent results suggest that the nitrite reductase of R. sullae strains HCNT1 and A4 can reduce other physiologically important oxyanions, such as selenite, to elemental selenium, and that strains of the same species that lack Nir (e.g. strain CC1335) do not show this property. Moreover, inactivation of nirK in strain HCNT1 resulted in the loss of selenite reduction and the mobilization of nirK into CC1335 produced a phenotype able to reduce selenite. An investigation was also carried out on other rhizobial species known to possess nitrite reductase
How nirK of R. sullae HCNT1 is involved in selenite reduction
No full textA nitrogen fixing symbiotic strain of Rhizobium sullae, HCNT1, contains a nitrite reductase producing nitric oxide, but not a complementary nitric oxide reductase and the other enzymes required for a complete denitrification pathway. The function of this enzyme is still unknown. Recently this strain was found to be able to grow on selenite concentrations as high as 50 mM and during growth selenite was reduced to the less toxic elemental selenium. A mutant of HCNT1 lacking nitrite reductase showed no evidence of selenite reduction, grew poorly in the presence of 5 mM selenite and was unable to grow in the presence of 25 or 50 mM selenite. Other strains isolated from the same site where HCNT1 was originally collected, showed a similar behaviour of HCNT1.
A naturally occurring nitrite reductase deficient R. sullae strain, CC1335, isolated from a quite different site, was found unable to grow in the presence of selenite. Mobilization of a plasmid containing the HCNT1 gene encoding nitrite reductase into CC1335 increased its resistance to this oxyanion.
In the presence of nitrite, increasing concentrations of selenite into the buffer containing induced cells of HCNT1 and the nirK+ mutant strain of CC1335 result in a gradual reduction of nitric oxide production.
These data suggest that the nitrite reductase of R. sullae provides resistance to selenite indicating a possible explanation for the radically truncated denitrification chain found uniquely in this bacterium
- …
