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

Ribonucleotide reductase: Implications of thiol S-nitrosylation and tyrosine nitration for different subunits

Ribonucleotide reductase (RNR) is a multi-subunit enzyme responsible for catalyzing the rate-limiting step in the production of deoxyribonucleotides essential for DNA synthesis and repair. The active RNR complex is composed of multimeric R1 and R2 subunits. The RNR catalysis involves the formation of tyrosyl radicals in R2 subunits and thiyl radicals in R1 subunits. Despite the quaternary structure and cofactor diversity, all the three classes of RNR have a conserved cysteine residue at the active site which is converted into a thiyl radical that initiates the substrate turnover, suggesting that the catalytic mechanism is somewhat similar for all three classes of the RNR enzyme. Increased RNR activity has been associated with malignant transformation, cancer cell growth, and tumorigenesis. Efforts concerning the understanding of RNR inhibition in designing potent RNR inhibitors/drugs as well as developing novel approaches for antibacterial, antiviral treatments, and cancer therapeutics with improved radiosensitization have been made in clinical research. This review highlights the precise and potent roles of NO in RNR inhibition by targeting both the subunits. Under nitrosative stress, the thiols of the R1 subunits have been found to be modified by S-nitrosylation and the tyrosyl radicals of the R2 subunits have been modified by nitration. In view of the recent advances and progresses in the field of nitrosative modifications and its fundamental role in signaling with implications in health and diseases, the present article focuses on the regulations of RNR activity by S-nitrosylation of thiols (R1 subunits) and nitration of tyrosyl residues (R2 subunits) which will further help in designing new drugs and therapies.

RRM1 predicts clinical outcome of high-and intermediate-risk non-muscle-invasive bladder cancer patients treated with intravesical gemcitabine monotherapy

Background

The expression level of ribonucleotide reductase subunit M1 (RRM1) is closely related to the effect of gemcitabine-based therapy in advanced bladder cancer. However, the value of RRM1 expression in predicting progression-free survival in non-muscle-invasive bladder cancer (NMIBC) patients treated with intravesical gemcitabine chemotherapy has not been elucidated.

Methods

This study randomly assigned 162 patients to either the RRM1-known group or the unknown group. We collected cancer tissues from 81 patients to evaluate the mRNA expression of RRM1 by using liquid chip technology. All patients were diagnosed and then treated with intravesical gemcitabine monotherapy immediately after transurethral resection of the bladder tumour (TURBT).

Results

RRM1 expression was high in 21% (17/81) of patients. The RRM1 mRNA level was not correlated with sex, age, weight, performance status, or CUA/EAU risk (p > 0.05). Progression-free survival (PFS) was significantly longer for patients with low RRM1 expression than for patients with high and unknown RRM1 expression (p = 0.009). Additionally, the 1- and 2-year relapse rates also differed according to RRM1 expression level. The 1-year relapse rates for RRM1-low, RRM1-high and RRM1-unknown patients were 0, 17.7 and 6.2% (p = 0.009), while the 2-year relapse rates for these groups were 3.1, 29.4, and 11.1% (p = 0.005), respectively.

Conclusions

This preliminary study showed that low RRM1 expression was associated with longer progression-free survival and lower 1-year/2-year relapse rates in NMIBC patients treated with intravesical gemcitabine monotherapy, despite the need for further verification with large sample sizes and considering more mixed factors and biases.

RRM1 *151A>T, RRM1 -756T>C, and RRM1 -585T>Gis associated with increased susceptibility of lung cancer in Chinese patients

