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Query: UNIPROT:P06889 (Mol)
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Mammalian ribonucleotide reductase catalyzes the rate-limiting for the de novo synthesis 2'-deoxyribonucleoside 5'-triphosphates. There is some suggestion that this step may also be the rate-limiting step of DNA synthesis. It is apparent that the level of the enzyme, ribonucleotide reductase, varies through the cell cycle and is highest in those tissues with the greatest proliferation rate. This increase in activity is associated with increased protein synthesis. The purified enzyme has been shown to be subject to strict allosteric regulation by the various nucleoside triphosphates and it has been proposed that allosteric regulation plays an important role in the level of ribonucleotide reductase activity which is expressed. All experimental data relating to this point, however, do not support the role of deoxyribonucleoside triphosphates as a major factor in determining cellular reductase activity during normal cell division. Several naturally occurring factors have been isolated from cells which lower ribonucleotide reductase activity in vitro. These factors have been found in tissues of low growth fraction and appear to be absent or low in tissues or high growth fraction such as tumor, regenerating liver and embryonic tissues. The expression of intracellular ribonucleotide reductase activity is therefore controlled at various levels and by various factors and the prevailing mode of regulation may vary throughout the cell cycle transverse and also in the various types of cells.
Mol Cell Biochem 1983
PMID:Regulation of ribonucleotide reductase activity in mammalian cells. 635 95

9-beta-D-Arabinofuranosyl-2-fluoroadenine (2-F-araA) inhibited the growth in vitro of HeLa cells by 50% at a concentration of 0.25 microM and depressed the replication of herpes simplex virus Types 1 and 2 by 99% at 25 microM. The analogue served as a substrate for cytoplasmic but not mitochondrial deoxycytidine (dCyd) kinase partially purified from human peripheral chronic lymphocytic leukemic blast cells. The Km values of dCyd and 2-F-araA for the cytoplasmic enzyme were 5 microM and 213 microM, respectively. However, at concentrations of 0.4 mM, the analogue was phosphorylated 2.9 times faster than dCyd. The 5'-triphosphate of 2-F-araA was examined for its biochemical effects on partially purified ribonucleotide reductase and highly purified DNA alpha- and beta-polymerases from HeLa cells. 2-F-araATP was a potent inhibitor of ribonucleotide reductase; the concentration required for 50% inhibition of ADP reduction (0.3 mM ADP; 5 mM GTP or dGTP) was 1 microM and for CDP reduction (0.15 mM CPD; 5 mM ATP) was 8.5 microM. Furthermore, 2-F-araATP was a competitive inhibition (Ki = 1.2 microM) with respect to dATP (Km = 3.8 microM) of DNA alpha-polymerase, whereas DNA beta-polymerase was relatively insensitive to the drug. The results suggest that the cytotoxic actions of 2-F-araA may be due, in part, to a "self-potentiating" inhibition of DNA synthesis. This is, by inhibiting the formation of competing dATP, 2-F-araATP may potentiate its inhibition of DNA synthesis.
Mol Pharmacol 1982 Mar
PMID:In vitro biological activity of 9-beta-D-arabinofuranosyl-2-fluoroadenine and the biochemical actions of its triphosphate on DNA polymerases and ribonucleotide reductase from HeLa cells. 704 62

Although a number of transfection experiments have suggested potential targets for the action of the E2F1 transcription factor, as is the case for many transcriptional regulatory proteins, the actual targets in their normal chromosomal environment have not been demonstrated. We have made use of a recombinant adenovirus containing the E2F1 cDNA to infect quiescent cells and then measure the activation of endogenous cellular genes as a consequence of E2F1 production. We find that many of the genes encoding S-phase-acting proteins previously suspected to be E2F targets, including DNA polymerase alpha, thymidylate synthase, proliferating cell nuclear antigen, and ribonucleotide reductase, are indeed induced by E2F1. Several other candidates, including the dihydrofolate reductase and thymidine kinase genes, were only minimally induced by E2F1. In addition to the S-phase genes, we also find that several genes believed to play regulatory roles in cell cycle progression, such as the cdc2, cyclin A, and B-myb genes, are also induced by E2F1. Moreover, the cyclin E gene is strongly induced by E2F1, thus defining an autoregulatory circuit since cyclin E-dependent kinase activity can stimulate E2F1 transcription, likely through the phosphorylation and inactivation of Rb and Rb family members. Finally, we also demonstrate that a G1 arrest brought about by gamma irradiation is overcome by the overexpression of E2F1 and that this coincides with the enhanced activation of key target genes, including the cyclin A and cyclin E genes.
Mol Cell Biol 1995 Aug
PMID:Cellular targets for activation by the E2F1 transcription factor include DNA synthesis- and G1/S-regulatory genes. 762 16

