Inactivation of ribonucleotide reductase by nitric oxide

Ribonucleotide reductase has been demonstrated to be inhibited by NO synthase product(s). The experiments reported here show that nitric oxide generated from sodium nitroprusside, S-nitrosoglutathione and the sydnonimine SIN-1 inhibits ribonucleotide reductase activity present in cytosolic extracts of TA3 mammary tumor cells. Stable derivatives of these nitric oxide donors were either inactive or much less inhibitory. EPR experiments show that the tyrosyl radical of the small subunit of E. Coli or mammalian ribonucleotide reductase is efficiently scavenged by these NO donors.

Demonstration of apolipoprotein CII in guinea pigs. Functional characteristics, cDNA sequence, and tissue expression

In contrast to plasma from other mammals, guinea pig plasma does not stimulate the activity of lipoprotein lipases in vitro. This had led previously to the conclusion that guinea pigs lack an analogue to apolipoprotein CII (apoCII). By adsorption of lipid-binding proteins to lipid droplets, thereby separating them from other plasma components, we could demonstrate apoCII-like activity in guinea pig plasma. On electrophoresis, the CII-like activity co-migrated with one isoform of guinea pig apolipoprotein CIII, identified by amino-terminal amino acid sequence determination (40 residues). By isoelectric focusing in a narrow pH gradient, the activating protein was separated sufficiently from the dominating apoCIII isoform to allow sequence determination of 8 residues from the amino terminus. Six of these were identical to corresponding residues in apoCII from dog and monkey. With the aid of a human apoCII cDNA probe we identified one cross-hybridizing mRNA species (approximately 600 nucleotides) on Northern blots of guinea pig liver. Three positive clones were isolated from a guinea pig liver cDNA library using the same cDNA probe. The nucleotide sequence showed extensive similarities to the previously known human, monkey, and canine sequences, but the signal peptide was 3 amino acid residues longer in the guinea pig protein, and there was a deletion of 4 residues in the putative lipid binding domain. Northern blot analyses indicated that guinea pig apoCII is mainly expressed in the liver with little or no contribution from the intestine.

Purification and characterization of recombinant mouse and herpes simplex virus ribonucleotide reductase R2 subunit

Overexpression of recombinant mouse and herpes simplex virus ribonucleotide reductase small subunit (protein R2) has been obtained by using the T7 RNA polymerase expression system. Both proteins, which constitute about 30% of the soluble Escherichia coli proteins, have been purified to homogeneity by a rapid and simple procedure. At this stage, few of the molecules contain the iron-tyrosyl free-radical center necessary for activity; however, addition of ferrous iron and oxygen under controlled conditions resulted in a mouse R2 protein containing 0.8 radical and 2 irons per polypeptide chain. In this reaction, one oxygen molecule was needed to generate each tyrosyl radical. Both proteins had full enzymatic activity. EPR spectroscopy showed that iron-center/radical interactions are considerably stronger in both mouse and viral proteins than in E. coli protein R2. CD spectra showed that the bacterial protein contains 70% alpha-helical structure compared to only about 50% in the mouse and viral proteins. Light absorption spectra between 310 and 600 nm indicate close similarity of the mu-oxo-bridged binuclear iron centers in all three R2 proteins. Furthermore, the paramagnetically shifted iron ligand proton NMR resonances show that the antiferromagnetic coupling and ligand arrangement in the iron center are nearly identical in all three species.

Tyrosyl free radical formation in the small subunit of mouse ribonucleotide reductase

