Enzymic modification of a tyrosine residue to a stable free radical in ribonucleotide reductase

Designed Metal-ATCUN Derivatives: Redox- and Non-redox-Based Applications Relevant for Chemistry, Biology, and Medicine

The designed "ATCUN" motif (amino-terminal copper and nickel binding site) is a replica of naturally occurring ATCUN site found in many proteins/peptides, and an attractive platform for multiple applications, which include nucleases, proteases, spectroscopic probes, imaging, and small molecule activation. ATCUN motifs are engineered at periphery by conjugation to recombinant proteins, peptides, fluorophores, or recognition domains through chemically or genetically, fulfilling the needs of various biological relevance and a wide range of practical usages. This chemistry has witnessed significant growth over the last few decades and several interesting ATCUN derivatives have been described. The redox role of the ATCUN moieties is also an important aspect to be considered. The redox potential of designed M-ATCUN derivatives is modulated by judicious choice of amino acid (including stereochemistry, charge, and position) that ultimately leads to the catalytic efficiency. In this context, a wide range of M-ATCUN derivatives have been designed purposefully for various redox- and non-redox-based applications, including spectroscopic probes, target-based catalytic metallodrugs, inhibition of amyloid-β toxicity, and telomere shortening, enzyme inactivation, biomolecules stitching or modification, next-generation antibiotic, and small molecule activation.

Crosstalk between chromatin structure, cohesin activity and transcription

BACKGROUND

A complex interplay between chromatin and topological machineries is critical for genome architecture and function. However, little is known about these reciprocal interactions, even for cohesin, despite its multiple roles in DNA metabolism.

RESULTS

We have used genome-wide analyses to address how cohesins and chromatin structure impact each other in yeast. Cohesin inactivation in scc1-73 mutants during the S and G2 phases causes specific changes in chromatin structure that preferentially take place at promoters; these changes include a significant increase in the occupancy of the - 1 and + 1 nucleosomes. In addition, cohesins play a major role in transcription regulation that is associated with specific promoter chromatin architecture. In scc1-73 cells, downregulated genes are enriched in promoters with short or no nucleosome-free region (NFR) and a fragile "nucleosome - 1/RSC complex" particle. These results, together with a preferential increase in the occupancy of nucleosome - 1 of these genes, suggest that cohesins promote transcription activation by helping RSC to form the NFR. In sharp contrast, the scc1-73 upregulated genes are enriched in promoters with an "open" chromatin structure and are mostly at cohesin-enriched regions, suggesting that a local accumulation of cohesins might help to inhibit transcription. On the other hand, a dramatic loss of chromatin integrity by histone depletion during DNA replication has a moderate effect on the accumulation and distribution of cohesin peaks along the genome.

CONCLUSIONS

Our analyses of the interplay between chromatin integrity and cohesin activity suggest that cohesins play a major role in transcription regulation, which is associated with specific chromatin architecture and cohesin-mediated nucleosome alterations of the regulated promoters. In contrast, chromatin integrity plays only a minor role in the binding and distribution of cohesins.

Homologous Recombination: To Fork and Beyond

Accurate completion of genome duplication is threatened by multiple factors that hamper the advance and stability of the replication forks. Cells need to tolerate many of these blocking lesions to timely complete DNA replication, postponing their repair for later. This process of lesion bypass during DNA damage tolerance can lead to the accumulation of single-strand DNA (ssDNA) fragments behind the fork, which have to be filled in before chromosome segregation. Homologous recombination plays essential roles both at and behind the fork, through fork protection/lesion bypass and post-replicative ssDNA filling processes, respectively. I review here our current knowledge about the recombination mechanisms that operate at and behind the fork in eukaryotes, and how these mechanisms are controlled to prevent unscheduled and toxic recombination intermediates. A unifying model to integrate these mechanisms in a dynamic, replication fork-associated process is proposed from yeast results.

