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At least 19 recordsLinked to original sources
PubMed1981

Possible regulation of ribonucleoside diphosphate reductase.

Two inhibitors of ribonucleoside diphosphate reductase (RR) (EC 1.17.4.1) in vitro were isolated from normal rat liver: they were a nondialyzable, heat-labile, high-molecular-weight ribonucleoside diphosphate reductase inhibitor (HRRI, and a dialyzable, heat-stable, low-molecular-weight ribonucleoside diphosphate reductase inhibitor (LRRI). The activities of both inhibitors varied inversely with the cell growth rate. HRRI from the cytosol fraction of rat liver was partially purified by ammonium sulfate fractionation (0 - 50%), and gel filtration on a Sepharose 6B column. It was eluted in the void volume from this column, together with ATP-hydrolyzing activity. The HRRI fraction also contained CDP kinase and CDPase activities, suggesting that HRRI is a complex of several enzymes that reduce the concentrations of the substrate of RR, CDP, and of the allosteric activator, ATP. LRRI was extracted from the cytosol of rat liver with ethanol (80% final concentration) and purified further by washing with organic solvent, and be chromatographies of Amberlite IR-45 and Dowex 50. Finally, it was identified as glucose, which was phosphorylated to glucose 6-phosphate by hexokinase present tin the RR enzyme solution ( 0 - 35% ammonium sulfate fraction of AH-130 cytosol), thus causing ATP depletion. Thus, neither inhibitor reacted directly with the RR enzyme, but both may regulate the enzyme activity in vivo by reducing the intracellular levels of substrates or cofactors.

Animals↗
PubMed1984

A quantitative analysis of steric and hydrophobic effects in ribonucleoside diphosphate reductase inhibition by thiosemicarbazones.

Ribonucleoside diphosphate reductase (RDR) inhibitory activity of 2-formylpyridine and 1-formylisoquinoline thiosemicarbazones is quantitatively analysed in relation to a steric parameter (van der Waals volume, VW) and the hydrophobic parameter logP. The activity is found to be significantly correlated with VW and very poorly with logP. On the basis of this, it is inferred that RDR inhibition by thiosemicarbazones is very sensitive to steric effects and is little influenced by the hydrophobic character of the molecules.

Chemical Phenomena↗
PubMed1980

Fluoride, pyrophosphate, and base release from 2'-deoxy-2'-fluoronucleoside 5'-diphosphates by ribonucleoside-diphosphate reductase.

Ribonucleoside-diphsophate reductase from Escherichia coli catalyzes release of fluoride, inorganic pyrophosphate, and base from 2'-deoxy-2'-fluoronucleoside diphosphates. This reaction is accompanied by inactivation of the enzyme and an increase in absorbance at 314 nm of the inactivated protein. 2'-Deoxy-2'-fluoroadenosine 5'-diphosphate requires two turnovers per inactivation, whereas 2'-deoxy-2'-fluorocytidine 5'-diphosphate requires 100 turnovers per inactivation.

Adenosine Diphosphate↗
PubMed1979

Glutathione-dependent hydrogen donor system for calf thymus ribonucleoside-diphosphate reductase.

Purified calf thymus ribonucleoside-diphosphate reductase (2'-deoxyribonucleoside-diphosphate:oxidized-thioredoxin 2'-oxidoreductase, EC 1.17.4.1), showed an absolute requirement for a dithiol as hydrogen donor, whereas the natural monothiol glutathione (GSH) was inactive per se. However, a protein partially purified from thymus coupled the oxidation of GSH to the formation of deoxyribonucleotides by ribonucleotide reductase. In analogy with the ribonucleotide reductase system of Escherichia coli this protein was called glutaredoxin [Holmgren, A. (1976) Proc. Natl. Acad. Sci. USA 73, 2275-2279]. Thymus glutaredoxin had the following properties: (i) its molecular weight determined by gel chromatography was about 12,000; (ii) it was active iwth ribonucleotide reductase in the presence of GSH, NADPH, and glutathione reductase but had no activity with NADPH and thioredoxin reductase; and (iii) it was immunologically different from thioredoxin because it did not bind to antithioredoxin immunoadsorbents. Experiments on the crossreactivity of thymus and E. coli ribonucleotide reductases and the corresponding thioredoxin and glutaredoxin systems showed essentially no specificity for the homologous thioredoxin but a high species specificity for the homologous glutaredoxin.

