Mössbauer studies of Escherichia coli sulfite reductase complexes with carbon monoxide and cyanide. Exchange coupling and intrinsic properties of the [4Fe-4S] cluster

Molecular understanding of heteronuclear active sites in heme-copper oxidases, nitric oxide reductases, and sulfite reductases through biomimetic modelling

Heme-copper oxidases (HCO), nitric oxide reductases (NOR), and sulfite reductases (SiR) catalyze the multi-electron and multi-proton reductions of O2, NO, and SO32-, respectively. Each of these reactions is important to drive cellular energy production through respiratory metabolism and HCO, NOR, and SiR evolved to contain heteronuclear active sites containing heme/copper, heme/nonheme iron, and heme-[4Fe-4S] centers, respectively. The complexity of the structures and reactions of these native enzymes, along with their large sizes and/or membrane associations, make it challenging to fully understand the crucial structural features responsible for the catalytic properties of these active sites. In this review, we summarize progress that has been made to better understand these heteronuclear metalloenzymes at the molecular level though study of the native enzymes along with insights gained from biomimetic models comprising either small molecules or proteins. Further understanding the reaction selectivity of these enzymes is discussed through comparisons of their similar heteronuclear active sites, and we offer outlook for further investigations.

A [4Fe-4S]-Fe(CO)(CN)-L-cysteine intermediate is the first organometallic precursor in [FeFe] hydrogenase H-cluster bioassembly

Biosynthesis of the [FeFe] hydrogenase active site (the 'H-cluster') requires the interplay of multiple proteins and small molecules. Among them, the radical S-adenosylmethionine enzyme HydG, a tyrosine lyase, has been proposed to generate a complex that contains an Fe(CO)₂(CN) moiety that is eventually incorporated into the H-cluster. Here we describe the characterization of an intermediate in the HydG reaction: a [4Fe-4S][(Cys)Fe(CO)(CN)] species, 'Complex A', in which a CO, a CN^- and a cysteine (Cys) molecule bind to the unique 'dangler' Fe site of the auxiliary [5Fe-4S] cluster of HydG. The identification of this intermediate-the first organometallic precursor to the H-cluster-validates the previously hypothesized HydG reaction cycle and provides a basis for elucidating the biosynthetic origin of other moieties of the H-cluster.

Oxidative stress and disruption of labile iron generate specific auxotrophic requirements in Salmonella enterica

The response of a cell to integrated stresses was investigated using environmental and/or genetic perturbations that disrupted labile iron homeostasis and increased oxidative stress. The effects of the perturbations were monitored as nutritional requirements, and were traced to specific enzymic targets. A yggX gshA cyaY mutant strain required exogenous thiamine and methionine for growth. The thiamine requirement, which had previously been linked to the Fe-S cluster proteins ThiH and ThiC, was responsive to oxidative stress and was not directly affected by manipulation of the iron pool. The methionine requirement was associated with the activity of sulfite reductase, an enzyme that appeared responsive to disruption of labile iron homeostasis. The results are incorporated in a model to suggest how the activity of iron-containing enzymes not directly sensitive to oxygen can be decreased by oxidation of the labile iron pool.

Cobalt targets multiple metabolic processes in Salmonella enterica

Cobalt is essential for growth of Salmonella enterica and other organisms, yet this metal can be toxic when present in excess. Wild-type Salmonella exhibits several metabolic defects when grown in the presence of cobalt, some of which generate visible growth consequences. Work herein identifies sulfur assimilation, iron homeostasis, and Fe-S cluster metabolism as targets for cobalt toxicity. In each case it is proposed that cobalt exerts its effect by one of two mechanisms: direct competition with iron or indirectly through a mechanism that involves the status of reduced thiols in the cell. Cobalt toxicity results in decreased siroheme production, increased expression of the Fur regulon, and decreased activity of Fe-S cluster proteins. The consequences of reduced sulfite reductase activity in particular are exacerbated by the need for glutathione in cobalt resistance. Significantly, independent metabolic perturbations could be detected at cobalt concentrations below those required to generate a detectable growth defect.

