R. Gachhui

Bio: R. Gachhui is an academic researcher from University of Calcutta. The author has contributed to research in topics: Mercury(II) reductase & Mercury (element). The author has an hindex of 7, co-authored 17 publications receiving 124 citations.

Detoxification of mercury and organomercurials by nitrogen-fixing soil bacteria

Abstract: Minimal inhibitory concentration values of HgCl2 and 5 organomercurials were determined against 24 mercury-resistant N2-fixing soil bacteria previously isolated from soil and identified in our laboratory. These bacterial strains also displayed multiple antibiotic resistant properties. Typical growth pattern of a highly mercury-resistantBeijerinckia sp (KDr2) was studied in liquid broth supplemented with toxic levels of mercury compounds. Four bacterial strains were selected for determining their ability to volatilize mercury and their Hg-volatilizing capacity was different. Cell-free extracts prepared from overnight mercury-induced cells catalyzed Hg2+-induced NADPH oxidation. Specific activities of Hg2+-reductase which is capable of catalyzing conversion of Hg2+ →Hg(o) of 10 Hg-resistant bacterial strains are also reported.

Effect of thiol compounds and flavins on mercury and organomercurial degrading enzymes in mercury resistant aquatic bacteria.

Abstract: Plasmid-determined mercuric and organomercurial resistance in microorganisms has been studied by several workers. Mercury reductase, catalyzing the reduction of mercury depends on sulfhydryl compounds. Organomercurial lyase that catalyzes the splitting of C-Hg linkages also needs thiol compounds for its activity. Until recently, no study has been reported on thiol specificity of these enzymes from various sources. In the present study, the authors report on enzymatic volatilization of HgCl{sub 2} by fourteen Hg-resistant bacterial strains. They have also studied thiol specificity of Hg-reductases and organomercurial lyases isolated from the above bacterial species. Hg-reductase is known to have FAD-moiety which stimulates enzyme activity whereas FMN and riboflavin are ineffective in this regard. The effect of flavins, namely FAD, FMN and riboflavin, on Hg-reductase and organomercurial lyase activity is also reported here.

Effect of mercury and organomercurials on cellular glucose utilization: a study using resting mercury‐resistant yeast cells

Abstract: Aims: Mercury compounds are highly toxic to all types of living cells. Isolated yeast strains of Rhodotorula rubra showed high and low resistance pattern towards mercury and organomercurial compounds. To investigate the basis of differential sensitivity of these two types of strains, glucose utilization was measured in the presence of mercury compounds. Methods and Results: Glucose utilization process remained unaffected in resting cells of highly Hg2+-resistant strain in the presence of HgCl2 but not in the presence of phenylmercuric acetate and thimerosal. However, HgCl2 significantly affected glucose utilization in the case of low-resistant cells. The Hg-retaining ability of the cell wall of highly Hg2+-resistant yeast strain was greater than that of the weakly Hg2+-resistant strain. The spheroplast-bound Hg2+ was also significantly less in the highly Hg2+-resistant strain than in the weakly Hg2+-resistant strain. Conclusions: Glucose uptake machinery was not affected in the presence of toxic metal ions in the case of high-resistant strains. But in the case of low Hg2+-resistant strain, glucose transport system may be affected either by inactivation of sensor proteins containing –SH group associated with glucose uptake. Significance and Impact of the Study: Cell wall of mercury-resistant yeast cells may play an important role in heavy metal bioremediation process.

Volatilization of mercury compounds and utilization of various aromatic compounds by a broad-spectrum mercury resistant Bacillus pasteurii strain

Abstract: Aquatic ecosystems may receive aromatic compounds through various routes These compounds can cause cancerous diseases in aquatic animals and enhance mutagenicity of the sediments The persistence of aromatic compounds deposited in sediments is affected by microbial degradation Plasmid-determined mercuric and organomercurial resistance in microorganisms has also been studied by several workers Utilization of various aromatic compounds as sole sources of carbon by an Hg-resistant bacterial strain has not been reported The author isolated a broad-spectrum Hg-resistant Bacillus pasteurii strain DR{sub 2} which could volatilize different mercury compounds and utilize various aromatic compounds as sole sources of carbon This strain preferentially utilized benzene in a medium containing both glucose and benzene To their knowledge, until recently there has been no report on preferential utilization of other compounds, particularly an aromatic compound to glucose in a mixture