Ribonucleotide reductase M1 (RRM1) is a crucial gene in DNA repair. Recent studies have shown that RRM1 expression can be a powerful predictor of survival or chemotherapy sensitivity in patients presenting with carcinomas who are treated with adjuvant gemcitabine-based chemotherapy including lung cancer. However, the relationship between the single nucleotide polymorphisms (SNP) of RRM1 and the susceptibility of lung cancer to chemotherapy has not been well addressed. We detected six tag SNPs of RRM1 genotypes in a cohort of 1007 patients with primary lung cancer and 1007 age- and sex-matched population controls using SNaPshot detection technology. Logistic regression, odds ratios (OR), and 95% confidence intervals were calculated to estimate lung cancer risk associated with SNP genotypes and haplotypes, after adjusting for case-control matching factors. Compared with the T/T and A/T genotype of RRM1 *151A>T, the A/A genotype had an increased risk for overall lung cancer (adjusted OR, 1.33). Additionally, the T/T+T/C genotypes of RRM1 -756T>C were risk factors that increased the susceptibility to lung cancer (adjusted OR 1.54, as compared with the C/C genotype). While the T/T+G/T genotypes of RRM1 -585T>G behaved as protective factors to increase the susceptibility to lung cancer (adjusted OR 0.44, as compared with the C/C genotype). In summary, this is the first study to systematically identify the relationship between the polymorphisms of RRM1 and individual susceptibility to lung cancer. It is anticipated that the RRM1 *151A>T, RRM1 -756T>C, and RRM1 -585T>G polymorphisms will improve the predictive prognosis of lung cancer sensitivity.

Targeting ribonucleotide reductase for cancer therapy

INTRODUCTION

Ribonucleotide reductase (RR) is a unique enzyme, because it is responsible for reducing ribonucleotides to their corresponding deoxyribonucleotides, which are the building blocks required for DNA replication and repair. Dysregulated RR activity is associated with genomic instability, malignant transformation and cancer development. The use of RR inhibitors, either as a single agent or combined with other therapies, has proven to be a promising approach for treating solid tumors and hematological malignancies.

AREAS COVERED

This review covers recent publications in the area of RR, which include: i) the structure, function and regulation of RR; ii) the roles of RR in cancer development; iii) the classification, mechanisms and clinical application of RR inhibitors for cancer therapy and iv) strategies for developing novel RR inhibitors in the future.

EXPERT OPINION

Exploring the possible nonenzymatic roles of RR subunit proteins in carcinogenesis may lead to new rationales for developing novel anticancer drugs. Updated information about the structure and holoenzyme models of RR will help in identifying potential sites in the protein that could be targets for novel RR inhibitors. Determining RR activity and subunit levels in clinical samples will provide a rational platform for developing personalized cancer therapies that use RR inhibitors.

Control of ribonucleotide reductase localization through an anchoring mechanism involving Wtm1

The control of deoxyribonucleotide levels is essential for DNA synthesis and repair. This control is exerted through regulation of ribonucleotide reductase (RNR). One mode of RNR regulation is differential localization of its subunits. In Saccharomyces cerevisiae, the catalytic subunit hererodimer, Rnr2/Rnr4, is localized to the nucleus while its regulatory subunit, Rnr1, is cytoplasmic. During S phase and in response to DNA damage, Rnr2-Rnr4 enters the cytoplasm, where it presumably combines with Rnr1 to form an active complex. The mechanism of its nuclear localization is not understood. Here, we report the isolation of the WTM (WD40-containing transcriptional modulator) proteins as regulators of Rnr2/Rnr4 localization. Overproduction of Wtm2 increased Rnr2/Rnr4. Deletion of WTM1, a homolog of WTM2, leads to the cytoplasmic localization of Rnr2/Rnr4, and increased hydroxyurea (HU)-resistance in mec1 mutants. Wtm1 binds Rnr2/4 complexes and release them to the cytoplasm in response to DNA damage. Forced localization of Wtm1 to the nucleolus causes Rnr2/Rnr4 complexes to relocalize to the nucleolus. Thus, Wtm1 acts as a nuclear anchor to maintain nuclear localization of Rnr2/4 complexes outside of S phase. In the presence of DNA damage this association is disrupted and Rnr2/Rnr4 become cytoplasmic, where they join with Rnr1 to form an intact complex.