Animals↗
PubMed1980

The purification and characterization of an adenosylcobalamin-dependent ribonucleoside diphosphate reductase from Corynebacterium nephridii.

A deoxyadenosylcobalamin-dependent ribonucleoside diphosphate reductase has been purified to homogeneity from cell free extracts of Corynebacterium nephridii. Ion exchange chromatography of th extract on DEAE-Sephadex and DEAE-Bio-Gel A, followed by affinity chromatography on dGTP-Sepharose, yielded two forms of the reductase. The first reductase, which was weakly bound to the affinity column, was eluted with 0.1 M citrate buffer, pH 6.5, while the second, more tightly bound form required 2 M urea for its elution. The weakly bound form is homogeneous as judged by gel filtration, equilibrium sedimentation, and polyacrylamide gel electrophoresis. This enzyme is a dimeric protein with a molecular weight of 196,000, composed of two identical or very similar subunits with molecular weights of 100,000. The second protein appears to be a polymeric form of the reductase. Both forms of the enzyme catalyze the reduction of the four common ribonucleoside diphosphates as well as the hydrogen exchange between adenosylcobalamin and the solvent. The ribonucleoside diphosphate reductase system of C. nephridii appears to be intermediate between the more "primitive" adenosylcobalamin-dependent reductase system of Lactobacillus leichmannii and the more "advanced" nonheme iron system of Escherichia coli.

Amino Acid Sequence↗
PubMed1976

Active site of ribonucleoside diphosphate reductase from Escherichia coli. Inactivation of the enzyme by 2'-substituted ribonucleoside diphosphates.

Ribonucleoside diphosphate reductase is an allosteric enzyme consisting of two nonidentical subunits, proteins B1 and B2. B1 contains dithiols which participate in the oxidation-reduction reactions of electron transport, while B2 contains a free radical essential for activity. Ribonucleoside diphosphates are bound to B1 but not to B2. Addition of 2'-deoxy-2'-chloro ribonucleoside diphosphates to ribonucleotide reductase irreversibly inactivates B1 without affecting B2. The reaction is specific since (a) it requires the presence of active B2, (b) it is controlled by allosteric effectors, (c) B1 is protected against inactivation by the normal substrates, and (d) the chloro-substituted nucleoside monophosphates have no effect. The inactivation of B1 is caused by a modification of the oxidation-reduction dithiols. The chloro derivatives decompose into free base, chloride ion, and 2-deoxyribose 5-diphosphate as a consequence of reaction with the enzyme. 2'-Deoxy-2"-azido ribonucleoside diphosphates cause an irreversible inactivation of B2 without affecting B1. The reaction is specific by the above criteria, indicating that the azido derivatives also bind to the active site of the enzyme. On reduction by ribonucleotide reductase, the azido derivatives function as radical scavengers and selectively destroy the free radical of B2, indicating that this radical participates in ribonucleotide reduction directly. On the basis of these results, a model of the active site of ribonucleotide reductase is proposed in which the site is formed from both B1 and B2. In the site the electron-donating oxidation-reduction active dithiols of B1 are in close contact with the free radical of B2. The azido derivative also inactivates the adenosylcobalamin-dependent ribonucleoside triphosphate reductase from Lactobacillus leichmannii and a ribonucleotide reductase preparation from calf thymus, indicating a general involvement of free radical intermediates in enzyme-catalyzed ribonucleotide reduction.

Escherichia coli↗
PubMed1995

High-performance liquid chromatographic purification, optimization of the assay, and properties of ribonucleoside diphosphate reductase from rabbit bone marrow.

Partial purification of ribonucleoside diphosphate reductase from rabbit bone marrow was achieved by size exclusion HPLC of the crude homogenate. This step, requiring < 15 min, led to 9- to 13-fold purification of the reductase and removal of 64% of the contaminating kinase/phosphatase activities, which in the crude extract degrade > 95% of substrate CDP when reductase is assayed. A systematic study was conducted to evaluate the influence of contaminating kinase/phosphatase activities on CDP concentration during the reductase-catalyzed reaction with either ATP or its kinase-inhibiting analog, 5'-adenylylimidodiphosphate (AMP-PNP), as the allosteric effector. Our studies demonstrated that in the presence of ATP, CDP levels fell instantly to < 24% but thereafter remained fairly constant due to recycling via CTP. In contrast, in the presence of AMP-PNP, CDP levels decreased continuously. The Km values of the reductase for CDP determined in the presence of ATP were significantly higher than those in the presence of AMP-PNP. Furthermore, we also found that the concentration of the ultimate electron donor dithiothreitol (DTT) required for optimum activity of the reductase varies significantly with the level of purity of the reductase preparation. Interestingly, DTT is an inhibitor of the reductase above the optimum concentration. This purification method and the optimized assay together with the understanding of the fate of CDP in partially purified preparations should find application in studies with reductases from other eukaryotic sources.