Each R2 subunit of mammalian ribonucleotide reductase contains a pair of high spin ferric ions and a tyrosyl free radical essential for activity. To study the mechanism of tyrosyl radical formation, substoichiometric amounts of Fe(II) were added to recombinant mouse R2 apoprotein under strictly anaerobic conditions and then the solution was exposed to air. Low temperature EPR spectroscopy showed that the signal from the generated tyrosyl free radical correlated well with the quantity of the Fe(II) added with a stoichiometry of 3 Fe(II) needed to produce 1 tyrosyl radical: 3 Fe(II) + P + O2 + Tyr-OH + H+----Fe(III)O2-Fe(III)-P + H2O. + Tyr-O. + Fe(III), where P is an iron-binding site of protein R2 and Tyr-OH is the active tyrosyl residue. The O-O bond of a postulated intermediate O2(2-)-Fe(III)2-P state is cleaved by the extra electron provided by Fe(II) leading to formation of OH., which in turn reacts with Tyr-OH to give Tyr-O.. In the presence of ascorbate, added to reduce the monomeric Fe(III) formed, 80% of the Fe(II) added produced a radical. The results strongly indicate that each dimeric Fe(III) center during its formation can generate a tyrosyl-free radical and that iron binding to R2 apoprotein is highly cooperative.

Paracetamol inhibits replicative DNA synthesis and induces sister chromatid exchange and chromosomal aberrations by inhibition of ribonucleotide reductase

Effects of paracetamol have been studied in a hydroxyurea (HU)-resistant mouse mammary tumour cell line TA3H2, shown to overproduce the small subunit of ribonucleotide reductase. These TA3H2 cells were much more resistant than the TA3H (wild-type) cells towards the inhibitory effect of paracetamol on cell growth, IC50 0.55 mM paracetamol for the wild-type compared to 2.7 mM for the HU-resistant cells. The reduced cell growth was due to an inhibition of replicative DNA synthesis, judged from an increased percentage of cells in S-phase measured by flow cytometry. Furthermore, in the wild-type cells, the increase in the number of cells in S phase was already observed at 0.1 mM while in the HU-resistant cell line this effect was first seen at 3.0 mM paracetamol. HU inhibits ribonucleotide reductase by destroying a tyrosyl free radical located on the small subunit of the enzyme. By electron paramagnetic resonance we demonstrate that paracetamol added to crude cell extracts of HU-resistant cells also immediately destroys this radical. These results show that paracetamol reduces DNA synthesis by a specific inhibition of ribonucleotide reductase. A concentration-dependent induction of sister chromatid exchanges was found both with paracetamol (1.0-10 mM) and HU (0.3-3 mM) in wild-type cells whereas no such increase was observed in HU-resistant cells. Paracetamol (1 mM for 2 h) also increased the number of chromosomal aberrations CAs in wild-type cells (i.e. chromatid breaks and chromatid exchanges). The frequency of CAs was not increased in HU-resistant cells at paracetamol concentrations up to 10 mM.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations of ribonucleotide reductase activity following induction of the nitrite-generating pathway in adenocarcinoma cells

The murine adenocarcinoma cell line TA 3 synthesized nitrite from L-arginine upon stimulation with gamma-interferon (IFN-gamma) associated with tumor necrosis factor (TNF), and/or bacterial lipopolysaccharide (LPS), but not with IFN-gamma, TNF, or LPS added separately. Induction of the NO2(-)-generating activity caused an inhibition of DNA synthesis in TA 3 cells. This inhibition was prevented by the L-arginine analog N omega-nitro-L-arginine, which inhibited under the same conditions nitrite production by TA 3 cells. The TA 3 M2 subclone, selected for enhanced ribonucleotide reductase activity, was found to be less sensitive than the wild phenotype TA 3 WT to the cytostatic activity mediated by the NO2(-)-generating system. Cytosolic preparations from TA 3 M2 cells treated for 24 or 48 h with IFN-gamma, TNF, and LPS exhibited a reduced ribonucleotide reductase activity, compared to untreated control cells. No reduction in ribonucleotide reductase activity was observed when N omega-nitro-L-arginine was added to treated cells. Addition of L-arginine, NADPH, and tetrahydrobiopterin into cytosolic extracts from 24-h treated TA 3 M2 cells triggered the synthesis of metabolic products from the NO2(-)-generating pathway. This resulted in a dramatic inhibition of the residual ribonucleotide reductase activity present in the extracts. The inhibition was reversed by NG-monomethyl-L-arginine, another specific inhibitor of the NO2(-)-generating activity. No L-arginine-dependent inhibition of ribonucleotide reductase activity was observed using extracts from untreated cells that did not express NO2(-)-generating activity. These results demonstrate that, in an acellular preparation, molecules derived from the NO2(-)-generating pathway exert an inhibitory effect on the ribonucleotide reductase enzyme. This negative action might explain the inhibition of DNA synthesis induced in adenocarcinoma cells by the NO2(-)-generating pathway.