Ribonucleotide reductase metallocofactor: assembly, maintenance and inhibition

Ribonucleotide reductase (RNR) supplies cellular deoxyribonucleotide triphosphates (dNTP) pools by converting ribonucleotides to the corresponding deoxy forms using radical-based chemistry. Eukaryotic RNR comprises α and β subunits: α contains the catalytic and allosteric sites; β houses a diferric-tyrosyl radical cofactor (FeIII2-Y•) that is required to initiates nucleotide reduction in α. Cells have evolved multi-layered mechanisms to regulate RNR level and activity in order to maintain the adequate sizes and ratios of their dNTP pools to ensure high-fidelity DNA replication and repair. The central role of RNR in nucleotide metabolism also makes it a proven target of chemotherapeutics. In this review, we discuss recent progress in understanding the function and regulation of eukaryotic RNRs, with a focus on studies revealing the cellular machineries involved in RNR metallocofactor biosynthesis and its implication in RNR-targeting therapeutics.

Evaluating the therapeutic potential of a non-natural nucleotide that inhibits human ribonucleotide reductase

Human ribonucleotide reductase (hRR) is the key enzyme involved in de novo dNTP synthesis and thus represents an important therapeutic target against hyperproliferative diseases, most notably cancer. The purpose of this study was to evaluate the ability of non-natural indolyl-2'-deoxynucleoside triphosphates to inhibit the activity of hRR. The structural similarities of these analogues with dATP predicted that they would inhibit hRR activity by binding to its allosteric sites. In silico analysis and in vitro characterization identified one particular analogue designated as 5-nitro-indolyl-2'-deoxyribose triphosphate (5-NITP) that inhibits hRR. 5-NITP binding to hRR was determined by isothermal titration calorimetry. X-ray crystal structure of 5-NITP bound to RR1 was determined. Cell-based studies showed the anti-cancer effects of the corresponding non-natural nucleoside against leukemia cells. 5-NITP binds to hRR with micromolar affinity. Binding does not induce hexamerization of hRR1 like dATP, the native allosteric inhibitor of hRR that binds with high affinity to the A-site. The X-ray crystal structure of Saccharomyces cerevisiae RR1-5-NITP (ScRR1-5-NITP) complex determined to 2.3 Å resolution shows that 5-NITP does not bind to the A-site but rather at the S-site. Regardless, 5-nitro-indolyl-2'-deoxynucleoside (5-NIdR) produces cytostatic and cytotoxic effects against human leukemia cells by altering cell-cycle progression. Our studies provide useful insights toward developing new inhibitors with improved potency and efficacy against hRR.

NrdI, a flavodoxin involved in maintenance of the diferric-tyrosyl radical cofactor in Escherichia coli class Ib ribonucleotide reductase

Ribonucleotide reductase (RNR) catalyzes the conversion of nucleotides to deoxynucleotides and is essential in all organisms. Class I RNRs consist of two homodimeric subunits: alpha2 and beta2. The alpha subunit contains the site of nucleotide reduction, and the beta subunit contains the essential diferric-tyrosyl radical (Y*) cofactor. Escherichia coli contains genes encoding two class I RNRs (Ia and Ib) and a class III RNR, which is active only under anaerobic conditions. Its class Ia RNR, composed of NrdA (alpha) and NrdB (beta), is expressed under normal aerobic growth conditions. The class Ib RNR, composed of NrdE (alpha) and NrdF (beta), is expressed under oxidative stress and iron-limited growth conditions. Our laboratory is interested in pathways of cofactor biosynthesis and maintenance in class I RNRs and modulation of Y* levels as a means of regulating RNR activity. Our recent studies have implicated a [2Fe2S]-ferredoxin, YfaE, in the NrdB diferric-Y* maintenance pathway and possibly in the biosynthetic and regulatory pathways. Here, we report that NrdI is a flavodoxin counterpart to YfaE for the class Ib RNR. It possesses redox properties unprecedented for a flavodoxin (E(ox/sq) = -264 +/- 17 mV and E(sq/hq) = -255 +/- 17 mV) that allow it to mediate a two-electron reduction of the diferric cluster of NrdF via two successive one-electron transfers. Data presented support the presence of a distinct maintenance pathway for NrdEF, orthogonal to that for NrdAB involving YfaE.