Adenosine Triphosphate↗
PubMed1988

Effect of bacteriophage T4 DNA topoisomerase gene 39 on level of beta chain of ribonucleoside diphosphate reductase in a T4 nrdB mutant.

Bacteriophage T4 ribonucleoside diphosphate reductase consists of alpha 2 and beta 2 subunits encoded by genes nrdA and nrdB, respectively, and plays a central role in the T4-induced deoxyribonucleotide synthetase complex. The accompanying paper describes the decreased rate of synthesis of deoxyribonucleotides after infection by the T4 mutant, nrdB93, and the suppression of this defect by a second mutation in gene 39, coding for one of the three protein chains of T4 DNA topoisomerase. In this study we examined these effects at the protein level. On infection by nrdB93 not only was the beta 93 protein chain altered, as shown by its migration relative to the wild type protein in electrophoretic gels and by its temperature sensitivity, but the infected cells showed very low levels of the protein. However, on infection with the double mutant of nrdB93 and 39-01 (gene 39) the concentration of beta 93 chain returned to the values of beta protein found with wild type phage. A double mutant bearing nrdB93 and an amber mutation of gene 39 also suppressed the nrdB93 defect. By contrast, a temperature-sensitive mutant of gene 39, A41, did not show suppression at either 30 or 41 degrees C. Amber mutations in the two other genes coding for T4 DNA topoisomerase, 52 and 60, did not suppress the defect. We propose that the deficiency in the quantity of beta 93 chain and the suppression of this defect occur at the transcriptional or translational expression of the nrdB93 gene and that a specific domain of the gene 39 protein, not acting in the capacity of T4 DNA topoisomerase, inhibits the expression.

DNA Topoisomerases, Type I↗
PubMed1973

Properties of ribonucleoside diphosphate reductase in nucleotide-permeable cells.

Ribonucleoside diphosphate (RDP) reductase activity can be readily assayed in ether-treated Escherichia coli cells. The rate of cytidine 5'-diphosphate (CDP) reduction observed in ether-treated cells by using saturating substrate concentrations is about 25% of the rate of de novo deoxyribonucleotide synthesis required to account for in vivo deoxyribonucleic acid synthesis. Optimal activity is observed in the presence of magnesium ions and a positive effector. Adenosine 5'-triphosphate (ATP), deoxy ATP (dATP), and deoxythimidine triphosphate serve as positive effectors, and dATP also serves as a negative effector. These effects on the activity in ether-treated cells resemble those observed in vitro with highly purified enzyme. When the RDP reductase activity in these cells is assayed by using high specific activity (3)H-CDP as substrate, even at nonsaturating substrate concentrations, the sensitivity of the assay is sufficient to make it useful for the assay of the low levels of reductase activity in cells not derepressed by thymine starvation or in cells containing mutationally altered RDP reductase. This assay is much easier to perform than the usual in vitro assay, since thioredoxin, thioredoxin reductase, and enzyme subunits B1 or B2 need not be first purified and added to the reaction mixtures.

Adenosine Triphosphate↗
PubMed1983

Cell cycle regulation of ribonucleoside diphosphate reductase activity in permeable mouse L cells and in extracts.

