Cellular adaptation to down-regulated iron transport into lymphoid leukaemic cells: effects on the expression of the gene for ribonucleotide reductase

Ribonucleotide reductase is an iron-containing enzyme that is essential for DNA synthesis. Whereas previous studies have used various iron chelators to examine the relationship between cellular iron metabolism and ribonucleotide reductase activity in cells, they have not elucidated the relationship between iron transport into cells and the expression of the gene for ribonucleotide reductase. To investigate this, we examined ribonucleotide reductase mRNA, protein and enzyme activity in a novel line of CCRF-CEM cells (DFe-T cells) that display an approx. 60% decrease in their uptake of iron compared with the parental wild-type cell line. We found that DFe-T cells displayed an approx. 40% decrease in ribonucleotide reductase specific enzyme activity relative to wild-type cells without a change in their proliferation. Kinetic analysis of CDP reductase activity revealed an approx. 60% decrease in V(max) in DFe-T cells without a change in K(m). Despite the decrease in enzyme activity, the mRNA and protein for the R1 and R2 subunits of ribonucleotide reductase in DFe-T cells were similar to those of wild-type cells. ESR spectroscopy studies revealed that DFe-T cells had a 22% decrease in the tyrosyl free radical of the R2 subunit, suggesting that a larger amount of R2 protein was present as functionally inactive apo-R2 in these cells. Our studies indicate that ribonucleotide reductase activity in CCRF-CEM cells can be down-regulated by more than 50% in response to down-regulated iron transport without an adverse effect on cell proliferation. Furthermore, our studies suggest a regulatory link between ribonucleotide reductase activity and iron transport into these cells.

Hydrogen abstraction from thiols by adenosyl radicals: chemical precedent for thiyl radical formation, the first catalytic step in ribonucleoside triphosphate reductase from Lactobacillus leichmannii

Aqueous solutions of adenosylcobalamin (AdoCbl) were thermolyzed with excess beta-mercaptoethanol under anaerobic conditions. The product studies reveal that approximately 90% Co-C bond homolysis occurs, to yield Co(II)cobalamin, 5'-deoxyadenosine, and the disulfide product from the combination of two HOCH2CH2S* radicals, 2,2'-dithiodiethanol; there is also approximately 10% Co-C bond heterolysis, yielding Co(III)cobalamin, adenine, and 2,3-dihydroxy-4-pentenal. The kinetic studies show there is a first-order dependence on AdoCbl and zero-order dependence on thiol under the higher [RSH] experimental conditions used, consistent with the rate-determining step at high [RSH] being the generation of Ado*. The kinetic results require that, in enzyme-free AdoCbl solution, adenosyl radical (Ado*) is formed as a discrete intermediate which then abstracts H* from the added thiol. The activation parameters for Co-C bond homolysis in the presence of thiol trap are the same within experimental error as the activation parameters for Co-C bond homolysis without trap, standard delta H(obs) = 29(2) kcal mol(-1) and standard delta S(obs) = -1(5) e.u. The results, in comparison to the rate of Co-C bond homolysis in ribonucleoside triphosphate reductase (RTPR), reveal that RTPR accelerates Co-C bond cleavage in AdoCbl by approximately 10(10+/-1). The recent literature evidence bearing on the exact mechanism of RTPR enzymic cleavage of the Co-C bond of AdoCbl is briefly discussed, notably the fact that this mechanism is presently controversial, but does involve at least coupled (and possibly concerted) Co-C cleavage, -S-H cleavage, and C-H (Ado-H) formation steps.