Importance of the maintenance pathway in the regulation of the activity of Escherichia coli ribonucleotide reductase

Ribonucleotide reductases (RNRs) catalyze the conversion of nucleotides to deoxynucleotides in all organisms. The Escherichia coli class Ia RNR is composed of α and β subunits that form an α₂β₂ active complex. β contains the diferric tyrosyl radical (Y^•) cofactor that is essential for the reduction process that occurs on α. [Y^•] in vitro is proportional to RNR activity, and its regulation in vivo potentially represents a mechanism for controlling RNR activity. To examine this thesis, N- and C-terminal StrepII-tagged β under the control of an l-arabinose promoter were constructed. Using these constructs and with [l-arabinose] varying from 0 to 0.5 mM in the growth medium, [β] could be varied from 4 to 3300 µM. [Y^•] in vivo and on affinity-purified Strep-β in vitro was determined by EPR spectroscopy and Western analysis. In both cases, there was 0.1–0.3 Y^• radical per β. To determine if the substoichiometric Y^• level was associated with apo β or diferric β, titrations of crude cell extracts from these growths were carried out with reduced YfaE, a 2Fe2S ferredoxin involved in cofactor maintenance and assembly. Each titration, followed by addition of O₂ to assemble the cofactor and EPR analysis to quantitate Y^•, revealed that β is completely loaded with a diferric cluster even when its concentration in vivo is 244 µM. These titrations, furthermore, resulted in 1 Y^• radical per β, the highest levels reported. Whole cell Mössbauer analysis on cells induced with 0.5 mM arabinose supports high iron loading in β. These results suggest that modulation of the level of Y^• in vivo in E. coli is a mechanism of regulating RNR activity.

The first crystal structure of a monomeric phenoxyl radical: 2,4,6-tri-tert-butylphenoxyl radical

Crystals of the 2,4,6-tri-tert-butylphenoxyl radical have been isolated and characterized by X-ray diffraction, and calculations have been performed that give the distribution of spin density in the radical.

Hemoglobin and red blood cells as tools for studying peroxynitrite biochemistry

Oxyhemoglobin represents a relevant intravascular sink of peroxynitrite. Indeed, peroxynitrite undergoes a fast isomerization (k = 1.7 x 10(4)M(-1)s(-1)) to nitrate in the presence of oxyhemoglobin; the reaction mechanism is complex and leads to methemoglobin and superoxide radical as additional products and a small amount (approximately 10%) of transient species, including ferrylhemoglobin, nitrogen dioxide, and globin-derived radicals. The mechanism of the reaction could be solved only after extensive quantitative analysis of reactants, intermediates, and products and setting up experimental conditions that favor direct reactions of peroxynitrite with hemoglobin versus peroxynitrite decay through proton- or carbon dioxide-catalyzed homolysis. Additionally, oxyhemoglobin has been used as a "reporter" molecule of peroxynitrite diffusion from extracellular to intracellular compartments, using red blood cells (RBCs) as a model system. In RBCs, peroxynitrite diffusion across the membrane is favored by the large abundance of anion channels, and average transit distances can vary as a function of cell density. Indeed, we have developed a mathematical model that incorporates competition between the extracellular consumption of peroxynitrite and the permeation to the erythrocytes as a function of the average diffusion distances. The RBC model presented herein serves to estimate biological diffusion distances of peroxynitrite in the presence of relevant molecular targets, and the theoretical approach can be successfully applied to study the diffusion of peroxynitrite in other cellular/tissue systems.

In vitro potentiation of antibiotic activities by a catecholate iron chelator against chloroquine-resistant Plasmodium falciparum

FR160, a catechol iron chelator, and tetracyclines or norfloxacin exert in vitro additive or synergistic activity against a chloroquine-resistant Plasmodium falciparum clone. FR160 shows antagonistic effects in association with macrolides, ofloxacin, and rifampin.

New mechanistic insights into the reactivity of the R2 protein of E. coli ribonucleotide reductase (RNR)