Previous studies have indicated that mutation of RAP1 (rap1s) or of the HMR-E silencer ARS consensus element leads to metastable repression of HMR. A number of extragenic suppressor mutations (sds, suppressors of defective silencing) that increase the fraction of repressed cells in rap1s hmr delta A strains have been identified. Here we report the cloning of three SDS genes. SDS11 is identical to SWI6, a transcriptional regulator of genes required for DNA replication and of cyclin genes. SDS12 is identical to RNR1, which encodes a subunit of ribonucleotide reductase. SDS15 is identical to CIN8, whose product is required for spindle formation. We propose that mutations in these genes improve the establishment of silencing by interfering with normal cell cycle progression. In support of this idea, we show that exposure to hydroxyurea, which increases the length of S phase, also restores silencing in rap1s hmr delta A strains. Mutations in different cyclin genes (CLN3, CLB5, and CLB2) and two cell cycle transcriptional regulators (SWI4 and MBP1) also suppress the silencing defect at HMR. The effect of these cell cycle regulators is not specific to the rap1s or hmr delta A mutation, since swi6, swi4, and clb5 mutations also suppress mutations in SIR1, another gene implicated in the establishment of silencing. Several mutations also improve the efficiency of telomeric silencing in wild-type strains, further demonstrating that disturbance of the cell cycle has a general effect on position effect repression in Saccharomyces cerevisiae. We suggest several possible models to explain this phenomenon.
Mol Cell Biol 1995 Jul
PMID:Disturbance of normal cell cycle progression enhances the establishment of transcriptional silencing in Saccharomyces cerevisiae. 779 68

The very effective (ID50 = 47 nM) and selective antimalarial compound (S)-9-(3-hydroxy-2-phosphonylmethoxypropyl) adenine (HPMPA) abruptly arrests Plasmodium falciparum-cultured schizonts at concentrations between 1 and 10 x ID50 as soon as their DNA content reaches 8 times that of the haploid ringform stage. Even very high HPMPA concentrations do not inhibit the first 2-3 rounds of schizogonic DNA replication. Also, in the presence of HPMPA, replication of the 6-kb mitochondrial and 35-kb chloroplast-like DNA proceeds normally and in close concert with each other, both to a 16-fold amount within 5 h during the trophozoite stage. Hence the in in vitro assays HPMPApp-sensitive plasmodial DNA polymerase gamma-like enzyme (IC50 = 1 microM)--assumed to be involved in mitochondrial DNA replication--is not the target of HPMPA in vivo (living parasites), nor seems to be the DNA polymerization activities of the--in vitro also HPMPA-sensitive (IC50 = 38 microM)--DNA polymerase alpha or of any other nuclear DNA polymerase of Plasmodium. In vitro assays demonstrated that HPMPApp does not act as an alternative substrate for plasmodial polymerases, contradicting the suggestion that the observed delayed inhibition of plasmodial schizogony might be the result of DNA strand breakage caused by HPMPApp incorporation. Neither do results support the idea that the HPMPA-induced arrest of DNA replication might be due to chain termination as a result of such incorporation. We investigated whether arrest of DNA replication by HPMPA in schizonts could be explained by inhibition of the DNA synthesis rate limiting ribonucleotide reductase enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)
Mol Biochem Parasitol 1994 Sep
PMID:The effect of (S)-9-(3-hydroxy-2-phosphonylmethoxypropyl) adenine on nuclear and organellar DNA synthesis in erythrocytic schizogony in malaria. 783 72