The anaerobic (class III) ribonucleotide reductase from Lactococcus lactis. Catalytic properties and allosteric regulation of the pure enzyme system

Lactococcus lactis contains an operon with the genes (nrdD and nrdG) for a class III ribonucleotide reductase. Strict anaerobic growth depends on the activity of these genes. Both were sequenced, cloned, and overproduced in Escherichia coli. The corresponding proteins, NrdD and NrdG, were purified close to homogeneity. The amino acid sequences of NrdD (747 residues, 84.1 kDa) and NrdG (199 residues, 23.3 kDa) are 53 and 42% identical with the respective E. coli proteins. Together, they catalyze the reduction of ribonucleoside triphosphates to the corresponding deoxyribonucleotides in the presence of S-adenosylmethionine, reduced flavodoxin or reduced deazaflavin, potassium ions, dithiothreitol, and formate. EPR experiments demonstrated a [4Fe-4S](+) cluster in reduced NrdG and a glycyl radical in activated NrdD, similar to the E. coli NrdD and NrdG proteins. Different from E. coli, the two polypeptides of NrdD and the proteins in the NrdD-NrdG complex were only loosely associated. Also the FeS cluster was easily lost from NrdG. The substrate specificity and overall activity of the L. lactis enzyme was regulated according to the general rules for ribonucleotide reductases. Allosteric effectors bound to two separate sites on NrdD, one binding dATP, dGTP, and dTTP and the other binding dATP and ATP. The two sites showed an unusually high degree of cooperativity with complex interactions between effectors and a fine-tuning of their physiological effects. The results with the L. lactis class III reductase further support the concept of a common origin for all present day ribonucleotide reductases.

An osmotic-remedial, temperature-sensitive mutation in the allosteric activity site of ribonucleotide reductase in Neurospora crassa

An osmotic-remedial, temperature-sensitive conditional mutant (un-24) was generated by Repeat Induced Point mutation (RIP) from a cross between a wild-type N. crassa strain and a strain carrying a approximately 250-kb duplication of the left arm of linkage group II (LGII). The mutation was mapped to the duplicated segment, within 2.6 map units of the heterokaryon incompatibility locus het-6. DNA transformation identified a 3.75-kb fragment that complemented the temperature-sensitive phenotype. A large ORF within this fragment was found to have a high degree of sequence identity to the large subunit of ribonucleotide reductase (RNR) from diverse organisms. Conserved amino acids at the active site and the allosteric activity sites are also evident. An unusual feature of the Neurospora sequence is a large insertion near the C-terminus relative to otherwise homologous sequences from other organisms. Three transition mutations, indicative of RIP, were identified in the N-terminal region of the temperature-sensitive mutant allele. One of these mutations results in a non-conservative amino acid substitution within the four-helix bundle that is important in the allosteric control of ribonucleotide reductase activity. This substitution appears to disrupt proper folding of the allosteric activity site during synthesis of the protein.

Yeast Sml1, a protein inhibitor of ribonucleotide reductase

Ribonucleotide reductase (RNR) catalyzes the reduction of ribonucleotides to deoxyribonucleotides; this step is rate-limiting in DNA precursor synthesis. A number of regulatory mechanisms ensure optimal deoxyribonucleotide pools, which are essential for cell viability. The best studied mechanisms are transcriptional regulation of the RNR genes during the cell cycle and in the response to DNA damage, and the allosteric regulation of ribonucleotide reductase by nucleoside triphosphates. Recently, another mode of RNR regulation has been hypothesized in yeast. A novel protein, Sml1, was shown to bind to the Rnr1 protein of the yeast ribonucleotide reductase; this interaction was proposed to inhibit ribonucleotide reductase activity when DNA synthesis is not required (Zhao, X., Muller, E.G.D., and Rothstein, R. (1998) Mol. Cell 2, 329-340). Here, we use highly purified recombinant proteins to directly demonstrate that the Sml1 protein is a strong inhibitor of yeast RNR. The Sml1p specifically binds to the yeast Rnr1p in a 1:1 ratio with a dissociation constant of 0.4 microM. Interestingly, Sml1p also specifically binds to the mouse ribonucleotide reductase R1 protein. However, the inhibition observed in an in vitro mouse ribonucleotide reductase assay is less pronounced than the inhibition in yeast and probably occurs via a different mechanism.