Further to a linear free-energy correlation of cross-reaction rate constants k12 for the reaction of eight organic radicals (OR), e.g. MV*+, from methyl viologen, with cytochrome c(III), we consider here similar studies for the reduction of the R2 protein of Escherichia coli ribonucleotide reductase, which has FeIII2 and Tyr* redox components. The same two techniques of pulse radiolysis and stopped-flow were used. Cross-reaction rate constants (22 degrees C) at pH 7.0, I=0.100 M (NaCl), were determined for the reduction of active-R2 with the eight ORs, reduction potentials E0(1) from -0.446 to +0.194 V. Samples of active-R2 have an FeIII2 met-R2 component, which in the present studies was close to 40%. Concurrent reactions have to be taken into account for the five most reactive ORs, corresponding to reduction of the FeIII2 of met-R2 and then of active-R2. Separate experiments on met-R2 reproduced the first of these rate constants, which on average is approximately 66% larger than the second rate constant. A single Marcus free-energy plot of log k12-0.5 log10f versus -E0(1)/0.059 describes all the data and the slope of 0.54 is in satisfactory agreement with the theoretical value of 0.50. Such behaviour is unexpected since the Tyr* is a much stronger oxidant (E0 approximately 1.0 V versus NHE) as compared to FeIII2 (E0 close to zero). X-ray structures of the met- and red-R2 states have indicated that electroneutrality of the approximately 10 A buried active site is maintained. Proton transfer is therefore proposed as a rapid sequel to electron transfer. Other reactions considered are the much slower conventional time-range reductions of active-R2 with hydrazine and dithionite. For these reactions one and/or two-equivalent changes are possible. With both reductants, met-R2 reacts about four-fold faster than active-R2, and as with the ORs the less strongly oxidising FeIII2 component is reduced before the Tyr*.

Identification of RNR4, encoding a second essential small subunit of ribonucleotide reductase in Saccharomyces cerevisiae

Ribonucleotide reductase (RNR), which catalyzes the rate-limiting step for deoxyribonucleotide production required for DNA synthesis, is an alpha2beta2 tetramer consisting of two large and two small subunits. RNR2 encodes a small subunit and is essential for mitotic viability in Saccharomyces cerevisiae. We have cloned a second essential gene encoding a homologous small subunit, RNR4. RNR4 and RNR2 appear to have nonoverlapping functions and cannot substitute for each other even when overproduced. The lethality of RNR4 deletion mutations can be suppressed by overexpression of RNR1 and RNR3, two genes encoding the large subunit of the RNR enzyme, indicating genetic interactions among the RNR genes. RNR2 and RNR4 may be present in the same reductase complex in vivo, since they coimmunoprecipitate from cell extracts. Like the other RNR genes, RNR4 is inducible by DNA-damaging agents through the same signal transduction pathway involving MEC1, RAD53, and DUN1 kinase genes. Analysis of DNA damage inducibility of RNR2 and RNR4 revealed partial inducibility in dun1 mutants, indicating a DUN1-independent branch of the transcriptional response to DNA damage.

Production of the R2 subunit of ribonucleotide reductase from herpes simplex virus with prokaryotic and eukaryotic expression systems: higher activity of R2 produced by eukaryotic cells related to higher iron-binding capacity

The R2 subunit of ribonucleotide reductase from herpes simplex virus type 2 was overproduced with prokaryotic and eukaryotic expression systems. The recombinant R2 purified by a two-step procedure exhibited a 3-fold higher activity when produced in eukaryotic cells. Precise quantification of the R2 concentration at each step of the purification indicated that the activity was not altered during the purification procedure. Moreover, we have observed that the level of R2 expression, in eukaryotic cells as well as in prokaryotic cells, did not influence R2 activity. Extensive characterization of the recombinant R2 purified from eukaryotic and prokaryotic expression systems has shown that both types of pure R2 preparations were similar in their 76 kDa dimer contents (more than 95%) and in their ability to bind the R1 subunit. However, we have found that the higher activity of R2 produced in eukaryotic cells is more probably related to a higher capability of binding the iron cofactor as well as a 3-fold greater ability to generate the tyrosyl free radical.

Reaction of NO with the reduced R2 protein of ribonucleotide reductase from Escherichia coli