Electronic structure of iron(II)-porphyrin nitroxyl complexes: Molecular mechanism of fungal nitric oxide reductase (P450nor)

Density functional calculations are employed to investigate key intermediates of the catalytic cycle of fungal nitric oxide reductase (P450nor). The formal Fe(II)-nitroxyl species Fe(II)--NO/(-) can principally exist in the two spin-states S = 0 and S = 1. In the S = 0 case, a very covalent Fe--NO sigma bond is present, which leads to an electronic structure description that is actually intermediate between Fe(I)--NO and Fe(II)--NO(-). In contrast, the S = 1 case shows a ferrous Fe(II)--NO complex with the extra electron being stored in the pi system of the porphyrin ligand. Importantly, the Fe(II)--NO/(-) species are very basic. The electronic structures and spectroscopic properties of the corresponding N- and O-protonated forms are very different, and unequivocally show that the Mb-HNO adduct (Mb-Myoglobin) prepared by farmer and coworkers is in fact N-protonated. The presence of an axial thiolate ligand enables a second protonation leading to the corresponding Fe(IV)--NHOH- species, which is identified with the catalytically active intermediate I of P450nor. This species reacts with a second molecule of NO by initial electron transfer from NO to Fe(IV) followed by addition of NO+ forming an N--N bond. This is accompanied by an energetically very favorable intramolecular proton transfer leading to the generation of a quite stable Fe(III)--N(OH)(NOH) complex. This way, the enzyme is able to produce dimerized HNO under very controlled conditions and to prevent loss of this ligand from Fe(III). The energetically disfavoured tautomer Fe(III)--N(OH2)(NO) is the catalytically productive species that spontaneously cleaves the N--OH2 bond forming N2O and H2O in a highly exergonic reaction.

Electrochemical and spectroscopic properties of the iron-sulfur flavoprotein from Methanosarcina thermophila

An iron-sulfur flavoprotein (Isf) from the methanoarchaeaon Methanosarcina thermophila, which participates in electron transfer reactions required for the fermentation of acetate to methane, was characterized by electrochemistry and EPR and Mössbauer spectroscopy. The midpoint potential (Em) of the FMN/FMNH2 couple was -0.277 V. No flavin semiquinone was observed during potentiometric titrations; however, low amounts of the radical were observed when Isf was quickly frozen after reaction with CO and the CO dehydrogenase/acetyl-CoA synthase complex from M. thermophila. Isf contained a [4Fe-4S]2+/1+ cluster with g values of 2.06 and 1.93 and an unusual split signal with g values at 1.86 and 1.82. The unusual morphology was attributed to microheterogeneity among Isf molecules. The Em value for the 2+/1+ redox couple of the cluster was -0.394 V. Extracts from H2-CO2-grown Methanobacterium thermoautotrophicum cells catalyzed either the H2- or CO-dependent reduction of M. thermophila Isf. In addition, Isf homologs were found in the genomic sequences of the CO2-reducing methanoarchaea M. thermoautotrophicum and Methanococcus jannaschii. These results support a general role for Isf in electron transfer reactions of both acetate-fermenting and CO2-reducing methanoarchaea. It is suggested that Isf functions to couple electron transfer from ferredoxin to membrane-bound electron carriers, such as methanophenazine and/or b-type cytochromes.

EPR and Mössbauer spectroscopic studies on enoate reductase

Enoate reductase (EC 1.3.1.31) is a protein isolated from Clostridium tyrobutyricum that contains iron, labile sulfide, FAD, and FMN. The enzyme reduces the alpha,beta carbon-carbon double bond of nonactivated 2-enoates and in a reversible way that of 2-enals at the expense of NADH or reduced methyl viologen. UV-visible and EPR potentiometric titrations detect a semiquinone species in redox intermediate states characterized by an isotropic EPR signal at g = 2.0 without contribution at 580 nm. EPR redox titration shows two widely spread mid-point redox potentials (-190 and -350 mV at pH 7. 0), and a nearly stoichiometric amount of this species is detected. The data suggest the semiquinone radical has an anionic nature. In the reduced form, the [Fe-S] moiety is characterized by a single rhombic EPR spectrum, observed in a wide range of temperatures (4. 2-60 K) with g values at 2.013, 1.943, and 1.860 (-180 mV at pH 7.0). The gmax value is low when compared with what has been reported for other iron-sulfur clusters. Mössbauer studies reveal the presence of a [4Fe-4S]+2/+1 center. One of the subcomponents of the spectrum shows an unusually large value of quadrupole splitting (ferrous character) in both the oxidized and reduced states. Substrate binding to the reduced enzyme induces subtle changes in the spectroscopic Mössbauer parameters. The Mössbauer data together with known kinetic information suggest the involvement of this iron-sulfur center in the enzyme mechanism.