Ribonucleotide reductase M1 gene promoter activity, polymorphisms, population frequencies, and clinical relevance

RRM1 is a gene crucial for determination of the tumor phenotype. It encodes the regulatory subunit of ribonucleotide reductase, and it is a molecular target of gemcitabine. In addition, RRM1 induces PTEN expression and inhibits cell migration, invasion, and metastasis formation. In patients with resected lung cancers, increased levels of RRM1 are highly associated with long survival. In contrast, patients on gemcitabine and cisplatin therapy for advanced disease have a poor survival if RRM1 expression is high presumably because of decreased efficacy of chemotherapy. We analyzed the RRM1 promoter for polymorphisms in an effort to develop a practical and inexpensive assay for RRM1 expression. Two single nucleotide polymorphisms, RR37 and RR524, were discovered. These polymorphisms impacted promoter activity in vitro and had different frequencies in various populations. Promoter allelotypes were highly associated with overall (P = 0.06) and disease-free (P = 0.03) patient survival. The allelotype with the highest predicted activity was associated with the best patient outcome. However, we did not find an association between allelotype and tumoral RRM1 expression. This is likely a result of the limited impact of the described promoter polymorphisms on overall in vivo gene expression. We conclude that clinical studies using RR37 and RR524 for decisions on chemotherapy are premature and that further functional studies on the RRM1 promoter are required to fully elucidate factors controlling RRM1 expression.

S Phase-specific transcription of the mouse ribonucleotide reductase R2 gene requires both a proximal repressive E2F-binding site and an upstream promoter activating region

Ribonucleotide reductase is essential for supplying a balanced pool of the four deoxyribonucleotides required for DNA synthesis and repair. The active enzyme consists of two non-identical subunits called proteins R1 and R2. There are multiple levels of regulation of ribonucleotide reductase activity, which is highest during the S and G(2) phases of an unperturbed cell cycle in mammalian cells. Previous reports in the literature have indicated that the S phase-specific transcription of the mammalian R2 gene is regulated by a transcriptional block, is dependent on the transcription factor E2F1, or is simply regulated by proteins that bind to promoter CCAAT boxes plus the TATA box. Here, we demonstrate that the S phase-specific transcription of the mouse R2 gene is dependent on an upstream promoter activating region (located at nucleotides (nt) -672 to -527 from the transcription start site) and one proximal promoter repressive element (located at nt -112 to -107) that binds E2F4. Binding to the E2F site is modulated by binding of nuclear factor-Y to an adjacent CCAAT element (nt -79 to -75). The upstream activating region is crucial for overall R2 promoter activity. Mutation of the E2F-binding site leads to premature promoter activation in G(1) and increases overall promoter activity but only when the upstream activating region is present and intact. Therefore, E2F-dependent repression is essential for cell cycle-specific R2 transcription.

A cryptic deletion of 2q35 including part of the PAX3 gene detected by breakpoint mapping in a child with autism and a de novo 2;8 translocation

We report a de novo, apparently balanced (2;8)(q35;q21.2) translocation in a boy with developmental delay and autism. Cross species (colour) paint (Rx) and SKY FISH, forward and reverse chromosome painting, and FISH with subtelomeric probes were used to examine the patient's karyotype, but further rearrangements were not detected. FISH with region specific clones mapping near 2q35 and 8q21.2 breakpoints and STS mapping performed on the isolated derivative chromosomes were used to refine the location of the breakpoints further. A cryptic deletion of between 4.23 and 4.41 Mb in extent and involving at least 13 complete genes or transcription units was found at the breakpoint on 2q35. The deletion includes the promoter and 5' untranslated region of the paired box 3 (PAX3) gene. The child has very mild dystopia canthorum which may be associated with the PAX3 haploinsufficiency. The 8q21.2 breakpoint is within MMP16 which encodes matrix metalloproteinase 16. We postulate that the cryptic deletion and rearrangement are responsible for the patient's phenotype and that a gene (or genes) responsible for autism lies at 2q35 or 8q21.2. The results present a step towards identifying genes predisposing to autism.