The active R2 protein of ribonucleotide reductase from Escherichia coli contains a catalytically essential tyrosine radical at position 122 (Tyr122.) that is formed during the reaction of dioxygen with the nearby diiron(II) center. To gain insight into the mode of dioxygen binding, the reaction of the O2 analog NO with the diiron(II) centers of R2red has been investigated by spectroscopic methods. R2red reacts with NO to form an adduct with visible absorption features at 450 and 620 nm and Mössbauer parameters (delta = 0.75 mm/s, delta EQ = -2.13 and -1.73 mm/s) typical of those observed for S = 3/2 [FeNO]7 complexes of other non-heme iron proteins. However, unlike other non-heme [FeNO]7 complexes, this adduct is EPR silent. Our Mössbauer studies show that each iron site of R2red binds one NO to form local S = 3/2 [FeNO]7 centers which then couple antiferromagnetically (J approximately 5 cm-1, H = JS1.S2) to afford an [FeNO]2 center (77% of total iron). This [FeNO]2 center decomposes with a first-order rate constant of 0.013 min-1 to form R2met, accompanied by the release of N2O. These observations suggest that both iron(II) ions of the two diiron(II) centers of R2red have available sites for NO binding, in agreement with the crystallographic results on R2red, and that the bound NO molecules are sufficiently close to each other to permit N-N bond formation to produce N2O. These observations support the proposal that dioxygen binding may also involve both metal ions of the diiron(II) center to form a (mu-1,1-, or mu-1,2-peroxo)-diiron(III) center. This observed reactivity of R2red with NO may contribute to the in vivo inhibition of ribonucleotide reductase by NO.

Escherichia coli and herpes-simplex-virus ribonucleotide reductase R2 subunit. Compared reactivities of the redox centres

Protein R2, the small subunit of ribonucleotide reductase, contains a diferric centre and a tyrosyl radical absolutely required for enzyme activity. The reduction of the tyrosyl radical and the mobilization of the iron centre result in the inhibition of the enzyme and thus of DNA synthesis. The chemical reactivity of the iron-radical centre of Escherichia coli and herpes simplex virus has been studied by u.v.-visible and e.p.r. spectroscopies. The tyrosyl radical is efficiently scavenged by hydroxamic acids and phenols during reactions controlled by steric hindrance and hydrophobic interactions. The reaction with o-disubstituted phenols yields the corresponding diphenoquinones. The reactivity of the bacterial radical greatly contrasts with that of the viral radical, and the iron centre in herpes-simplex-virus R2 is much more labile than that in E. coli R2, as shown from the facile mobilization of iron by chelators such as catechol. These results suggest that the active sites of the two enzymes are significantly different and might be useful for designing new antiviral agents.

Oxygen-sensitive ribonucleoside triphosphate reductase is present in anaerobic Escherichia coli

Ribonucleoside diphosphate reductase from Escherichia coli and mammalian cells provides the deoxyribonucleoside triphosphates for DNA synthesis. The active enzyme contains a tyrosyl free radical whose formation requires oxygen. Earlier genetic evidence suggested that the enzyme is not required for anaerobic growth of E. coli, implicating the activity of a different enzyme or enzyme system for deoxyribonucleotide synthesis in the absence of oxygen. We now conclude from isotope incorporation experiments that E. coli during anaerobiosis obtains its deoxyribonucleotides by reduction of ribonucleotides. Extracts from anaerobically grown bacteria contain a different enzyme activity capable of reducing CTP to dCTP. To obtain an active enzyme, strict anaerobiosis must be maintained during extract preparation and during assay of the enzyme. The reaction is stimulated by NADPH, Mg2+, and ATP. Inhibition by deoxyribonucleoside triphosphates suggests that the anaerobic enzyme has allosteric properties. Antibodies raised against the aerobic enzyme do not inhibit the new activity, and hydroxyurea, a potent scavenger of the tyrosyl radical of the aerobic enzyme, only weakly inhibits the anaerobic enzyme. The anaerobic enzyme has interesting evolutionary aspects since it might reflect on an activity that in the absence of oxygen made possible the transition from an "RNA world" into a "DNA world."

Superoxide dismutase participates in the enzymatic formation of the tyrosine radical of ribonucleotide reductase from Escherichia coli

One of the two nonidentical subunits of Escherichia coli ribonucleotide reductase, protein B2, contains in its active form two antiferromagnetically coupled Fe(III) ions and an organic free radical that arises by the one-electron oxidation of tyrosine-122 of the polypeptide chain. Protein B2 lacking the tyrosine radical but with the iron center intact (called protein B2/HU because it is produced by treatment with hydroxyurea) is enzymatically inactive. Previously, it was found that a crude extract from E. coli transforms B2/HU into B2 in the presence of dithiothreitol, Mg2+, and oxygen. On purification of the enzyme system, we now find that radical introduction requires three separate proteins as well as NADPH and FMN. One of the proteins is superoxide dismutase. We hypothesize that the overall reaction involves a reduction of the iron center followed by the oxidation of iron and tyrosine-122. Superoxide dismutase may then be involved in the second step to protect an oxidation-sensitive intermediate. Alternatively, the enzyme might be directly involved in the oxidation step.