S-phase-specific expression of mammalian ribonucleotide reductase R1 and R2 subunit mRNAs

Ribonucleotide reductase in mammalian cells is composed of two nonidentical subunits, proteins R1 and R2, each inactive alone. The R1 protein is present in excess in proliferating cells, and its levels are constant during the cell cycle. Expression of the R2 protein, which is limiting for enzyme activity, is strictly S-phase-correlated. In this paper, we have used antisense RNA probes in a solution hybridization assay to measure the levels of R1 and R2 mRNA during the cell cycle in centrifugally elutriated cells and in cells synchronized by isoleucine or serum starvation. The levels of both transcripts were very low or undetectable in G0/G1-phase cells, showed a pronounced increase as cells progressed into S phase, and then declined when cells progressed into G2 + M phase. The R1 and R2 transcripts increased in parallel, starting slightly before the rise in S-phase cells, and reached the same levels. The relative lack of cell cycle dependent variation in R1 protein levels, obtained previously, may therefore simply be a consequence of the long half-life of the R1 protein. Hydroxyurea-resistant, R2-overproducing mouse TA3 cells showed the same regulation of the R1 and R2 transcripts as the parental cells, but with R2 mRNA at a 40-fold higher level.

Molecular cloning and expression of the functional gene encoding the M2 subunit of mouse ribonucleotide reductase: a new dominant marker gene

Mammalian ribonucleotide reductase consists of two non-identical subunits, proteins M1 and M2. M2-related DNA sequences are present on mouse chromosomes 4, 7, 12 and 13. However, M2-overproducing mouse cells show amplification of a chromosome 12-specific, single 13 kb HindIII fragment, which probably represents the active gene. We have isolated this fragment from parental mouse cell DNA and used it to clone and characterize the functional M2 gene. The 5770 bp transcribed M2 sequence contains ten exons separated by nine 95-917 bp introns. The 501 bp of 5' flanking DNA is G + C rich and contains TTTAAA and CCAAT sequences as well as potential Sp1 binding sites. The M2-related sequence on chromosome 13, which contains only the last six exons and several internal rearrangements, is a pseudogene. Transfection of BALB/3T3 cells with the M2 gene resulted in stable transformants with a 10-fold reduction in sensitivity to hydroxyurea, compared to control cells. This confirmed that the cloned M2 genomic DNA represents the functional gene and conclusively establishes the link between hydroxyurea resistance and M2 expression in mammalian cells. M2 genomic DNA should be a valuable dominant, selectable marker for identifying and isolating stable co-transformants.

Herpes simplex virus ribonucleotide reductase: expression in Escherichia coli and purification to homogeneity of a tyrosyl free radical-containing, enzymatically active form of the 38-kilodalton subunit

Infection of mammalian cells with herpes simplex virus (HSV) induces a virus-encoded ribonucleotide reductase which is different from the cellular enzyme. This essential viral enzyme consists of two nonidentical subunits of 140 and 38 kilodaltons (kDa) which have not previously been purified to homogeneity. The small subunit of ribonucleotide reductases from other species contains a tyrosyl free radical essential for activity. We have cloned the gene for the small subunit of HSV-1 ribonucleotide reductase into a tac expression plasmid vector. After transfection of Escherichia coli, expression of the 38-kDa protein was detected in immunoblots with a specific monoclonal antibody. About 30 micrograms of protein was produced per liter of bacterial culture. The 38-kDa protein was purified to homogeneity in an almost quantitative yield by immunoaffinity chromatography. It contained a tyrosyl free radical which gave a specific electron paramagnetic resonance spectrum identical to that we have observed in HSV-infected mammalian cells and clearly different from that produced by the E. coli and mammalian ribonucleotide reductases. The recombinant 38-kDa subunit had full activity when assayed in the presence of HSV-infected cell extracts deficient in the native 38-kDa subunit.