Localization of a tumor suppressor gene in 11p15.5 using the G401 Wilms' tumor assay

Multiple studies have underscored the importance of loss of tumor suppressor genes in the development of human cancer. To identify these genes, we used somatic cell hybrids in a functional assay for tumor suppression in vivo. A tumor suppressor gene in 11p15.5 was detected by transferring single human chromosomes into the G401 Wilms' tumor cell line. In order to better map this gene, we created a series of radiation-reduced t(X;11) chromosomes and characterized them at 24 loci between H-RAS and beta-globin. Interestingly, three of the chromosomes were indistinguishable as determined by genomic and cytogenetic analyses. Each contains an interstitial deletion with one breakpoint in 11p14.1 and the other breakpoint between the D11S601 and D11S648 loci in 11p15.5. PFGE analysis localized the 11p15.5 breakpoints to a 175 kb MluI fragment that hybridized to D11S601 and D11S648 probes. Genomic fragments from this 175 kb region were hybridized to DNA from mouse hybrid lines containing the delta t(X;11) chromosomes. This analysis detected the identical 11p15.5 breakpoint which disrupts a 7.8 kb EcoRI fragment in all three of the delta t(X;11) chromosomes, suggesting they are subclones of the same parent colony. Upon transfer into G401 cells, one of the chromosomes suppressed tumor formation in nude mice, while the other two chromosomes lacked this ability. Thus, our mapping data indicate that the gene in 11p15.5 which suppresses tumor formation in G401 cells must lie telomeric to the D11S601 locus. Koi et al. (Science 260: 361-364, 1993) have used a similar functional assay to localize a growth suppressor gene for the RD cell line centromeric to the D11S724 locus. The combination of functional studies by our lab and theirs significantly narrows the location of the tumor suppressor gene in 11p15.5 to the approximately 500 kb region between D11S601 and D11S724.

The TATA-less promoter of mouse ribonucleotide reductase R1 gene contains a TFII-I binding initiator element essential for cell cycle-regulated transcription

Mammalian ribonucleotide reductase shows S-phase specific expression and consists of two non-identical subunits, proteins R1 (large subunit) and R2 (small subunit). A comparison between the human and mouse TATA-less R1 gene promoters revealed four highly conserved DNA regions, while the remaining sequence showed a low degree of conservation. Two regions, alpha and beta, were earlier identified as protein binding regions in the mouse R1 promoter by using DNase footprinting technique. The two new regions are located to the transcription start and to a DNA sequence about 40 base pairs downstream from the start. Gel shift assays using TFII-I antibodies and competition with an oligonucleotide representing the terminal deoxynucleotidyl transferase inhibitor element identified the start region as a TFII-I binding initiator element. The conserved downstream region, called gamma, also formed specific DNA-protein complexes in gel shift assays. Functional studies, using synchronized cells stably transformed by R1 promoter-luciferase reporter gene constructs, indicated that the initiator and the gamma elements together were necessary for cell cycle-regulated R1 promoter activity. Earlier published data, indicating Sp1 binding to the R1 alpha/beta regions, could not be confirmed, suggesting that the R1 initiator element may function independent of Sp1.

Structure and promoter characterization of the gene encoding the large subunit (R1 protein) of mouse ribonucleotide reductase

Mammalian ribonucleotide reductase (EC 1.17.4.1) is composed of two nonidentical subunits, proteins R1 and R2, both required for enzyme activity. The structure of the genomic mouse ribonucleotide reductase R1 gene was compiled from a number of overlapping lambda clones isolated from a Charon 4A mouse sperm genomic library. The R1-encoding gene covers 26 kb and consists of 19 exons. All exon-intron boundaries were located by dideoxynucleotide sequencing, showing that intron 7 starts with the variant GC instead of GT. About 3.5 kb of DNA from the 5'-flanking region of the R1-encoding gene were cloned and sequenced, and the transcriptional start site was determined by nuclease S1 mapping of RNA. DNase I footprinting assays on the R1 promoter identified two nearly identical 23-bp-long protein-binding regions. Three protein complexes binding to one of the 23-mer regions were resolved and partially identified by using gel-retardation mobility-shift assays and UV crosslinking. One complex most likely contained Sp1, and another complex showed S-phase-specific binding, suggesting a direct role in the cell-cycle-dependent R1 gene expression.