Fast control of DNA replication in response to hypoxia and to inhibited protein synthesis in CCRF-CEM and HeLa cells

In order to elucidate whether data about the fast regulation of DNA replication in dependence on oxygen supply and on a functioning protein synthesis, previously elaborated with Ehrlich ascites cells, are valid for human cells too, we repeated key experiments with CCRF-CEM and HeLa cells. The most important techniques employed were DNA fibre autoradiography and alkaline sedimentation analyses of growing (pulse-labeled) daughter strand DNA. It was found that CCRF-CEM and HeLa cells responded to transient hypoxia and to transient inhibition of protein synthesis in an almost identical fashion. Scheduled replicon initiations were reversibly suppressed and the progress rates of replication forks, which were already active before the respective inhibitory conditions were established, were reversibly slowed down. The inclusion of the fork progress rate in the response differs from Ehrlich ascites cells, which respond only by suppressing initiation. Further circumstances of the fast oxygen dependent response, concerning the behaviour of ribonucleotide reductase and of the dNTP pools, revealed no significant differences among the three cell lines. The striking identity of the response of each of the cell lines to hypoxia and to inhibited protein synthesis prompts the suspicion that converging fast regulatory pathways act on the cellular replication machinery. The phenomena as such seem to be rather widespread among mammalian cells.

Defective DNA replication and repair associated with decreases in deoxyribonucleotide pools in a mouse cell mutant with thermolabile ubiquitin-activating enzyme E1

Upon shift-up in temperature, mouse tsFS20 mutant cells with thermolabile ubiquitin-activating enzyme E1 immediately stopped DNA replication and showed cell cycle arrest in S-phase. In contrast, when the cells were permeabilized with lysolecithin after culture at the nonpermissive temperature, they exhibited a normal level of replicative DNA synthesis in vitro. In agreement with this, intracellular pools of deoxyribonucleoside triphosphates were significantly reduced in the cells cultured at the nonpermissive temperature. Even under the permissive conditions, tsFS20 cells were more sensitive to hydroxyurea and alkylating agents, and induced less mutation than the wild-type cells. These results suggest that the ubiquitin system affects DNA replication and repair.

Thermolysis of coenzymes B12 at physiological temperatures: activation parameters for cobalt-carbon bond homolysis and a quantitative analysis of the perturbation of the homolysis equilibrium by the ribonucleoside triphosphate reductase from Lactobacillus leichmannii

The kinetics of the thermolysis of 5'-deoxyadenosylcobalamin (AdoCbl, coenzyme B12) in aqueous solution, pH 7.5, have been studied in the temperature range 30-85 degrees C using AdoCbl tritiated at the adenine C2 position and the method of initial rates. Combined with a careful analysis of the distribution of adenine-containing products, the results permit the dissection of the competing rate constants for carbon-cobalt bond homolysis and heterolysis. After correction for the temperature-dependent occurrence of the much less reactive base-off species of AdoCbl, the activation parameters for homolysis of the base-on species were found to be delta H++homo,on = 33.8 +/- 0.2 kcal mol-1 and delta S++homo,on = 13.5 +/- 0.7 cal mol-1 K-1, values not significantly different from those determined by Hay and Finke (J. Am. Chem. Soc. 108 (1986) 4820), in the temperature range 85-115 degrees C. In contrast, the heterolysis of base-on AdoCbl was characterized by a much smaller enthalpy of activation (delta H++het,on = 18.5 +/- 0.2 kcal mol-1) and a negative entropy of activation (delta S++het,on = -34.0 +/- 0.7 cal mol-1 K-1) so that heterolysis, which is minor pathway at elevated temperatures, is the dominant pathway for AdoCbl decomposition at physiological temperatures. Using literature values for the rate constant for the reverse reaction, the equilibrium constant for AdoCbl homolysis at 37 degrees C was calculated to be 7.9 x 10(-18). Comparison with the equilibrium constant for this homolysis at the active site of the ribonucleoside triphosphate reductase from Lactobacillus leichmannii shows that the enzymes shifts the equilibrium constant towards homolysis products by a factor of 2.9 x 10(12) (17.7 kcal mol-1) by binding the thermolysis products with an equilibrium constant of 7.1 x 10(16) M-2, compared to the bonding constant for AdoCbl of 2.4 x 10(4) M-1.