Molecular mechanisms responsible for the drug-induced posttranscriptional modulation of ribonucleotide reductase levels in a hydroxyurea-resistant mouse L cell line

Ribonucleotide reductase, which catalyzes the formation of deoxyribonucleotides from ribonucleoside diphosphate precursors, is the rate-limiting enzyme in DNA synthesis. The enzyme consists of two nonidentical subunits called M1 and M2, both of which are required for activity. Hydroxyurea, a specific inhibitor of DNA synthesis, acts by destroying the unique tyrosyl free radical of protein M2. Previously, we have described a mouse L cell line which exhibited a stable resistance to high concentrations of hydroxyurea. This mutant cell line contains elevated quantities of both proteins M1 and M2 as a result of corresponding increases in the levels of mRNAs for both subunits. Interestingly, both M1 and M2 protein levels were further elevated when mutant cells were cultured in the presence of hydroxyurea, and this elevation was not accompanied by increases in their corresponding mRNAs. These results indicated that hydroxyurea can modulate ribonucleotide reductase expression posttranscriptionally. In this report, we show that the level of both subunits of ribonucleotide reductase responds to hydroxyurea in a drug concentration dependent manner. Furthermore, results from kinetic studies indicate that protein M2 levels rise much more rapidly than protein M1. Pulse-chase experiments indicated that the half-lives of both the M1 and M2 polypeptides are increased by approximately 2-fold when the mutant cells are cultured in the presence of hydroxyurea. We also present evidence indicating that exposure of these cells to hydroxyurea leads to a relatively slow but specific increase in the rate of biosynthesis of both proteins M1 and M2, as assayed by pulse labeling.(ABSTRACT TRUNCATED AT 250 WORDS)

Ribonucleotide reductase M1 subunit in cellular proliferation, quiescence, and differentiation

Ribonucleotide reductase catalyzes the first unique, rate-limiting step in DNA synthesis; both its large (M1) and small (M2) subunits are necessary for activity. While direct studies of M2 expression have previously shown a tight correlation with S phase, the kinetic features of M1-expressing cells have remained ill defined. Therefore we have, using immunofluorescence flow cytometry, analyzed changes in whole cell M1 levels and DNA content during various cell cycle and differentiation events. In asynchronous cultures M1 levels are sustained throughout the cell cycle, including G1 phase when M2 levels and ribonucleotide reductase catalytic activity are known to be very low. In contrast M1 is virtually absent from quiescent lymphocytes but is expressed following mitogen stimulation, shortly before S phase cells appear. M1 declines to low levels in "plateau phase" cultures, the major reduction occurring in cells with 2n (G0/G1) DNA content. HL-60 promyelocytic leukemia cells, induced into either myeloid or monocyte-macrophage differentiation, show a similar marked decrease in M1 levels concomitant with the cessation of cell division. We conclude that the M1 subunit of ribonucleotide reductase is constitutively expressed by cycling cells. It is acquired during stimulated transition from G0 to G1 and is lost during exit to G0 or terminal differentiation. This pattern of expression suggests that determination of cellular M1 content may be useful in distinguishing proliferating (including G1) and quiescent (including G0) cells in vivo.