Ribonucleoside diphosphate reductase (EC1.17.4.1) was previously characterized in exponentially growing mouse L cells selectively permeabilized to small molecules by treatment with dextran sulfate (Kucera and Paulus, 1982b). This characterization has now been extended to cells in specific phases of the cell cycle and in transition between cell cycle phases, with activity studied both in situ (permeabilized cells) and in cell extracts. Cells at various stages in the cell cycle were obtained by unit-gravity sedimentation employing a commercially available reorienting chamber device, by G1 arrest induced by isoleucine limitation, and by metaphase arrest induced by Colcemid. G1 cells from both cycling and noncycling populations had negligible levels of ribonucleotide reductase activity as measured by CDP reduction both in situ and in extracts. When G1 arrested cells were allowed to progress to S phase, ribonucleotide reductase activity increased in parallel with [3H]thymidine incorporation into DNA. Ribonucleotide reductase activity in extracts increased at a somewhat greater rate than in situ activity. S phase ribonucleotide reductase activity measured in situ resembled the previously characterized activity in exponentially growing cells with respect to an absolute dependence on ATP or its analogs as positive allosteric effector, sensitivity to the negative allosteric effector dATP, and low susceptibility to stimulation by NADPH, dithiothreitol, and FeCl3. Disruption of permeabilized cells caused reductase activity to become highly dependent on the presence of both dithiothreitol and FeCl3. As synchronized cultures progressed from S into G2/M phase, no significant change in ribonucleotide reductase activity was seen. On the other hand, when cells that had been arrested in metaphase by Colcemid were allowed to resume cell cycle traversal by removing the drug, in situ ribonucleotide reductase activity decreased by 75% within 2.5 h. This decrease seemed to be a late mitotic event, since it was not correlated with the percentage of cells entering G1 phase. The cause of a subsequent slight increase of in situ ribonucleotide reductase activity is not clear. Parallel measurements of ribonucleotide reductase activity in cell extracts indicated also an initial decline accompanied by increasing dependence on added dithiols and FeCl3, followed by complete activity loss. Our results suggest a cell cycle pattern of ribonucleotide reductase activity that involves negligible levels in G1 phase, a progressive increase of activity upon entry into S phase paralleling overall DNA synthesis, continued retention of significant ribonucleotide reductase activity well into the metaphase period of mitosis, and a very rapid decline in activity during the later phases of mitosis. The periods of increase and decrease of ribonucleotide reductase activity were accompanied by modulation of the properties of the enzyme as indicated by differential changes in enzyme activity measured in situ and in extracts.

Animals↗
PubMed1986

Selective inhibition of herpes simplex virus ribonucleoside diphosphate reductase by derivatives of 2-acetylpyridine thiosemicarbazone.

The effects of thiosemicarbazone derivatives of 2-acetylpyridine on mammalian and viral ribonucleoside diphosphate reductases were investigated. The enzymes were partially purified from uninfected and herpes simplex virus type-1 (HSV-1)-infected KB cells by sequential salt fractionation with streptomycin sulfate and ammonium sulfate and by affinity chromatography on ATP-agarose. The five thiosemicarbazone derivatives investigated were all potent inhibitors of the virus-induced reductase. Fifty percent inhibitory concentrations (IC50 values) range from 2 to 13 microM. Four of the five derivatives also were inhibitors of the host cell reductase (IC50 values = 7-34 microM). A semicarbazone was inactive against the cellular enzyme and relatively weak as an inhibitor of the viral enzyme (IC50 = 340 microM). Four of six compounds were preferential inhibitors of the viral reductase based on a comparison of IC50 values (5- to greater than 85-fold difference). Kinetic experiments revealed that inhibition of the HSV-1 reductase by the thiosemicarbazones was noncompetitive with respect to CDP and dithiothreitol. A comparison of the inhibitory effects of 2-acetylpyridine thiosemicarbazone itself on viral reductase and on virus replication in vitro demonstrated a similarity in the dose-response relationships for the two parameters. This observation supports the hypothesis that the HSV-induced ribonucleoside diphosphate reductase is an important target for the design of antiviral drugs.

Antiviral Agents↗
PubMed1993

2'-C-methyladenosine and 2'-C-methyluridine 5'-diphosphates are mechanism-based inhibitors of ribonucleoside diphosphate reductase from Corynebacterium nephridii.