An active ribonucleotide reductase from Arabidopsis thaliana cloning, expression and characterization of the large subunit

In all living organisms, deoxyribonucleotides, the DNA precursors, are produced by reduction of the corresponding ribonucleotides catalyzed by ribonucleotide reductase. In mammals as in Escherichia coli, the enzyme consists of two proteins. Protein R1 is the proper reductase as it contains, in the substrate binding site, the reducing active cysteine pair. Protein R2 provides a catalytically essential organic radical. Here we report the cloning, expression, purification and characterization of protein R1 from Arabidopsis thaliana. Expression in E. coli was made possible by coexpression of tRNAArg4 which is required for the utilization of AGA and AGG as codons for arginines. Protein R1 shows extensive similarities with protein R1 from mammals: (a) it shows 69% amino-acid sequence identity to human and mouse R1 protein; (b) it is active during CDP reduction by dithiothreitol, in the presence of protein R2 [Sauge-Merle, S., Laulhère, J.-P., Coves, J., Ménage, S., Le Pape, L. & Fontecave, M. (1997) J. Biol. Inorg. Chem. 2, 586-594]; (c) activity is stimulated by thioredoxin and ATP and is inhibited by dATP, showing that as in the mammalian enzyme, the plant ribonucleotide reductase seems to be allosterically regulated by positive (ATP) and negative (dATP) effectors.

The anaerobic ribonucleotide reductase from Escherichia coli. The small protein is an activating enzyme containing a [4fe-4s](2+) center

For deoxyribonucleotide synthesis during anaerobic growth, Escherichia coli cells depend on an oxygen-sensitive class III ribonucleotide reductase. The enzyme system consists of two proteins: protein alpha, on which ribonucleotides bind and are reduced, and protein beta, of which the function is to introduce a catalytically essential glycyl radical on protein alpha. Protein beta can assemble one [4Fe-4S] center per polypeptide enjoying both the [4Fe-4S](2+) and [4Fe-4S](1+) redox state, as shown by iron and sulfide analysis, Mössbauer spectroscopy (delta = 0.43 mm.s(-1), DeltaE(Q) = 1.0 mm.s(-1), [4Fe-4S](2+)), and EPR spectroscopy (g = 2. 03 and 1.93, [4Fe-4S](1+)). This iron center is sensitive to oxygen and can decompose into stable [2Fe-2S](2+) centers during exposure to air. This degraded form is nevertheless active, albeit to a lesser extent because of the conversion of the cluster into [4Fe-4S] forms during the strongly reductive conditions of the assay. Furthermore, protein beta has the potential to activate several molecules of protein alpha, suggesting that protein beta is an activating enzyme rather than a component of an alpha(2)beta(2) complex as previously claimed.

The U28 ORF of human herpesvirus-7 does not encode a functional ribonucleotide reductase R1 subunit

Herpesvirus ribonucleotide reductases, essential for the de novo synthesis of viral DNA, are composed of two non-identical subunits, termed R1 and R2. The U28 ORF from human herpesvirus-7 has been classified, by sequence comparisons, as a homologue of the R1 subunit from ribonucleotide reductase but no R2 ORF is present. Detailed analysis of the U28 amino acid sequence indicated that a number of essential R1 catalytic residues are absent. Cloning and expression of the U28 protein in E. coli and its subsequent characterization in subunit interaction and enzyme activity assays confirmed that it is not a functional equivalent of a herpesvirus R1. In the absence of the R2 gene, we propose that the R1 ORF has evolved a distinct, as yet unidentified, function not only in human herpesvirus-7 but also in other human betaherpes-viruses.