dfp Gene of Escherichia coli K-12, a locus affecting DNA synthesis, codes for a flavoprotein

The cloned dfp gene complements dna-707 (now designated dfp-707), a temperature-sensitive conditionally lethal mutation that results in a slow cessation of DNA synthesis while protein synthesis is maintained. In vitro and in vivo experiments failed to demonstrate a specific defect in the initiation of DNA replication, and turn-off of DNA synthesis at high temperature was slower than that of a typical initiation (dnaA) mutant. The gene was localized, and its product was identified through the construction and analysis of deletion and insertion mutants of dfp-containing plasmids. dfp is located between the rpmB and dut genes at 81 min on the linkage map of Escherichia coli K-12. It is transcribed clockwise, independently of dut. The ability of a plasmid to complement a chromosomal dfp-707 mutation was correlated with its ability to produce a 45-kilodalton polypeptide. The purified protein contained 1 mol of flavin mononucleotide per mol of polypeptide.

The tyrosyl free radical in ribonucleotide reductase

The enzyme, ribonucleotide reductase, catalyses the formation of deoxyribonucleotides from ribonucleotides, a reaction essential for DNA synthesis in all living cells. The Escherichia coli ribonucleotide reductase, which is the prototype of all known eukaryotic and virus-coded enzymes, consists of two nonidentical subunits, proteins B1 and B2. The B2 subunit contains an antiferromagnetically coupled pair of ferric ions and a stable tyrosyl free radical. EPR studies show that the tyrosyl radical, formed by loss of ferric ions and a stable tyrosyl free radical. EPR studies show that the tyrosyl radical, formed by loss of an electron, has its unpaired spin density delocalized in the aromatic ring of tyrosine. Effects of iron-radical interaction indicate a relatively close proximity between the iron center and the radical. The EPR signal of the radical can be studied directly in frozen packed cells of E. coli or mammalian origin, if the cells are made to overproduce ribonucleotide reductase. The hypothetic role of the tyrosyl free radical in the enzymatic reaction is not yet elucidated, except in the reaction with the inhibiting substrate analogue 2'-azido-CDP. In this case, the normal tyrosyl radical is destroyed with concomitant appearance of a 2'-azido-CDP-localized radical intermediate. Attempts at spin trapping of radical reaction intermediates have turned out negative. In E. coli the activity of ribonucleotide reductase may be regulated by enzymatic activities that interconvert a nonradical containing form and the fully active protein B2. In synchronized mammalian cells, however, the cell cycle variation of ribonucleotide reductase, studied by EPR, was shown to be due to de novo protein synthesis. Inhibitors of ribonucleotide reductase are of medical interest because of their ability to control DNA synthesis. One example is hydroxyurea, used in cancer therapy, which selectively destroys the tyrosyl free radical.

Modulation of transferrin receptor expression by inhibitors of nucleic acid synthesis

We investigated the effects of the iron chelator desferrioxamine on the expression of transferrin receptors (TfR) by CCRF-CEM human T-cell leukaemia and B16 mouse melanoma cells growing in tissue culture. Desferrioxamine (DFOA) enhanced TfR expression when added in the dose range of 10(-5)-10(-4) to CCRF-CEM cells, but was toxic to these cells, the lower concentrations producing a slowing of cell growth with a build up in S-phase, while higher concentrations caused cell death with a block at the G1/S-phase interface. These toxic effects are compatible with its previously reported inhibition of the non-haem iron containing (M2) subunit of ribonucleotide reductase. In marked contrast, DFOA caused the growth of B16 melanoma cells to arrest in G1, without loss of cloning efficiency, and resulted in a fall in TfR expression to approximately 50% of control values. These results suggested that the effects of DFOA on TfR expression were linked to DNA synthesis rather than to a more generalised inhibition of iron-dependent cellular processes. It was subsequently found that inhibition of the M2 subunit of ribonucleotide reductase in CCRF-CEM cells with 5 X 10(-5) M hydroxyurea, which is not an iron chelator, also enhanced TfR expression, as did thymidine and cytosine arabinoside, which have different enzyme targets. By measuring cellular DNA and RNA content simultaneously it was shown that all of these agents caused unbalanced growth, i.e., inhibited DNA synthesis more than RNA synthesis. In contrast, 6-thioguanine was more inhibitory to RNA synthesis, and treatment with this drug caused a fall in TfR expression. Thus, although CCRF-CEM cells treated with DFOA show enhanced TfR expression, similar effects are also seen with other inhibitors of DNA synthesis, provided that RNA synthesis is allowed to continue. These results provide further evidence that the regulation of TfR expression by proliferating cells is specifically linked to DNA synthesis rather than to the iron requirements of other cellular processes.