The interaction of the adenyosylcobalamin-dependent ribonucleoside diphosphate reductase of Cornyebacterium nephridii with 2'-C-methyladenosine 5'-diphosphate (2'-MeADP) and 2'-C-methyluridine 5'-diphosphate (2'-MeUDP) has been investigated. The nucleotide analogs are converted to adenine and uracil, respectively, suggesting that they may be mechanism-based inhibitors. In addition, both analogs generate nucleotides with properties expected for the 2'-deoxy-2'-C-methylnucleotides. The nucleoside obtained after enzymatic dephosphorylation of the product formed from 2'-MeADP has been identified as 2'-deoxy-2'-C-methyladenosine by 1H NMR and mass spectroscopies. Adenine is the major product derived from 2'-MeADP, indicating that the degradation pathway predominates. During the reaction, the carbon-cobalt bond of the coenzyme is cleaved irreversibly to yield 5'-deoxyadenosine and cob(II)alamin. 2'-MeADP is a potent competitive inhibitor of the reduction of the purine nucleotides ADP and GDP, while 2'-MeUDP competitively inhibits the reduction of the pyrimidine nucleotides UDP and CDP. 2'-MeADP is a very effective promoter of the tritium exchange reaction between [5'-3H2]adenosylcobalamin and the solvent, indicating that the exchange reaction is an integral part of the overall reduction. All these observations are consistent with the reaction mechanism proposed by Stubbe and co-workers [Harris, G., Ashley, G. W., Robins, M. J., Tolman, R. L., & Stubbe, J. (1987) Biochemistry 26, 1895-1902 (1987); Stubbe, J. (1990) J. Biol. Chem. 265, 5329-5332] in which they suggest that the partitioning between reduction and inactivation occurs at the level of the 2'-deoxy-3'-ketoribonucleotide intermediate.

Adenosine Diphosphate↗
PubMed1992

Tandem cloning of bacteriophage T4 nrdA and nrdB genes and overproduction of ribonucleoside diphosphate reductase (alpha 2 beta 2) and a mutationally altered form (alpha 2 beta 2(93)).

To investigate the role of ribonucleoside diphosphate reductase in the deoxyribonucleoside triphosphate synthetase multienzyme complex induced by bacteriophage T4 infection and to study the expression of the T4 nrdA and nrdB genes, we have constructed separate plasmid expression strains overproducing their respective alpha 2 and beta 2 protein products. Because complementation of the two proteins to form an active alpha 2 beta 2 enzyme presented complications, nrdA and nrdB, each with its own tac promoter, were also cloned in tandem into a single expression vector. The resulting plasmid (pnrdAB) overproduces ribonucleoside diphosphate reductase. Phage T4 nrdB93, described by Wirak et al. (D. O. Wirak, K. S. Cook, and G. R. Greenberg, J. Biol. Chem. 263:6193-6201, 1988) contains a lesion in exon II of the gene. The mutation causes not only a temperature-sensitive inactivation of the catalytic structure of the beta 2(93) protein and of its ability to interact with alpha 2 protein to form the alpha 2 beta 2(93) enzyme but also a profound non-temperature-sensitive decrease in the formation of the beta 2(93) protein. An expression vector overproducing active alpha 2 beta 2(93) was constructed by site-directed mutagenesis of the nrdB gene.

Base Sequence↗
PubMed1983

Regulation of ribonucleoside diphosphate reductase mRNA synthesis in Escherichia coli.

A RNA-DNA hybridization assay for ribonucleoside diphosphate reductase (RDP reductase) mRNA was used to determine whether control of RDP reductase synthesis in Escherichia coli is at the level of RNA transcription. The correlation observed between the level of RDP reductase enzymatic activity and the rate of RDP reductase mRNA synthesis suggested that the control is at the level of RNA transcription. No increase in RDP reductase enzymatic activity or RDP reductase mRNA was observed during the first 15 min after removal of thymine from a thymine-requiring culture. Thereafter, the rate of RDP reductase mRNA synthesis increased linearly for approximately 75 min before beginning to level off. The addition of thymine to a culture starved for thymine resulted in a decreasing rate of RDP reductase mRNA synthesis. However, 30 min of growth in the presence of thymine was required before the rate of RDP reductase mRNA synthesis dropped to the rate observed in an exponentially growing culture. Inhibition of DNA synthesis caused by shifting a culture of a polC mutant or a dnaB mutant to nonpermissive growth conditions resulted in an increase in the rate of RDP reductase mRNA synthesis similar to that observed for a thymine-starved culture.

DNA, Bacterial↗
PubMed1979

Schedule-dependency assessments of ribonucleoside diphosphate reductase inhibitors when used in combination with platinum compounds plus cyclophosphamide in the treatment of advanced L1210 leukemia.