Iron-sulfur interconversions in the anaerobic ribonucleotide reductase from Escherichia coli

The anaerobic ribonucleotide reductase from Escherichia coli contains an iron-sulfur cluster which, in the reduced [4Fe-4S](+) form, serves to reduce S-adenosylmethionine and to generate a catalytically essential glycyl radical. The reaction of the reduced cluster with oxygen was studied by UV-visible, EPR, NMR, and Mössbauer spectroscopies. The [4Fe-4S](+) form is shown to be extremely sensitive to oxygen and converted to [4Fe-4S](2+), [3Fe-4S](+/0), and to the stable [2Fe-2S](2+) form. It is remarkable that the oxidized protein retains full activity. This is probably due to the fact that during reduction, required for activity, the iron atoms, from 2Fe and 3Fe clusters, readily reassemble to generate an active [4Fe-4S] center. This property is discussed as a possible protective mechanism of the enzyme during transient exposure to air. Furthermore, the [2Fe-2S] form of the protein can be converted into a [3Fe-4S] form during chromatography on dATP-Sepharose, explaining why previous preparations of the enzyme were shown to contain large amounts of such a 3Fe cluster. This is the first report of a 2Fe to 3Fe cluster conversion.

Synthesis and characterization of isolable thiolatocobalamin complexes relevant to coenzyme B12-dependent ribonucleoside triphosphate reductase

The syntheses, isolation and characterization of cyclohexylthiolatocobalamin (C6H11SCbl), glutathionylcobalamin (GluSCbl), and cysteinylcobalamin (CysSCbl) are reported in 75, 55, and 65% yield, respectively. Characterization was achieved using elemental analyses, L-SIMS (liquid secondary ion mass spectrometry), UV-visible spectroscopy and, for the more stable C6H11SCbl and GluSCbl, our recently established 1H NMR method (which emphasizes the readily interpreted aromatic region of the cobalamin's 1H NMR spectrum). Preliminary evidence is presented for clean homolysis of the RS-Co bond in C6H11SCbl, GluSCbl, and CysSCbl to give RS. and .Co(II)Cbl radical pairs analogous to those that are intermediates in ribonucleoside triphosphate reductase (RTPR). A summary is provided which emphasizes the seven variables identified to date, underlying the successful syntheses and isolation of thiolatocobalamins, variables which make the one-step syntheses of RSCbls considerably more complex than they initially appear. Also briefly discussed are the analogous protein-S-Cbl complexes that are seen as side-products in RTPR, and the probability that such side-products are formed when HOCbl.HX is used as a possible 'active-site inhibitor' complex with B12-dependent enzymes.

Common resistance mechanisms to nucleoside analogues in variants of the human erythroleukemic line K562

Variants of the human K562 were developed against the nucleoside analogues cytosine arabinoside, 2 chlorodeoxyadenosine, fludarabine and gemcitabine. The resistant lines displayed a high degree of cross-resistance to all nucleoside analogues, with little or no cross resistance to other agents. There was a profound accumulation defect of the different nucleoside analogues in all of the variants. There was a strong overexpression of 5'nucleotidase, measured by rt-PCR and enzyme activity, in all resistant variants. There was a two fold increase of ribonucleotide reductase in the fludarabine resistant line and increased expression of purine nucleoside phosphorylase in the 2 chlorodeoxyadenosine selected line. Karyotypic analysis revealed the loss of a 6(q16;q22) deletion present in the parental line in all of the resistant lines. This portion of chromosome 6 has been shown to contain the gene for 5'nucleotidase. Early events in the transport and metabolism appear to be involved in the resistance mechanisms to nucleoside analogues and are responsible for broad cross resistance to this family of compounds.