Desulfoviridin, a multimeric-dissimilatory sulfite reductase from Desulfovibrio vulgaris (Hildenborough). Purification, characterization, kinetics and EPR studies

Conditions for the rigorous purification of desulfoviridin, the dissimilatory sulfite reductase from the sulfate-reducing bacterium Desulfovibrio vulgaris (Hildenborough) have been established. A final purification by fast protein liquid chromatography yields at least three distinct bands that each exhibit the characteristic absorption spectrum of desulfoviridin. Two of these have been extensively characterized by amino acid analysis, isoelectric focusing, polyacrylamide gel electrophoresis, and formulation of the prosthetic centers. Each contains two pairs of [Fe4S4] and siroheme units. These results stand in marked contrast to recent work claiming significant demetallation of siroheme, excess iron content, and the presence of Fe6S6 clusters. These proposals are critically assessed in light of our results and other published work. Steady-state kinetic parameters have been determined: kcat(SO3(2-) = 0.31 mol SO3(2-).s-1.mol heme-1, Km = 0.06 mM; kcat(NO2-) = 0.038 mol NO2-.s-1.mol heme-1, Km = 0.028 mM; kcat(NH2OH) = 29 mol NH2OH.s-1.mol heme-1, Km = 48 mM. A detailed comparison is made with the Escherichia coli and spinach assimilatory sulfite reductase enzymes and spinach nitrite reductase. Highly purified samples of dissimilatory sulfite reductase display an electron paramagnetic resonance spectrum characteristic of rhombic high spin ferric heme centers, while the fully reduced enzyme shows EPR features typical of [Fe4S4] clusters. The magnetic properties of the prosthetic centers are further characterized by variable temperature experiments and spin quantitation.

Proton NMR of Escherichia coli sulfite reductase: studies of the heme protein subunit with added ligands

The heme protein subunit of sulfite reductase (SiR-HP; M(r) 64,000) from Escherichia coli as isolated contains the isobacteriochlorin siroheme exchange-coupled to a [4Fe-4S] cluster in the 2+ oxidation state. SiR-HP in the presence of a suitable electron donor can catalyze the six-electron reductions of sulfite to sulfide and nitrite to ammonia. Paramagnetic 1H NMR was used to study the low-spin complexes of SiR-HP formed by binding the exogenous inhibitor cyanide or the substrates sulfite and nitrite. As a model, the cyanide complex of purified siroheme was also prepared. The NMR spectrum of isolated ferric low-spin siroheme-CN is consistent with spin density being transferred into the a2u molecular orbital, an interaction which is symmetry-forbidden in porphyrins. The pattern of proton NMR shifts observed for isolated ferric low-spin siroheme-CN is very similar to those obtained for the protein-cyanide complex. NMR spectra of the cyanide complex of SiR-HP were obtained in all three accessible redox states. The pattern of hyperfine shifts observed for the one-electron and two-electron reduced cyanide complexes is typical of those seen for [4Fe-4S] clusters in the 2+ and 1+ oxidation states, respectively. Resonances arising from the beta-CH2 protons of cluster cysteines have been assigned for all complexes studied utilizing deuterium substitution. The cyanide-, sulfite-, and nitrite-ligated states possessed an almost identically shifted upfield cluster cysteine resonance whose presence indicates that covalent coupling exists between siroheme and cluster in solution. Data are also presented for the existence of a secondary anion binding site, the occupancy of which perturbs the oxidized SiR-HP NMR spectrum, where binding occurs at a rate much faster than that of ligand binding to heme.