Mutationally altered ribonucleotide reductase from Escherichia coli: characterization of mutations isolated on multicopy plasmids

The Escherichia coli ribonucleotide reductase genes (nrd genes) were mutagenized at random. Point mutations were introduced in vitro into a recombinant nrd plasmid. Transformants were initially screened for altered tolerance toward the drug hydroxyurea and further characterized by enzymatic and immunological methods. The screening procedure could pick out defects in either of the two subunits of ribonucleotide reductase. Cells carrying the nrd plasmid pPS2 were earlier shown to have levels of ribonucleotide reductase molecules that were 10 to 20 times higher than those in wild-type cells. We now demonstrate that the enzymatic activity in gently lysed pPS2-containing cells on cellophane disks is six times higher than in wild-type cells. Supplementation of the pPS2-containing lysates with a purified thioredoxin system results in a further 4.5-fold stimulation of the enzymatic activity, which implies a functional shortage of the electron donor system(s) for ribonucleotide reduction in pPS2-containing cells.

Deoxyribonucleotide biosynthesis in yeast (Saccharomyces cerevisiae). A ribonucleotide reductase system of sufficient activity for DNA synthesis

Ribonucleotide reductase, the central enzyme of DNA precursor biosynthesis, has been isolated and characterized from baker's yeast. The enzyme activity, measured in extracts from three different, exponentially growing yeast strains, is high enough to meet the substrate requirement of DNA replication, in contrast to very low activities found in most other organisms. In thymidylate-permeable yeast cells ribonucleotide reductase activity is stimulated under both starvation and excess of intracellular dTMP. On the other hand growth of yeast in presence of 20 mM hydroxyurea did not increase enzyme activity. Yeast ribonucleotide reductase is composed of two non-identical subunits, inactive separately, of which one binds to immobilized dATP. The relative molecular mass of the holoenzyme is about 250 000. The enzyme reduces all four natural ribonucleoside diphosphates with comparable efficacy. GDP reduction requires dTTP as effector, ADP reduction is stimulated by dGTP, whereas pyrimidine nucleotide reduction is stimulated by any deoxyribonucleotide and ATP. Enzyme activity is independent of exogenous metal ions and is insensitive towards chelating agents. Hydroxyurea inactivates yeast ribonucleotide reductase in a slow reaction; half-inhibition (I50) is reached only at 2-6 mM hydroxyurea concentration. Up to 50% reactivation occurs spontaneously after removal of the inhibitor. In accord with previous attempts by others, extensive purification of the yeast enzyme has failed owing to its extreme instability in solution; the half-life of about 11 h could not be influenced by any protective measure. Taken together, yeast ribonucleotide reductase combines features known from Escherichia coli and mammalian enzymes with differing, individual properties.

Post-translational activation introduces a free radical into pyruvate formate-lyase

Pyruvate formate-lyase (formate acetyltransferase; EC 2.3.1.54) of Escherichia coli cells is post-translationally interconverted between inactive and active forms. Conversion of the inactive to the active form is catalyzed by an Fe2+-dependent activating enzyme and requires adenosylmethionine and dihydroflavodoxin. This process is shown here to introduce a paramagnetic moiety into the structure of pyruvate formate-lyase. It displays an EPR signal at g = 2 with a doublet splitting of 1.5 mT and could comprise an organic free radical located on an amino acid residue of the polypeptide chain. Hypophosphite was discovered as a specific reagent that destroys both the enzyme radical and the enzyme activity; it becomes covalently bound to the protein. The enzymatic generation of the radical, which is linked to adenosylmethionine cleavage into 5'-deoxyadenosine and methionine, possibly occurs through an Fe-adenosyl complex. These results suggest a radical mechanism for the catalytic cycle of pyruvate formate-lyase.