Molecular mechanisms of drug resistance involving ribonucleotide reductase: hydroxyurea resistance in a series of clonally related mouse cell lines selected in the presence of increasing drug concentrations

Mammalian ribonucleotide reductase is a highly regulated, rate-limiting activity responsible for converting ribonucleoside diphosphates to the deoxyribonucleotide precursors of DNA. The enzyme consists of two nonidentical proteins often called M1 and M2, both of which are required for activity. Hydroxyurea is an antitumor agent which inhibits ribonucleotide reductase by interacting with the M2 component specifically at a unique tyrosyl free radical. To obtain further information about drug resistance mechanisms, we have used M1 and M2 complementary DNAs and monoclonal antibodies to investigate the properties of a series of clonally related drug-resistant mouse cell lines, selected by a step-wise procedure for increasing levels of resistance to the cytotoxic effects of hydroxyurea. Several interesting mechanisms have been identified. Each successive drug selection step leading to the isolation of highly resistant cells was accompanied by stable elevations in cellular resistance and ribonucleotide reductase activities. The changes that occurred at each step involved the M2 component. A very early event, occurring at the first step in the selection process, was the amplification of the M2 gene accompanied by an increase in M2 messenger RNA. Although cellular resistance and M2 protein levels increased significantly during drug selection, only a modest change in M2 gene copy number was observed after the initial selection step. Analysis of wild type, moderately resistant, and highly resistant cells indicated that, in addition to M2 gene amplification, posttranscriptional modification also occurred during drug selection. This second mechanism was not due to alterations in protein M2 half-life, but involved an increase in translational efficiency. By increasing the rate of M2 synthesis, without altering degradation rates, resistant cells were able to accumulate high levels of this key regulatory protein. Cells selected for the ability to proliferate in concentrations of drug as high as 4 mM exhibited changes that involved M2, without detectable changes to M1. These results provide further evidence that M1 and M2 levels are controlled by different mechanisms in mammalian cells. Eventually, however, cells required an elevation in the M1 protein, as well as the M2 protein, to survive in a hydroxyurea concentration of 5 mM. These results illustrate the complexity of the drug-resistant phenotype and provide further information about the molecular processes that lead to the development of cells resistant to low, intermediate, and high concentrations of hydroxyurea.

The structural gene for the M1 subunit of ribonucleotide reductase maps to chromosome 11, band p15, in human and to chromosome 7 in mouse

The genes for the M1 subunit of the enzyme ribonucleotide reductase have been mapped in the human and the murine species by use of two independently derived mouse cDNA clones. Southern blot analysis of rodent x human somatic cell hybrid DNAs confirmed the assignment of RRM1 to the short arm of human chromosome 11. In situ hybridization to human metaphase chromosomes revealed a peak of silver grains over the distal third of band 11p15, a region corresponding to subbands p15.4----p15.5. The mouse Rrml locus was assigned to chromosome 7, where it forms part of a conserved syntenic group of at least seven other genes assigned to human chromosome band 11p15.

Elevated expression of M1 and M2 components and drug-induced posttranscriptional modulation of ribonucleotide reductase in a hydroxyurea-resistant mouse cell line

Ribonucleotide reductase, a rate-limiting enzyme in the synthesis of DNA, consists of two nonidentical subunits, proteins M1 and M2. Hydroxyurea, a specific inhibitor of DNA synthesis, acts by destroying the unique tyrosyl free radical of protein M2. In the past, we have described a mouse L cell line which exhibited a stable resistance to high concentrations of hydroxyurea [McClarty, G. A., Chan, A., & Wright, J.A. (1986) Somat. Cell Mol. Genet. 12, 121-131]. When this line was grown in the absence of hydroxyurea, the cells contained a modest but stable elevation in ribonucleotide reductase activity. However, the activity was further increased on the addition of drug to the culture medium. This was accompanied by an increase in protein M2 activity as shown by activity titration experiments. Likewise, removal of hydroxyurea resulted in a decrease in M2 activity. In the present study, we make use of recently isolated cDNAs and monoclonal antibodies for both the M1 and M2 proteins to further our understanding of the mechanism of hydroxyurea resistance at the molecular level in a subclone of this cell line. Our results indicated that protein M1 levels were elevated 2-3-fold and protein M2 levels were increased about 50-fold in the mutant cells when they were grown in the absence of hydroxyurea, compared to wild-type cells. These protein increases were accompanied by corresponding elevations in the levels of mRNAs for both subunits and increased rates of transcription of both genes. There was a 6-fold amplification in the gene copy number for protein M2.(ABSTRACT TRUNCATED AT 250 WORDS)