Cloning, sequence determination, and regulation of the ribonucleotide reductase subunits from Plasmodium falciparum: a target for antimalarial therapy

Malaria remains a leading cause of morbidity and mortality worldwide, accounting for more than one million deaths annually. We have focused on the reduction of ribonucleotides to 2'-deoxyribonucleotides, catalyzed by ribonucleotide reductase, which represents the rate-determining step in DNA replication as a target for antimalarial agents. We report the full-length DNA sequence corresponding to the large (PfR1) and small (PfR2) subunits of Plasmodium falciparum ribonucleotide reductase. The small subunit (PfR2) contains the major catalytic motif consisting of a tyrosyl radical and a dinuclear Fe site. Whereas PfR2 shares 59% amino acid identity with human R2, a striking sequence divergence between human R2 and PfR2 at the C terminus may provide a selective target for inhibition of the malarial enzyme. A synthetic oligopeptide corresponding to the C-terminal 7 residues of PfR2 inhibits mammalian ribonucleotide reductase at concentrations approximately 10-fold higher than that predicted to inhibit malarial R2. The gene encoding the large subunit (PfR1) contains a single intron. The cysteines thought to be involved in the reduction mechanism are conserved. In contrast to mammalian ribonucleotide reductase, the genes for PfR1 and PfR2 are located on the same chromosome and the accumulation of mRNAs for the two subunits follow different temporal patterns during the cell cycle.

The M1 subunit of ribonucleotide reductase refines mapping of genetic rearrangements at chromosome 11p15

We report the first use of the ribonucleotide reductase M1 subunit (RRM1) locus as a marker to assist in defining genetic rearrangements at 11p15. Our sample consisted of 21 Wilms' tumors from 18 patients, and one adrenal adenoma from a patient with Beckwith-Wiedemann syndrome, preexisting chromosome 11 maps being refined by the use of the RRM1 locus in all cases. Significantly, one Wilms' tumor showed loss of heterozygosity at the RRM1 locus only, whereas the adrenal adenoma showed a maintenance of heterozygosity at the RRM1 locus, loss having been previously demonstrated at the c-Ha-ras locus. The relevance of this finding to the location of one or more disease-associated loci at 11p15 is discussed.

Synergistic and coordinate expression of the genes encoding ribonucleotide reductase subunits in Swiss 3T3 cells: effect of multiple signal-transduction pathways

Ribonucleoside-diphosphate reductase (ribonucleotide reductase, EC 1.17.4.1) is the enzyme responsible for the in vivo production of deoxyribonucleotides for DNA synthesis and is essential for cell proliferation. We examined the signal transduction pathways leading to expression of the M1 and M2 subunits of this enzyme in Swiss 3T3 mouse fibroblasts by Northern blot analysis. Stimulation of quiescent cells resulted in coordinate expression of both subunits, beginning at 8 hr after serum addition, in late G1 phase, and peaking at 18-24 hr. Serum increased M2 message to 30 to 50 times that of quiescent cells, in contrast with M1 message, which was increased 10 times. Agents that elevated cAMP, including forskolin, and the cAMP analogue 8-bromo-cAMP modestly stimulated gene expression. Each of these agents was synergistic with insulin, and these combinations induced expression equivalent to that induced by serum stimulation. Likewise, agents that activate protein kinase C such as phorbol 12,13-dibutyrate, bombesin, and vasopressin were also synergistic with insulin with respect to ribonucleotide reductase gene expression, as was epidermal growth factor, which stimulates receptor tyrosine kinase activity. The time course for induction of mRNA expression by each of these agents alone or in combination was identical to that for induction stimulated by serum. Finally, the synergistic effects apparent in Northern analysis of ribonucleotide reductase gene expression were mirrored in parallel determinations of DNA synthesis. Thus, the combinatorial nature of signal transduction pathways resulting in proliferation of Swiss 3T3 cells is expressed at the level of ribonucleotide reductase gene expression.