Proton NMR of Escherichia coli sulfite reductase: the unligated hemeprotein subunit

The isolated hemeprotein subunit of sulfite reductase (SiR-HP) from Escherichia coli consists of a high spin ferric isobacteriochlorin (siroheme) coupled to a diamagnetic [4Fe-4S]2+ cluster. When supplied with an artificial electron donor, such as methyl viologen cation radical, SiR-HP can catalyze the six electron reductions of sulfite to sulfide and nitrite to ammonia. Thus, the hemeprotein subunit appears to represent the minimal protein structure required for multielectron reductase activity. Proton magnetic resonance spectra are reported for the first time on unligated SiR-HP at 300 MHz in all three redox states. The NMR spectrum of high spin ferric siroheme at pH 6.0 was obtained for the purpose of comparing its spectrum with that of oxidized SiR-HP. On the basis of line widths, T1 measurements, and 1D NOE experiments, preliminary assignments have been made for the oxidized enzyme in solution. The pH profile of oxidized SiR-HP is unusual in that a single resonance shows a 9 ppm shift over a range of only 3 pH units with an apparent pK = 6.7 +/- 0.2. Resonances arising from the beta-CH2 protons of cluster cysteines have been assigned using deuterium substitution for all redox states. One beta-CH2 resonance has been tentatively assigned to the bridging cysteine on the basis of chemical shift, T1, line width, and the presence of NOEs to protons from the siroheme ring. The observed pattern of hyperfine shifts can be used as a probe to measure the degree of coupling between siroheme and cluster in solution. The cluster iron sites of the resting (oxidized) enzyme are found to possess both positive and negative spin density which is in good agreement with Mossbauer results on frozen enzyme. The NMR spectrum of the 1-electron reduced form of SiR-HP is consistent with an intermediate spin (S = 1) siroheme. Intermediate spin Fe(II) hemes have only been previously observed in 4-coordinate model compounds. However, the amount of electron density transferred to the cluster, as measured by the isotropic shift of beta-CH2 resonances, is comparable to that present in the fully oxidized enzyme despite diminution of the total amount of unpaired spin density available. Addition of a second electron to SiR-HP, besides generating a reduced S = 1/2 cluster with both upfield and downfield shifted cysteine resonances, converts siroheme to the high spin (S = 2) ferrous state.(ABSTRACT TRUNCATED AT 400 WORDS)

Primary structure of the assimilatory-type sulfite reductase from Desulfovibrio vulgaris (Hildenborough): cloning and nucleotide sequence of the reductase gene

The nucleotide sequence encoding the structural gene (651 bp) and flanking regions for the assimilatory-type sulfite reductase from the sulfate-reducing bacterium Desulfovibrio vulgaris (Hildenborough) was determined after cloning a 1.4 kb HindIII/SalI genomic fragment possessing the gene into Bluescript pBS(+)KS. The primary structure of the protein was deduced, and the molecular mass of the apoprotein was estimated as 24 kDa. The amino acid sequence of the polypeptide shows some similarities at putative [Fe4S4] cluster binding sites in comparison with the heme protein subunit of the larger Escherichia coli and Salmonella typhimurium sulfite reductases and spinach nitrite reductase. This is the first reported sequence of a member of a new class of low molecular weight assimilatory sulfite-reducing enzymes recently identified in a number of anaerobic bacteria [Moura, I., Lina, A. R., Moura, J. J. G., Xavier, A. V., Fauque, G., Peck, H. D., & Le Gall, J. (1986) Biochem. Biophys. Res. Commun. 141, 1032-1041].

S = 9/2 EPR signals are evidence against coupling between the siroheme and the Fe/S cluster prosthetic groups in Desulfovibrio vulgaris (Hildenborough) dissimilatory sulfite reductase

Sulfite reductases contain siroheme and iron-sulfur cluster prosthetic groups. The two groups are believed to be structurally linked via a single, common ligand. This chemical model is based on a magnetic model for the oxidized enzyme in which all participating iron ions are exchange coupled. This description leads to two serious discrepancies. Although the iron-sulfur cluster is assumed to be a diamagnetic cubane, [4Fe-4S]2+, all iron appears to be paramagnetic in Mössbauer spectroscopy. On the other hand, EPR spectroscopy has failed to detect anything but a single high-spin heme. We have re-addressed this problem by searching for new EPR spectroscopic clues in concentrated samples of dissimilatory sulfite reductase from Desulfovibrio vulgaris (Hildenborough). We have found several novel signals with effective g values of 17, 15.1, 11.7, 9.4, 9.0, 4. The signals are interpreted in terms of an S = 9/2 system with spin-Hamiltonian parameters g = 2.00, D = -0.56 cm-1, magnitude of E/D = 0.13 for the major component. In a reductive titration with sodium borohydride the spectrum disappears with Em = -205 mV at pH 7.5. Contrarily, the major high-spin siroheme component has S = 5/2, g = 1.99, D = +9 cm-1, magnitude of E/D = 0.042, and Em = -295 mV. The sum of all siroheme signals integrates to 0.2 spin/half molecule, indicating considerable demetallation of this prosthetic group. Rigorous quantification procedures for S = 9/2 are not available, however, estimation by an approximate method indicates 0.6 S = 9/2 spin/half molecule. The S = 9/2 system is ascribed to an iron-sulfur cluster. It follows that this cluster is probably not a cubane, is not necessarily exchange-coupled to the siroheme, and, therefore, is not necessarily structurally close to the siroheme. It is suggested that this iron-sulfur prosthetic group has a novel structure suitable for functioning in multiple electron transfer.