We recently demonstrated that the plant amino acid, mimosine, is an extremely efficacious inhibitor of DNA replication in mammalian cells [P. A. Dijkwel and J. L. Hamlin (1992) Mol. Cell. Biol. 12, 3715-3722; P. J. Mosca et al. (1992) Mol. Cell. Biol. 12, 4375-4383]. Several of its properties further suggested that mimosine might target initiation at origins of replication, which would make it a unique and very useful inhibitor for studying the regulation of DNA synthesis. However, mimosine is known to chelate iron, a cofactor for ribonucleotide reductase. Thus, the possibility arose that mimosine functions in vivo simply by lowering intracellular deoxyribonucleotide pools. In the present study, we show that, in fact, it is possible to override mimosine inhibition in vivo by adding excess iron; however, copper, which is not a substitute for iron in ribonucleotide reductase, is equally effective. Evidence is presented that mimosine functions instead by binding to an intracellular protein. We show that radiolabeled mimosine can be specifically cross-linked to a 50 kDa polypeptide (termed p50) in vitro. Binding to p50 is virtually undetectable in CHO cells selected for resistance to 1 mM mimosine, arguing that p50 is the biologically relevant target. p50 is not associated with the cellular membrane fraction and, hence, is probably not a channel protein. Furthermore, the binding activity does not vary markedly as a function of cell cycle position, arguing that p50 is not a cyclin. Finally, both iron and copper are able to reverse the mimosine-p50 interaction in vitro, probably explaining why both metal ions are able to overcome mimosine's inhibitory effect on DNA synthesis in vivo.
 ...
PMID:Mimosine, a novel inhibitor of DNA replication, binds to a 50 kDa protein in Chinese hamster cells. 786 31

The effects of hydroxyurea (HU), an inhibitor of ribonucleotide reductase, on the replication of human immunodeficiency virus type 1 (HIV-1) in activated peripheral blood mononuclear cells were studied. The inhibition of HIV-1 replication by HU alone was dose dependent, with a 90% inhibitory concentration of 0.4 mM, a plasma concentration tolerated by patients with oncological diseases. HU at lower concentrations (< 0.1 mM) was found to potentiate the antiviral activity of 2',3'-dideoxyinosine (ddl), 3'-azido-2',3'- dideoxythymidine, and 2',3'-dideoxycytidine against HIV-1, with the potentiation being ddl greater than 3'-azido-2',3'- dideoxythymidine = 2',3'-dideoxycytidine. In the presence of 0.1 mM HU, the 90% inhibitory concentration of ddl was reduced by 6-fold in activated peripheral blood mononuclear cells. The potentiating effect of HU on ddl action was time dependent, with the greatest inhibition of HIV-1 growth being seen when HU was present during and after virus adsorption, i.e., apparently coinciding with the time of proviral DNA synthesis. A brief incubation of activated cells with HU and ddl at low concentrations before virus exposure reduced p24 production by > 50%. Analyses using high performance liquid chromatography and enzymatic assays suggested that the greater degree of potentiation by HU of the action of ddl, compared with the other dideoxynucleosides, is due to the more effective inhibition by HU of dATP synthesis, compared with the synthesis of the other deoxynucleoside triphosphates (dGTP, dTTP, and dCTP). The present study suggests that, for appropriate agents, pharmacological reduction of deoxynucleoside triphosphate levels represents a potential therapeutic approach for inhibition of HIV-1 replication.
Mol Pharmacol 1994 Oct
PMID:Anti-human immunodeficiency virus type 1 activity of hydroxyurea in combination with 2',3'-dideoxynucleosides. 796 58

Phylogenetic trees were derived for the Alphaherpesvirinae subfamily of the Herpesviridae using molecular sequences. Sequences from the families of genes encoding glycoprotein B, thymidine kinase, S region protein kinase, immediate-early transcriptional regulator IE175 and ribonucleotide reductase large subunit were examined by means of both maximum parsimony and distance methods, and for both protein and DNA alignments. Trees obtained were evaluated by bootstrap analysis. A clear consensus tree was obtained, with most detail coming from 14 sequences in the glycoprotein B gene set. The tree showed two avian viruses branching first from the lineage leading to the mammalian alphaherpesviruses. The mammalian viruses were split into two groups, which corresponded to the Simplexvirus and Varicellovirus genera. A timescale for events in alphaherpesvirus evolution was tested, based on the proposition that most of the lineages arose by ancient cospeciation with hosts. The virus phylogenetic tree was unambiguously compatible with cospeciation for ten of the 12 mammalian viruses. The tree was also supported by demonstration of an approximate proportionality between magnitudes of pairwise divergences of viral sequences and times since lineages of corresponding pairs of hosts split. On the basis of this timescale it was estimated that the two mammalian alphaherpesvirus groups diverged around the period of the mammalian radiation, and that alphaherpesviral genome sequences have evolved faster than those of mammals by a factor of one to two orders of magnitude.
J Mol Biol 1994 Apr 22
PMID:Molecular phylogeny of the alphaherpesvirinae subfamily and a proposed evolutionary timescale. 814 60