Expression and cellular localization of ribonucleotide reductase small subunit M2 protein in hydroxyurea-resistant Leishmania mexicana amazonensis

We raised a specific antiserum against the recombinant M2 subunit protein of ribonucleotide reductase of Leishmania mexicana amazonensis in rabbit. This antiserum was used to study the expression and cellular location of the M2 protein in wildtype as well as hydroxyurea-resistant variants (HuR) of the parasite. The protein increased with increasing dose of the drug used for selection of resistance. The increase in protein level was accompanied by an increase in the copy numbers of mRNA of the M2 gene in the variants. In contrast to mammalian cells, the M2 protein of Leishmania is located in the nucleus rather than in the cytoplasm. The number of cells expressing M2 protein is also different in mammalian cells versus Leishmania. In mammalian cells, expression of M2 protein is a strictly S-phase-correlated event and in exponentially growing cells only approximately 50% of the cells are in S-phase and only these cells synthesize M2 protein. In L. m. amazonensis, however, almost all exponentially growing cells are positive for M2 protein. This makes it unlikely that M2 protein expression in Leishmania is S-phase dependent. In view of these findings, a fresh look in the future into the regulatory mechanisms of synthesis and the site of action of RNR in L. m. amazonensis is warranted.

High-field EPR detection of a disulfide radical anion in the reduction of cytidine 5'-diphosphate by the E441Q R1 mutant of Escherichia coli ribonucleotide reductase

Class I ribonucleotide reductases (RNRs) are composed of two subunits, R1 and R2. The R2 subunit contains the essential diferric cluster-tyrosyl radical (Y.) cofactor and R1 is the site of the conversion of nucleoside diphosphates to 2'-deoxynucleoside diphosphates. A mutant in the R1 subunit of Escherichia coli RNR, E441Q, was generated in an effort to define the function of E441 in the nucleotide-reduction process. Cytidine 5'-diphosphate was incubated with E441Q RNR, and the reaction was monitored by using stopped-flow UV-vis spectroscopy and high-frequency (140 GHz) time-domain EPR spectroscopy. These studies revealed loss of the Y. and formation of a disulfide radical anion and present experimental mechanistic insight into the reductive half-reaction catalyzed by RNR. These results support the proposal that the protonated E441 is required for reduction of a 3'-ketodeoxynucleotide by a disulfide radical anion. On the minute time scale, a second radical species was also detected by high-frequency EPR. Its g values suggest that this species may be a 4'-ketyl radical and is not on the normal reduction pathway. These experiments demonstrate that high-field time-domain EPR spectroscopy is a powerful new tool for deconvolution of a mixture of radical species.

Ribonucleotide reductase: A new target for antiparasite therapies

New treatments are required urgently for the control of parasitic protozoan diseases of humans and animals. One approach is the development of subunit vaccines; the other focuses on identifying and exploiting specific differences in essential functions between the host and parasite. One enzyme currently attracting attention is ribonucleotide reductase, an essential component in the biosynthesis of DNA. In this article, Geoffrey Ingram and Jane Kinnaird examine differences between the host and parasite enzymes and assess possible means of therapeutic intervention.

A megaplasmid-borne anaerobic ribonucleotide reductase in Alcaligenes eutrophus H16

The conjugative 450-kb megaplasmid pHG1 is essential for the anaerobic growth of Alcaligenes eutrophus H16 in the presence of nitrate as the terminal electron acceptor. We identified two megaplasmid-borne genes (nrdD and nrdG) which are indispensable under these conditions. Sequence alignment identified significant similarity of the 76.2-kDa gene product NrdD and the 30.9-kDa gene product NrdG with anaerobic class III ribonucleotide reductases and their corresponding activases. Deletion of nrdD and nrdG in A. eutrophus abolished anaerobic growth and led to the formation of nondividing filamentous cells, a typical feature of bacteria whose DNA synthesis is blocked. Enzyme activity of NrdD-like ribonucleotide reductases is dependent on a stable radical at a glycine residue in a conserved C-terminal motif. A mutant of A. eutrophus with a G650A exchange in NrdD showed the DNA-deficient phenotype as the deletion strain, suggesting that G650 forms the glycyl radical. Analysis of transcriptional and translational fusions indicate that nrdD and nrdG are cotranscribed and that the translation efficiency of nrdD is 40-fold higher than that of nrdG. Thus, the two proteins NrdD and NrdG are not synthesized at a stoichiometric level.