The active centers of adenylylsulfate reductase from Desulfovibrio gigas. Characterization and spectroscopic studies

In order to utilize sulfate as the terminal electron acceptor, sulfate-reducing bacteria are equipped with a complex enzymatic system in which adenylylsulfate (AdoPSO4) reductase plays one of the major roles, reducing AdoPSO4 (the activated form of sulfate) to sulfite, with release of AMP. The enzyme has been purified to homogeneity from the anaerobic sulfate reducer Desulfovibrio gigas. The protein is composed of two non-identical subunits (70 kDa and 23 kDa) and is isolated in a multimeric form (approximately 400 kDa). It is an iron-sulfur, flavin-containing protein, with one FAD moiety, eight iron atoms and a minimum molecular mass of 93 kDa. Low-temperature EPR studies were performed to characterize its redox centers. In the native state, the enzyme showed an almost isotropic signal centered at g = 2.02 and only detectable below 20 K. This signal represented a minor species (0.10-0.25 spins/mol) and showed line broadening in the enzyme isolated from 57Fe-grown cells. Addition of sulfite had a minor effect on the EPR spectrum, but caused a major decrease in the visible region of the optical spectrum (around 392 nm). Further addition of AMP induced only a minor change in the visible spectrum whereas major changes were seen in the EPR spectrum; the appearance of a rhombic signal at g values 2.096, 1.940 and 1.890 (reduced Fe-S center I) observable below 30 K and a concomitant decrease in intensity of the g = 2.02 signal were detected. Effects of chemical reductants (ascorbate, H2/hydrogenase-reduced methyl viologen and dithionite) were also studied. A short time reduction with dithionite (15 s) or reduction with methyl viologen gave rise to the full reduction of center I (with slightly modified g values at 2.079, 1.939 and 1.897), and the complete disappearance of the g = 2.02 signal. Further reduction with dithionite produces a very complex EPR spectrum of a spin-spin-coupled nature (observable below 20 K), indicating the presence of at least two iron-sulfur centers, (centers I and II). Mössbauer studies on 57Fe-enriched D. gigas AdoPSO4 reductase demonstrated unambiguously the presence of two 4Fe clusters. Center II has a redox potential less than or equal to 400 mV and exhibits spectroscopic properties that are characteristic of a ferredoxin-type [4Fe-4S] cluster. Center I exhibits spectra with atypical Mössbauer parameters in its reduced state and has a midpoint potential around 0 mV, which is distinct from that of a ferredoxin-type [4Fe-4S] cluster, suggesting a different structure and/or a distinct cluster-ligand environment.

Substrate recognition by UDP-N-acetyl-alpha-D-galactosamine: polypeptide n-acetyl-alpha-D-galactosaminyltransferase. Effects of chain length and disulphide bonding of synthetic peptide substrates

A synthetic peptide AcTPPP, based on a threonine-containing sequence present in bovine myelin basic protein, is a potent acceptor of glycosyl transfer from UDP-N-acetylgalactosamine catalyzed by extracts of baby hamster kidney (BHK) cells or rabbit lymph node tissue. In contrast, the disulphide-linked peptide (AcTCPPP)2, based on a glycosylated sequence present in the hinge region of rabbit immunoglobulin G, is not an acceptor and inhibits glycosylation of AcTPPP. Extension of the cystine-containing peptide at the N-terminus produced weak acceptors but strong acceptors resulted when the cystine residue was reduced to form monomeric peptides. The acceptor specificity of the N-acetylgalactosaminyl-transferase activity of BHK cells is very similar to that of rabbit lymph node tissue. The results indicated that tissues actively secreting immunoglobulin do not contain a transferase activity adapted specifically for glycosylation of sequences containing cystine residues, and suggested that addition of an N-acetylgalactosamine to a threonine residue in the hinge region of rabbit immunoglobulin takes place during biosynthesis prior to the formation of the inter-chain disulphide bridge of fully assembled immunoglobulins.