Ribonucleotide reductase M2 subunit sequences mapped to four different chromosomal sites in humans and mice: functional locus identified by its amplification in hydroxyurea-resistant cell lines

The sites of sequences homologous to a murine cDNA for ribonucleotide reductase (RR) subunit M2 were determined on human and murine chromosomes by Southern blot analysis of interspecies somatic cell hybrid lines and by in situ hybridization. In the human genome, four chromosomal sites carrying RRM2-related sequences were identified at 1p31----p33, 1q21----q23, 2p24----p25, and Xp11----p21. In the mouse, M2 sequences were found on chromosomes 4, 7, 12, and 13 by somatic cell hybrid studies. By Southern analysis of human hydroxyurea-resistant cells that overproduce M2 because of gene amplification, we have identified the amplified restriction fragments as those that map to chromosome 2. To further confirm the site of the functional RRM2 locus, two other cDNA clones, p5-8 and S7 (coding for ornithine decarboxylase; ODC), which are coamplified with RRM2 sequences in human and rodent hydroxyurea-resistant cell lines, were mapped by Southern and in situ hybridization. Their chromosomal map positions coincided with the region of human chromosome 2 (p24----p25) that also contains one of the four RRM2-like sequences. Since this RRM2 sequence and p5-8 and ODC are most likely part of the same amplification unit, the RRM2 structural gene can be assigned to human chromosome 2p24----p25. This region is homologous to a region of mouse chromosome 12 that also carries one of numerous ODC-like sequences. In an RRM2-overproducing mouse cell line, we found amplification of the chromosome 12-specific restriction fragments. Thus, we conclude that mouse chromosome 12 carries the functional locus for RRM2.

Magnetic interaction between the tyrosyl free radical and the antiferromagnetically coupled iron center in ribonucleotide reductase

Ribonucleotide reductases from Escherichia coli and from mammalian cells are heterodimeric enzymes. One of the subunits, in the bacterial enzyme protein B2 and in the mammalian enzyme protein M2, contains iron and a tyrosyl free radical that both are essential for enzyme activity. The iron center in protein B2 is an antiferromagnetically coupled pair of high-spin ferric ions. This study concerns magnetic interaction between the tyrosyl radical and the iron center in the two proteins. Studies of the temperature dependence of electron paramagnetic resonance (EPR) relaxation and line shape reveal significant differences between the free radicals in proteins B2 and M2. The observed temperature-dependent enhanced EPR relaxation and line broadening of the enzyme radicals are furthermore completely different from those of a model UV-induced free radical in tyrosine. The results are discussed in terms of magnetic dipolar as well as exchange interactions between the free radical and the iron center in both proteins. The free radical and the iron center are thus close enough in space to exhibit magnetic interaction. For protein M2 the effects are more pronounced than for protein B2, indicating a stronger magnetic interaction.

Altered expression of ribonucleotide reductase and role of M2 gene amplification in hydroxyurea-resistant hamster, mouse, rat, and human cell lines

Five hamster, mouse, and rat cell lines resistant to the cytotoxic effects of hydroxyurea have been characterized. All cell lines contained increased ribonucleotide reductase activity, elevated levels of the M2 component of ribonucleotide reductase as judged by electron paramagnetic resonance spectroscopy, and increased copies of M2 mRNA as determined by Northern blot analysis. Two species of M2 mRNA were detected in rodent cell lines, a high-molecular-weight species of approximately 3.4 kb in hamster and rat cells and about 2.1 kb in mouse cells. The low molecular-weight M2 mRNA was about 1.6 kb in all rodent lines. Northern blot analysis showed that the mRNA for the other component of ribonucleotide reductase, M1, was not markedly elevated in the drug-resistant cells and existed as a single 3.1-kb species. Four of the five resistant lines contained an M2 gene amplification as determined by Southern blot analysis, providing direct evidence to support earlier suggestions that hydroxyurea resistance is often accompanied by amplification of a ribonucleotide reductase gene. An increase in gene dosage was detected even in cells exhibiting only modest drug-resistance properties. No evidence for amplification of the M1 gene of ribonucleotide reductase was found. In keeping with these observations with drug-resistant rodent lines, a human (HeLa) cell line resistant to hydroxyurea was also found to contain increased levels of two M2 mRNA species (about 3.4 and 1.6 kb) and exhibited M2 gene amplification. One hamster cell line resembled the other resistant rodent lines in cellular characteristics but did not show amplification of either the M1 or M2 gene, providing an example of a drug-resistant mechanism in which an elevation of M2 mRNA has occurred without a concomitant increase in M2 gene copy number.