Regulation of the ribonucleotide reductase small subunit gene by DNA-damaging agents in Dictyostelium discoideum

In Escherichia coli, yeast and mammalian cells, the genes encoding ribonucleotide reductase, an essential enzyme for de novo DNA synthesis, are up-regulated in response to DNA damaging agents. We have examined the response of the rnrB gene, encoding the small subunit of ribonucleotide reductase in Dictyostelium discoideum, to DNA damaging agents. We show here that the accumulation of rnrB transcript is increased in response to methyl methane sulfonate, 4-nitroquinoline-1-oxide and irradiation with UV-light, but not to the ribonucleotide reductase inhibitor hydroxyurea. This response is rapid, transient and independent of protein synthesis. Moreover, cells from different developmental stages are able to respond to the drug in a similar fashion, regardless of the basal level of expression of the rnrB gene. We have defined the cis -acting elements of the rnrB promoter required for the response to methyl methane sulfonate and 4-nitroquinoline-1-oxide by deletion analysis. Our results indicate that there is one element, named box C, that can confer response to both drugs. Two other boxes, box A and box D, specifically conferred response to methyl methane sulfonate and 4-nitroquinoline-1-oxide, respectively.

De novo proteins as models of radical enzymes

Catalytically essential side-chain radicals have been recognized in a growing number of redox enzymes. Here we present a novel approach to study this class of redox cofactors. Our aim is to construct a de novo protein, a radical maquette, that will provide a protein framework in which to investigate how side-chain radicals are generated, controlled, and directed toward catalysis. A tryptophan and a tyrosine radical maquette, denoted alpha(3)W(1) and alpha(3)Y(1), respectively, have been synthesized. alpha(3)W(1) and alpha(3)Y(1) contain 65 residues each and have molecular masses of 7.4 kDa. The proteins differ only in residue 32, which is the position of their single aromatic side chain. Structural characterization reveals that the proteins fold in water solution into thermodynamically stable, alpha-helical conformations with well-defined tertiary structures. The proteins are resistant to pH changes and remain stable through the physiological pH range. The aromatic residues are shown to be located within the protein interior and shielded from the bulk phase, as designed. Differential pulse voltammetry was used to examine the reduction potentials of the aromatic side chains in alpha(3)W(1) and alpha(3)Y(1) and compare them to the potentials of tryptophan and tyrosine when dissolved in water. The tryptophan and tyrosine potentials were raised considerably when moved from a solution environment to a well-ordered protein milieu. We propose that the increase in reduction potential of the aromatic residues buried within the protein, relative to the solution potentials, is due to a lack of an effective protonic contact between the aromatic residues and the bulk solution.

Ribonucleotide reductase (RNR) of Corynebacterium glutamicum ATCC 13032--genetic characterization of a second class IV enzyme

Ribonucleotide reductases (RNRs) encoded by nrd (nucleotide reduction) genes are unique enzymes providing the DNA precursors in all living organisms and several viruses. The designation of four classes of RNRs reflects their use of diverse metallo-cofactors. Using oligonucleotide primers derived from conserved domains of the primary structure of known NrdA and NrdE proteins, an internal 938 bp fragment of the nrdE gene was amplified from genomic DNA of Corynebacterium glutamicum. With this PCR product a 4.36 kb fragment was identified and cloned containing the nrdHIE genes of C. glutamicum. A probe derived from nrdF2 of Mycobacterium tuberculosis allowed the cloning and sequencing of the nrdF gene located 3.1 kb further downstream, encoding the small subunit of the C. glutamicum RNR. Conjugative introduction of nrdE from C. glutamicum complemented thermosensitive mutants of Corynebacterium ammoniagenes which had a defective catalytic subunit of the Mn-RNR. The authors provide arguments for allocation of the C. glutamicum NrdEF proteins to class IV in the RNR classification scheme of Stubbe & van der Donk (1995) [Chem Biol 2, 793-801].