New TaqI polymorphism in the RRM1 gene

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Human polymorphic probe pE1.8 detects SacI polymorphism in the ribonucleotide reductase M1 subunit gene

A new human polymorphic probe to the ribonucleotide reductase M1 subunit gene is described. The location of this gene at chromosome 11p15 makes it a useful marker for studying DNA rearrangements in embryonal tumours.

Temperature-sensitive DNA mutant of Chinese hamster ovary cells with a thermolabile ribonucleotide reductase activity

JB3-B is a Chinese hamster ovary cell mutant previously shown to be temperature sensitive for DNA replication (J. J. Dermody, B. E. Wojcik, H. Du, and H. L. Ozer, Mol. Cell. Biol. 6:4594-4601, 1986). It was chosen for detailed study because of its novel property of inhibiting both polyomavirus and adenovirus DNA synthesis in a temperature-dependent manner. Pulse-labeling studies demonstrated a defect in the rate of adenovirus DNA synthesis. Measurement of deoxyribonucleoside triphosphate (dNTP) pools as a function of time after shift of uninfected cultures from 33 to 39 degrees C revealed that all four dNTP pools declined at similar rates in extracts prepared either from whole cells or from rapidly isolated nuclei. Ribonucleoside triphosphate pools were unaffected by a temperature shift, ruling out the possibility that the mutation affects nucleoside diphosphokinase. However, ribonucleotide reductase activity, as measured in extracts, declined after cell cultures underwent a temperature shift, in parallel with the decline in dNTP pool sizes. Moreover, the activity of cell extracts was thermolabile in vitro, consistent with the model that the JB3-B mutation affects the structural gene for one of the ribonucleotide reductase subunits. The kinetics of dNTP pool size changes after temperature shift are quite distinct from those reported after inhibition of ribonucleotide reductase with hydroxyurea. An indirect effect on ribonucleotide reductase activity in JB3-B has not been excluded since human sequences other than those encoding the enzyme subunits can correct the temperature-sensitive growth defect in the mutant.

Temperature-sensitive DNA mutant of Chinese hamster ovary cells with a thermolabile ribonucleotide reductase activity

JB3-B is a Chinese hamster ovary cell mutant previously shown to be temperature sensitive for DNA replication (J. J. Dermody, B. E. Wojcik, H. Du, and H. L. Ozer, Mol. Cell. Biol. 6:4594-4601, 1986). It was chosen for detailed study because of its novel property of inhibiting both polyomavirus and adenovirus DNA synthesis in a temperature-dependent manner. Pulse-labeling studies demonstrated a defect in the rate of adenovirus DNA synthesis. Measurement of deoxyribonucleoside triphosphate (dNTP) pools as a function of time after shift of uninfected cultures from 33 to 39 degrees C revealed that all four dNTP pools declined at similar rates in extracts prepared either from whole cells or from rapidly isolated nuclei. Ribonucleoside triphosphate pools were unaffected by a temperature shift, ruling out the possibility that the mutation affects nucleoside diphosphokinase. However, ribonucleotide reductase activity, as measured in extracts, declined after cell cultures underwent a temperature shift, in parallel with the decline in dNTP pool sizes. Moreover, the activity of cell extracts was thermolabile in vitro, consistent with the model that the JB3-B mutation affects the structural gene for one of the ribonucleotide reductase subunits. The kinetics of dNTP pool size changes after temperature shift are quite distinct from those reported after inhibition of ribonucleotide reductase with hydroxyurea. An indirect effect on ribonucleotide reductase activity in JB3-B has not been excluded since human sequences other than those encoding the enzyme subunits can correct the temperature-sensitive growth defect in the mutant.

Localization of the gene for human proliferating nuclear antigen/cyclin by in situ hybridization

Proliferating cell nuclear antigen (PCNA)/cyclin has been localized by in situ hybridization to the short arm of human chromosome 20 with a peak of grains over band 20p13. In addition, there were two strong secondary peaks of grains over 11p15.1 and Xp11.4 indicating the presence of two related genes in man.