Each of three ribonucleoside diphosphate reductase inhibitors was used as a third drug in combination with selected antitumor platinum (Pt) agents and cyclophosphamide (CY) in the treatment of advanced L1210 leukemia in C57BL/6 x DBA/2 mice. Each was synergistic with the various Pt plus CY combinations but the effect was highly schedule dependent. The collective cure rate was 68% when hydroxyurea (HU) was given as a single injection with Pt plus CY; the cure rate was 15% when HU was administered on a divided-dose schedule with Pt plus CY. The collective cure rate was 53% when guanazole was given as a single injection with Pt plus CY, but was only 8% when it was given on a divided-dose schedule with Pt plus CY. The effect of 4-methyl-5-amino-1-formylisoquinoline thiosemicarbazone, when used as a third drug with the various Pt plus CY regimens, was not schedule dependent as assessed by the collective cure rate. A therapeutic synergy between CY and each of the three ribonucleoside diphosphate reductase inhibitors was also observed.

Animals↗
PubMed2001

Kinetics in the pre-steady state of the formation of cystines in ribonucleoside diphosphate reductase: evidence for an asymmetric complex.

Two folded polypeptides, designated R1 and R2, respectively, combine in an as yet undefined stoichiometry to form ribonucleoside diphosphate reductase (ribonucleotide reductase) from Escherichia coli. Two pairs of cysteines in each R1 protomer have been implicated in the enzymatic mechanism. One pair, cysteines 225 and 462, is located in the active site of the enzyme and forms a cystine concomitant with the reduction of the ribonucleotide. The other pair, cysteines 754 and 759, is located near the carboxy terminus and is thought to reduce the cystine in the active site by disulfide interchange; either thioredoxin or glutaredoxin is then thought to reduce the cystine that results. Rapid quenching and site-directed immunochemistry have been used to follow the formation of the cystine in the active site and the peripheral cystine simultaneously during the pre-steady state. Prereduced R1 dimer of ribonucleoside diphosphate reductase, in the presence of ATP and CDP, was mixed with R2 dimer in an apparatus for quench flow. The reaction was quenched with a solution of acetic acid and N-ethylmaleimide, the protein was then precipitated with trichloroacetic acid, and the precipitate was separated into two portions. The percent of the cystine in the active site in one of the portions was determined as described previously [Erickson, H. K. (2000) Biochemistry 39, 9241-9250]. A similar method was employed to determine the percent of the peripheral cystine in the other portion of the precipitate. It was found that while the formation of both of these cystines was initiated by the addition of R2 dimer, presumably as products of the reduction of CDP, the peripheral cystine appeared to form more rapidly and in a higher yield than the cystine in the active site. These results demonstrate that the formation of the cystine between cysteines 754 and 759 of ribonucleotide reductase from E. coli is kinetically competent. A mechanism consistent with the prior formation of the cystine between cysteine 225 and cystene 462 as well as the kinetics for the formation of each cystine with time is presented. Because twice as much of the peripheral cystine than cystine in the active site had formed during the pre-steady state, it follows that the enzymatically competent complex between R1 dimers and R2 dimers cannot be symmetric.

Binding Sites↗
PubMed1975

Ribonucleoside diphosphate reductase induced by bacteriophage T4. III. Isolation and characterization of proteins B1 and B2.

Ribonucleoside diphosphate reductase determined by bacteriophage T4 consists of a tight complex (alpha2beta2) of the polypeptide chains alpha (Mr = 80,000 to 85,000) and beta (Mr = 35,000). The alpha2 dimer (= protein B1) was purified from Escherichia coli B infected with T4 mutant nrdB55 (Yeh, Y.C., and Tessman, I. (1972) Virology 47, 767-772) which carries an amber mutation in the gene coding for the beta polypeptide chain. Protein B1 contained binding sites for dATP, an allosteric effector of the reductase. The beta2 dimer (= protein B2) was purified by selective desorption with 1 M guanidine HCl from a dATP-Sepharose affinity column containing adsorbed native T4 ribonucleotide reductase. Protein B2, isolated this way, was enzymatically inactive due to partial loss of its iron but it could be reactivated by treatment with ferrous iron. Active protein B2 contained two atoms of non-heme iron per molecule and exhibited the optical and electron spin resonance spectra previously demonstrated in the native enzyme. The T4-induced proteins B1 and B2 were unable to reduce ribonucleotides when assayed separately but were active in combination. The proteins did not form catalytically functional hybrids with proteins B1 and B2 of Escherichia coli ribonucleotide reductase, neither did they cross-react immunologically with the latter. 5-Hydroxymethyl-dCTP, at concentrations above 10 muM, was a positive allosteric effector of T4 ribonucleotide reductase promoting the reduction of the pyrimidine ribonucleotides CDP and UDP. The nucleotide had little effect on E. coli ribonucleotide reductase.

Coliphages↗