Ribonucleotide reduction in Pseudomonas species: simultaneous presence of active enzymes from different classes

Three separate classes of ribonucleotide reductases exist in nature. They differ widely in protein structure. Class I enzymes are found in aerobic bacteria and eukaryotes; class II enzymes are found in aerobic and anaerobic bacteria; class III enzymes are found in strict and facultative anaerobic bacteria. Usually, but not always, one organism contains only one or two (in facultative anaerobes) classes. Surprisingly, the genomic sequence of Pseudomonas aeruginosa contains sequences for each of the three classes. Here, we show by DNA hybridization that other species of Pseudomonas also contain the genes for three classes. Extracts from P. aeruginosa and P. stutzeri grown aerobically or microaerobically contain active class I and II enzymes, whereas we could not demonstrate class III activity. Unexpectedly, class I activity increased greatly during microaerobic conditions. The enzymes were separated, and the large proteins of the class I enzymes were obtained in close to homogeneous form. The catalytic properties of all enzymes are similar to those of other bacterial reductases. However, the Pseudomonas class I reductases required the continuous presence of oxygen during catalysis, unlike the corresponding Escherichia coli enzyme but similar to the mouse enzyme. In similarity searches, the amino acid sequence of the class I enzyme of P. aeruginosa was more related to that of eukaryotes than to that of E. coli or other proteobacteria, with the large protein showing 42% identity to that of the mouse, suggesting the possibility of a horizontal transfer of the gene. The results raise many questions concerning the physiological function and evolution of the three classes in Pseudomonas species.

EPR properties of mixed-valent mu-oxo and mu-hydroxo dinuclear iron complexes produced by radiolytic reduction at 77 K

Radiolytic reduction at 77 K of oxo/hydroxo-bridged dinuclear iron(III) complexes in frozen solutions forms kinetically stabilized, mixed-valent species in high yields that model the mixed-valent sites of non-heme, diiron proteins. The mixed-valent species trapped at 77 K retain ligation geometry similar to the initial diferric clusters. The shapes of the mixed-valent EPR signals depend strongly on the bridging ligands. Spectra of the Fe(II)OFe(III) species reveal an S = 1/2 ground state with small g-anisotropy as characterized by the uniaxial component (gz-gav/2 < 0.03) observable at temperatures as high as approximately 100 K. In contrast, hydroxo-bridged mixed-valent species are characterized by large g-anisotropy (gz-gav/2 > 0.03) and are observable only below 30 K. Annealing at higher temperatures causes structural relaxation and changes in the EPR characteristics. EPR spectral properties allow the oxo- and hydroxo-bridged, mixed-valent diiron centers to be distinguished from each other and can help characterize the structure of mixed-valent centers in proteins.

The unique N terminus of herpes simplex virus type 1 ribonucleotide reductase large subunit is phosphorylated by casein kinase 2, which may have a homologue in Escherichia coli

Studies were performed to determine if the unique N-terminal domain of the R1 subunit from herpes simplex virus (HSV) type 1 ribonucleotide reductase is a substrate for casein kinase 2 (CK2). Transphosphorylation assays demonstrated that R1 was highly phosphorylated by this enzyme with multiple phosphorylation sites mapped to the N terminus between residues 1 and 245. Immunoprecipitation pull-down assays using R1-specific antisera failed to demonstrate a stable interaction between R1 and CK2 but residual amounts of CK2 present after immunoprecipitation efficiently transphosphorylated R1. Activity assays with a peptide substrate identified CK2 in R1 immunoprecipitated from infected-cell extracts but did not detect activity in R1 proteins immunoprecipitated from bacterial extracts. However, Western blotting identified potential E. coli homologues of the CK2 alpha and beta subunits. These results support conclusions that the N-terminal domain of HSV R1 is not a protein kinase and that all previous results can be explained by contaminating kinases, principally CK2.