The inhibition by different p-alkoxyphenol derivatives of the growth-regulating enzyme ribonucleotide reductase (RR) in purified Escherichia coli and mouse R2 protein preparations was studied by EPR spectroscopy. The inhibitor-induced inactivation of the catalytic subunit protein R2 was measured at 77 degrees K by observing the decrease of the typical EPR signal from the functionally essential protein-linked tyrosyl free radical. p-Methoxy-, p-ethoxy-, p-propoxy-, and p-allyloxyphenol were about 2 orders of magnitude more effective in inhibiting mouse R2, compared with E. coli R2. Among the p-alkoxyphenols studied, p-propoxyphenol was the most effective inhibitor of mouse R2 (IC50, 0.7 microM) and p-methoxyphenol was the least effective (IC50, 11 microM); p-ethoxy- and p-allyloxyphenol were intermediate. The observed half-maximal inhibition values characterized p-alkoxyphenols as a new class of strong inhibitors of the R2 protein of mammalian RR. p-Propoxy-, p-ethoxy-, and p-allyloxyphenol could be considered as new candidates for anticancer drugs. A special cellular inhibition assay of RR in proliferating tumor cells, in which the tyrosyl radical of R2 at natural concentration was monitored by EPR, showed that the four para-substituted alkoxyphenols also inhibited the enzyme with high efficiency in tumor cells (IC50, between 0.5 microM and 5 microM). Our results with inactivation of protein R2 of RR imply that the cytostatic effect of p-alkoxyphenols on melanoma cells, which has been hitherto explained by inhibition of tyrosinase [Melanoma Res. 2:295-304 (1992)], may be caused at least partly by inhibition of RR. Protein R2 of RR may be considered as an additional target that could be used for future cancer chemotherapy.
Mol Pharmacol 1994 Apr
PMID:p-Alkoxyphenols, a new class of inhibitors of mammalian R2 ribonucleotide reductase: possible candidates for antimelanotic drugs. 818 56

The crystal structure of the ribonucleotide reductase free radical protein R2 from Escherichia coli has been determined by multiple isomorphous replacement and twofold molecular averaging. The structure has been refined at 2.2 A resolution to R = 0.175. The subunit structure of the R2 protein has a novel fold where the basic motif is a bundle of eight long helices. The R2 dimer has two equivalent dinuclear iron centers. Each iron center is well buried in the subunit. The iron atoms have both histidine and carboxyl acid ligands and are bridged by an oxide ion and the carboxylate group of Glu115. One iron atom is octahedrally coordinated with small deviations from ideal values, while the coordination of the other iron ion is more distorted, mainly due to the fact that Asp84 is a bidental ligand to this iron atom. The oxidation of the enzymatically essential tyrosine residue (Tyr122) and the dinuclear iron center by molecular oxygen is suggested to take part in a suitable conserved oxygen-binding pocket between the iron center and the tyrosine zeta-oxygen 5.3 A away from the closest iron ion. The tyrosine proton can be abstracted by the dioxygen and the deprotonated tyrosine residue is then more easily oxidized to a radical species. Tyr122 is buried inside the protein about 10 A from the surface. This has the consequence that the tyrosyl radical cannot participate directly in hydrogen abstraction from the substrate ribose at the active site of the holoenzyme located on the R1 subunit. The radical must then be indirectly involved in the mechanism of the enzyme and an electron transfer reaction between the active site and the tyrosine must take place. Based on the analysis of the available ribonucleotide reductase sequences, the binding surface for the large ribonucleotide reductase protein R1, and a possible route for an electron transport between the buried radical and this surface is described.
J Mol Biol 1993 Jul 05
PMID:Structure and function of the Escherichia coli ribonucleotide reductase protein R2. 833 55


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