Bacteriophage T4 nrdA and nrdB genes, encoding ribonucleotide reductase, are expressed both separately and coordinately: characterization of the nrdB promoter

We examined the expression of the bacteriophage T4 nrdA and nrdB genes, which encode the alpha 2 and beta 2 subunits, respectively, of ribonucleoside diphosphate reductase, the first committed enzyme in the pathway of synthesis of the deoxyribonucleoside triphosphates. T4 nrdA, located 700 bp upstream from nrdB, has been shown previously to be transcribed by two major transcripts: a prereplicative, polycistronic message, TU, orginating at an immediate-early promoter, PE, that is 3.5 kb upstream from nrdA, and a postreplicative message commencing from a late promoter in its 5' flank. We have found a third promoter initiating a transcript at 159 nucleotides upstream from the reading frame of nrdB. PnrdB functions only in the presence of the T4 motA gene product, which is required for middle (time) promoters, and therefore the onset of nrdB transcription is delayed more than 2 min after infection. Because of the distance of nrdA from PE, the inception of nrdA transcription (delayed early) coincides closely with that of nrdB. An apparent termination site, tA, occurs about 80 bp downstream from nrdA. Some of the polycistronic mRNA reading through the site after 5 min contributes to nrdB transcription. nrdA and nrdB genes in an uninfected host have been reported to be transcribed only coordinately. In contrast, T4 nrdA and nrdB are initially transcribed separately onto the PE and PnrdB transcripts, respectively, but at about 5 min after infection are transcribed both coordinately and on separate transcripts. Evidence is presented that TU coordinately transcribes a deoxyribonucleotide operon in the order: frd, td, gene 'Y,' nrdA, nrdB. Since the beta 2 subunit is known to be formed after the alpha 2 subunit, the expression of the nrdB gene determines the onset of deoxyribonucleoside triphosphate synthesis and thus of T4 DNA replication.

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.

A common SacI polymorphism in the gene for the M1 subunit of ribonucleotide reductase (RRM1)

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Chromosome maps of man and mouse. IV

Current knowledge of man-mouse genetic homology is presented in the form of chromosomal displays, tables and a grid, which show locations of the 322 loci now assigned to chromosomes in both species, as well as 12 DNA segments not yet associated with gene loci. At least 50 conserved autosomal segments with two or more loci have been identified, twelve of which are over 20 cM long in the mouse, as well as five conserved segments on the X chromosome. All human and mouse chromosomes now have conserved regions; human 17 still shows the least evidence of rearrangement, with a single long conserved segment which apparently spans the centromere. The loci include 102 which are known to be associated with human hereditary disease; these are listed separately. Human parental effects which may well be the result of genomic imprinting are reviewed and the location of the factors concerned displayed in relation to mouse chromosomal regions which have been implicated in imprinting phenomena.

Human tyrosinase gene, mapped to chromosome 11 (q14----q21), defines second region of homology with mouse chromosome 7

The enzyme tyrosinase (monophenol,L-dopa:oxygen oxidoreductase; EC 1.14.18.1) catalyzes the first two steps in the conversion of tyrosine to melanin, the major pigment found in melanocytes. Some forms of oculocutaneous albinism, characterized by the absence of melanin in skin and eyes and by a deficiency of tyrosinase activity, may result from mutations in the tyrosinase structural gene. A recently isolated human tyrosinase cDNA was used to map the human tyrosinase locus (TYR) to chromosome 11, region q14----q21, by Southern blot analysis of somatic cell hybrid DNA and by in situ chromosomal hybridization. A second site of tyrosinase-related sequences was detected on the short arm of chromosome 11 near the centromere (p11.2----cen). Furthermore, we have confirmed the localization of the tyrosinase gene in the mouse at or near the c locus on chromosome 7. Comparison of the genetic maps of human chromosome 11 and mouse chromosome 7 leads to hypotheses regarding the evolution of human chromosome 11.