Mercury resistance in nitrogen-fixing soil bacteria

Abstract: A large number of mercury-resistant nitrogen-fixing bacteria were isolated by screening soil samples from agricultural farms of West Bengal, India. All of the samples were found to contain significant quantities of mercury. Twenty five of the pure isolates were identified to the generic level following Bergey's Manual of Determinative Bacteriology, 8th edition. These organisms were resistant to HgCl2, and a few of them were resistant to phenylmercuric acetate (PMA) as well. Their mercury resistance was also associated with antibiotic resistance properties. Growth of two mercury-resistant N2-fixing organisms was monitored in liquid media supplemented with HgCl2. The mercury volatilizing capacity of four mercury-resistant bacterial strains was also determined.

Mercury toxicity in plants

Abstract: Mercury poisoning has become a problem of current interest as a result of environmental pollution on a global scale. Natural emissions of mercury form two-thirds of the input; manmade releases form about one-third. Considerable amounts of mercury may be added to agricultural land with sludge, fertilizers, lime, and manures. The most important sources of contaminating agricultural soil have been the use of organic mercurials as a seed-coat dressing to prevent fungal diseases in seeds. In general, the effect of treatment on germination is favorable when recommended dosages are used. Injury to the seed increases in direct proportion to increasing rates of application. The availability of soil mercury to plants is low, and there is a tendency for mercury to accumulate in roots, indicating that the roots serve as a barrier to mercury uptake. Mercury concentration in aboveground parts of plants appears to depend largely on foliar uptake of Hg0 volatilized from the soil. Uptake of mercury has been found to be plant specific in bryophytes, lichens, wetland plants, woody plants, and crop plants. Factors affecting plant uptake include soil or sediment organic content, carbon exchange capacity, oxide and carbonate content, redox potential, formulation used, and total metal content. In general, mercury uptake in plants could be related to pollution level. With lower levels of mercury pollution, the amounts in crops are below the permissible levels. Aquatic plants have shown to be bioaccumulators of mercury. Mercury concentrations in the plants (stems and leaves) are always greater when the metal is introduced in organic form. In freshwater aquatic vascular plants, differences in uptake rate depend on the species of plant, seasonal growthrate changes, and the metal ion being absorbed. Some of the mercury emitted from the source into the atmosphere is absorbed by plant leaves and migrates to humus through fallen leaves. Mercury-vapor uptake by leaves of the C3 speciesoats, barley, and wheat is five times greater than that by leaves of the C4 species corn, sorghum, and crabgrass. Such differential uptake by C3 and C4 species is largely attributable to internal resistance to mercury-vapor binding. Airborne mercury thus seems to contribute significantly to the mercury content of crops and thereby to its intake by humans as food. Accumulation, toxicity response, and mercury distribution differ between plants exposed through shoots or through roots, even when internal mercury concentrations in the treated plants are similar. Throughfall and litterfall play a significant role in the cycling and deposition of mercury. The possible causal mechanisms of mercury toxicity are changes in the permeability of the cell membrane, reactions of sulphydryl (-SH) groups with cations, affinity for reacting with phosphate groups and active groups of ADP or ATP, and replacement of essential ions, mainly major cations. In general, inorganic forms are thought to be more available to plants than are organic ones.

Distribution, diversity and evolution of the bacterial mercury resistance (mer) operon