Magnetization of the sulfite and nitrite complexes of oxidized sulfite and nitrite reductases: EPR silent spin S = 1/2 states

The saturation magnetizations of the sulfite complex of oxidized sulfite reductase and the nitrite complex of oxidized nitrite reductase have been measured to determine their spin state. Each shows the saturation magnetization signal of a spin S = 1/2 state with sigma g2 = 16, which is typical of low-spin ferrihemes. However, the EPR spectra of these complexes lack the expected signal intensity of a spin S = 1/2 state. Indeed, one of these complexes is EPR silent. The reasons for this unexpectedly low EPR signal intensity are considered.

Spectroscopic properties of siroheme extracted from sulfite reductases

Siroheme has been extracted from sulfite reductases and its properties in aqueous solution have been investigated by optical absorption, electron paramagnetic resonance (EPR), and magnetic circular dichroism (MDC) spectroscopy. The absorption spectrum of siroheme exhibits a marked pH dependence, and two pK values, 4.2 and 9.0, were determined by pH titration in the range 2-12. The first pK (4.2) is thought to correspond to the ionization of the carboxylic acid side-chains on the tetrapyrrole rings, and the second pK (9.0) is attributed to displacement of the axial ligand chloride by hydroxide. The binding of the strong field ligands, CO, NO, and cyanide, were investigated by UV-visible absorption and, in the case of the cyanide complex, by low-temperature EPR and MCD spectroscopies. CO and NO were able to reduce and bind to siroheme without additional reducing agent. The EPR spectrum of the isolated siroheme (chloride-ferrisiroheme) exhibits an axial signal with g perpendicular = 6.0 and g parallel = 2.0, typical of high-spin ferric hemes (S = 5/2), whereas the cyanide-complexed siroheme exhibits an approximately axial signal with g perpendicular = 2.38 and g parallel = 1.76 that is indicative of a low-spin ferric heme (S = 1/2). The low-temperature MCD spectra and magnetization data for the as-isolated and cyanide-complexed ferrisiroheme are entirely consistent with the interpretation of the EPR spectra. The results for ferrosiroheme indicate that the siroheme remains high spin (S = 2) and low spin (S = 0) on reduction of the as-isolated and cyanide-complexed siroheme, respectively. The isolated siroheme expressed sulfite reductase activity but the assessable catalytic cycle was much less than that of the native enzyme, showing the importance of the protein environment.

57Fe and 1H electron-nuclear double resonance of three doubly reduced states Escherichia coli sulfite reductase

We have employed electron-nuclear double resonance (ENDOR) spectroscopy to study the 57Fe hyperfine interactions in the bridged-siroheme [4Fe-4S] cluster that forms the catalytically active center of the two-electron-reduced hemoprotein subunit of Escherichia coli NADPH-sulfite reductase (SiR2-). Previous electron paramagnetic resonance (EPR) and Mössbauer studies have shown that this enzyme oxidation state can exist in three distinct spectroscopic forms: (1) a "g = 2.29" EPR species that predominates in unligated SiR2-, in which the siroheme Fe2+ is believed to be in an S = 1 state; (2) a "g = 4.88" type of EPR species that predominates in SiR2- in the presence of small amounts of guanidinium sulfate, in which the siroheme Fe2+ is in an S = 2 state; and (3) a classical "g = 1.94" type of EPR species that is seen in SiR2- ligated with CO, in which the siroheme Fe2+ is in an S = 0 state. In all three species, the cluster is in the [4Fe-4S]1+ state, and two distinct types of Fe site are seen in Mössbauer spectroscopy. ENDOR studies confirm the Mössbauer assignments for the cluster 57Fe in the g = 1.94 state, with A values of 37, 37, and 32 MHz for site I and ca. 19 MHz for site II. The hyperfine interactions are not too different on the g = 2.29 state, with site I Fe showing more anisotropic A values of 32, 24, and 20 MHz (site II was not detected).(ABSTRACT TRUNCATED AT 250 WORDS)