Isolation and characterization of expressible cDNA clones encoding the M1 and M2 subunits of mouse ribonucleotide reductase

Mammalian ribonucleotide reductase consists of two nonidentical subunits, proteins M1 and M2, which are differentially regulated during the cell cycle. We have isolated expressible cDNA clones of both subunits from an Okayama-Berg cDNA library made with mRNA from hydroxyurea-resistant, M2 protein-overproducing mouse TA3 cells. Expression of M2 protein could be demonstrated by electron paramagnetic resonance spectroscopy after transfection of COS-7 monkey cells with the plasmid. Electrophoresis and blot analyses of the parent and hydroxyurea-resistant TA3 mRNA revealed two M2 transcripts, a major one of 2.1 kilobases and a minor one of about 1.6 kilobases. Restriction endonuclease mapping of the corresponding cDNAs indicated that the two mRNAs differed only in the length of the 3' untranslated ends. By contrast, there was only one mRNA corresponding to the M1 protein, and its mobility corresponded to about 3.1 kilobases. The hydroxyurea-resistant TA3 cells contained a 50- to 100-fold excess of the M2 mRNAs over that of the parent cells and a 10-fold excess of the M1 mRNA. However, a Southern blot analysis of the corresponding genomic DNA sequences showed that the M2 gene was amplified fivefold but the M1 gene was still single copy. The complete nucleotide sequence of the 2,111-base-pair-long M2 cDNA revealed an open reading frame coding for 390 amino acids, which corresponds to a molecular weight of 45,100. The mouse M2 protein sequence was quite homologous to the equivalent protein in the clam Spisula solidissima, while the homology to the smaller subunits of Epstein-Barr virus, herpes simplex virus type 2, and Escherichia coli ribonucleotide reductases were less pronounced.

Different cellular distribution of thioredoxin and subunit M1 of ribonucleotide reductase in rat tissues

The cellular distribution of thioredoxin and protein M1 of ribonucleotide reductase in adult rat tissues was investigated with immunohistochemical techniques using specific antisera. Tissues with high or low frequency of either mitotic or meiotic cell divisions were compared. Thioredoxin was demonstrated in many cells types that showed no detectable protein M1 of ribonucleotide reductase. A few cell types with protein M1 immunoreactivity also contained immunoreactive thioredoxin. However, in most cells no such co-localization could be demonstrated. This lack of correlation between cells containing subunit M1 of ribonucleotide reductase and the thioredoxin indicates that thioredoxin is not the physiologist hydrogen donor for ribonucleotide reductase in rat tissues and that the expression of two enzymes is differently regulated.

Cell cycle-dependent expression of mammalian ribonucleotide reductase. Differential regulation of the two subunits

Consistent with its specialized role in DNA synthesis, the activity of ribonucleotide reductase is cell cycle-dependent, reaching its maximum during S-phase. This paper demonstrates, however, the levels of the two protein subunits, M1 and M2, of this enzyme vary independently of one another. The level of protein M1 was determined by use of a two-site monoclonal antibody-enzyme immunoassay and found to be constant throughout the cell cycle in bovine kidney MDBK cells. Pulse-chase experiments showed that the half-life of protein M1 was 15 h. This contrasts with our previous results demonstrating an S-phase-correlated increase in the concentration of protein M2 and a half-life of this subunit of 3 h. Therefore, ribonucleotide reductase is controlled during the cell cycle by the level of protein M2.