Abstract: Mercury and its compounds are distributed widely across the earth. Many of the chemical forms of mercury are toxic to all living organisms. However, bacteria have evolved mechanisms of resistance to several of these different chemical forms, and play a major role in the global cycling of mercury in the natural environment. Five mechanisms of resistance to mercury compounds have been identified, of which resistance to inorganic mercury (HgR) is the best understood, both in terms of the mechanisms of resistance to mercury and of resistance to heavy metals in general. Resistance to inorganic mercury is encoded by the genes of the mer operon, and can be located on transposons, plasmids and the bacterial chromosome. Such systems have a worldwide geographical distribution, and furthermore, are found across a wide range of both Gram-negative and Gram-positive bacteria from both natural and clinical environments. The presence of mer genes in bacteria from sediment cores suggest that mer is an ancient system. Analysis of DNA sequences from mer operons and genes has revealed genetic variation both in operon structure and between individual genes from different mer operons, whilst analysis of bacteria which are sensitive to inorganic mercury has identified a number of vestigial non-functional operons. It is hypothesised that mer, due to its ubiquity with respect to geographical location, environment and species range, is an ancient system, and that ancient bacteria carried genes conferring resistance to mercury in response to increased levels of mercury in natural environments, perhaps resulting from volcanic activity. Models for the evolution of both a basic mer operon and for the Tn21-related family of mer operons and transposons are suggested. The study of evolution in bacteria has recently become dominated by the generation of phylogenies based on 16S rRNA genes. However, it is important not to underestimate the roles of horizontal gene transfer and recombinational events in evolution. In this respect mer is a suitable system for evaluating phylogenetic methods which incorporate the effects of horizontal gene transfer. In addition, the mer operon provides a model system in the study of environmental microbiology which is useful both as an example of a genotype which is responsive to environmental pressures and as a generic tool for the development of new methodology for the analysis of bacterial communities in natural environments.

Arginine conversion to nitroxide by tetrahydrobiopterin-free neuronal nitric-oxide synthase. Implications for mechanism.

Abstract: We studied catalysis by tetrahydrobiopterin (H4B)-free neuronal nitric-oxide synthase (nNOS) to understand how heme and H4B participate in nitric oxide (NO) synthesis. H4B-free nNOS catalyzed Arg oxidation to N(omega)-hydroxy-l-Arg (NOHA) and citrulline in both NADPH- and H(2)O(2)-driven reactions. Citrulline formation was time- and enzyme concentration-dependent but was uncoupled relative to NADPH oxidation, and generated nitrite and nitrate without forming NO. Similar results were observed when NOHA served as substrate. Steady-state and stopped-flow spectroscopy with the H4B-free enzyme revealed that a ferrous heme-NO complex built up after initiating catalysis in both NADPH- and H(2)O(2)-driven reactions, consistent with formation of nitroxyl as an immediate product. This differed from the H4B-replete enzyme, which formed a ferric heme-NO complex as an immediate product that could then release NO. We make the following conclusions. 1) H4B is not essential for Arg oxidation by nNOS, although it helps couple NADPH oxidation to product formation in both steps of NO synthesis. Thus, the NADPH- or H(2)O(2)-driven reactions form common heme-oxy species that can react with substrate in the presence or absence of H4B. 2) The sole essential role of H4B is to enable nNOS to generate NO instead of nitroxyl. On this basis we propose a new unified model for heme-dependent oxygen activation and H4B function in both steps of NO synthesis.

Bacterial nitric oxide synthases.

Abstract: Nitric oxide synthases (NOSs) are multidomain metalloproteins first identified in mammals as being responsible for the synthesis of the wide-spread signaling and protective agent nitric oxide (NO). Over the past 10 years, prokaryotic proteins that are homologous to animal NOSs have been identified and characterized, both in terms of enzymology and biological function. Despite some interesting differences in cofactor utilization and redox partners, the bacterial enzymes are in many ways similar to their mammalian NOS (mNOS) counterparts and, as such, have provided insight into the structural and catalytic properties of the NOS family. In particular, spectroscopic studies of thermostable bacterial NOSs have revealed key oxyheme intermediates involved in the oxidation of substrate L-arginine (Arg) to product NO. The biological functions of some bacterial NOSs have only more recently come to light. These studies disclose new roles for NO in biology, such as taking part in toxin biosynthesis, protection against oxidative stress, and regulation of recovery from radiation damage.

Glutathione in bacteria

Abstract: Glutathione metabolism and its role in vital functions of bacterial cells are considered, as well as common features and differences between the functions of glutathione in prokaryotic and eukaryotic cells. Particular attention is given to the recent data for the role of glutathione in bacterial redox-regulation and adaptation to stresses.