Ribonucleotide reductase of herpes simplex virus type 2 resembles that of herpes simplex virus type 1

Unveiling the Connection: Viral Infections and Genes in dNTP Metabolism

Deoxynucleoside triphosphates (dNTPs) are crucial for the replication and maintenance of genomic information within cells. The balance of the dNTP pool involves several cellular enzymes, including dihydrofolate reductase (DHFR), ribonucleotide reductase (RNR), and SAM and HD domain-containing protein 1 (SAMHD1), among others. DHFR is vital for the de novo synthesis of purines and deoxythymidine monophosphate, which are necessary for DNA synthesis. SAMHD1, a ubiquitously expressed deoxynucleotide triphosphohydrolase, converts dNTPs into deoxynucleosides and inorganic triphosphates. This process counteracts the de novo dNTP synthesis primarily carried out by RNR and cellular deoxynucleoside kinases, which are most active during the S phase of the cell cycle. The intracellular levels of dNTPs can influence various viral infections. This review provides a concise summary of the interactions between different viruses and the genes involved in dNTP metabolism.

Allosteric regulation of the class III anaerobic ribonucleotide reductase from bacteriophage T4

Ribonucleotide reductase (RNR) is an essential enzyme in all organisms. It provides precursors for DNA synthesis by reducing all four ribonucleotides to deoxyribonucleotides. The overall activity and the substrate specificity of RNR are allosterically regulated by deoxyribonucleoside triphosphates and ATP, thereby providing balanced dNTP pools. We have characterized the allosteric regulation of the class III RNR from bacteriophage T4. Our results show that the T4 enzyme has a single type of allosteric site to which dGTP, dTTP, dATP, and ATP bind competitively. The dissociation constants are in the micromolar range, except for ATP, which has a dissociation constant in the millimolar range. ATP and dATP are positive effectors for CTP reduction, dGTP is a positive effector for ATP reduction, and dTTP is a positive effector for GTP reduction. dATP is not a general negative allosteric effector. These effects are similar to the allosteric regulation of class Ib and class II RNRs, and to the class Ia RNR of bacteriophage T4, but differ from that of the class III RNRs from the host bacterium Escherichia coli and from Lactococcus lactis. The relative rate of reduction of the four substrates was measured simultaneously in a mixed-substrate assay, which mimics the physiological situation and illustrates the interplay between the different effectors in vivo. Surprisingly, we did not observe any significant UTP reduction under the conditions used. Balancing of the pyrimidine deoxyribonucleotide pools may be achieved via the dCMP deaminase and dCMP hydroxymethylase pathways.

Ribonucleotide reductases

Ribonucleotide reductases provide the building blocks for DNA replication in all living cells. Three different classes of enzymes use protein free radicals to activate the substrate. Aerobic class I enzymes generate a tyrosyl radical with an iron-oxygen center and dioxygen, class II enzymes employ adenosylcobalamin, and the anaerobic class III enzymes generate a glycyl radical from S-adenosylmethionine and an iron-sulfur cluster. The X-ray structure of the class I Escherichia coli enzyme, including forms that bind substrate and allosteric effectors, confirms previous models of catalytic and allosteric mechanisms. This structure suggests considerable mobility of the protein during catalysis and, together with experiments involving site-directed mutants, suggests a mechanism for radical transfer from one subunit to the other. Despite large differences between the classes, common catalytic and allosteric mechanisms, as well as retention of critical residues in the protein sequence, suggest a similar tertiary structure and a common origin during evolution. One puzzling aspect is that some organisms contain the genes for several different reductases.

In vitro inhibition of human cytomegalovirus replication by desferrioxamine

Desferrioxamine (DFO) is commonly used in therapy as a chelator of ferric ion in disorders of iron overload. We found that DFO inhibits human cytomegalovirus (HCMV) replication in infected cultures of human foreskin fibroblasts (HFF) at concentrations that have been achieved in humans with no significant adverse effects. The concentrations of DFO required for 50 and 90% reduction in the production of a HCMV-late antigen ranged for several HCMV strains from 3.1 to 4.9 microM and from 14.2 to 17.3 microM, respectively. DFO concentration of 60 microM had no significant effect on the viability of HFF cells. Inhibitory effects of DFO on HCMV replication were completely prevented by co-incubation with stoichiometric amounts of Fe3+.

2-Acetylpyridine 5-[(dimethylamino)thiocarbonyl]-thiocarbonohydrazone (1110U81) potently inhibits human cytomegalovirus replication and potentiates the antiviral effects of ganciclovir

We studied the effects of 2-acetylpyridine 5-[(dimethylamino)thiocarbonyl]-thiocarbonohydrazone (1110U81 or A1110U), a potent inhibitor of the ribonucleotide reductases encoded by herpes simplex virus types 1 and 2 and by varicella-zoster virus, against human cytomegalovirus (HCMV) replication in infected MRC-5 cells. We show that 1110U81 is a potent inhibitor of HCMV DNA replication (50% inhibitory concentration [IC50], 3.6 microM; IC90, 5.6 microM) and also potentiates the effects of ganciclovir (GCV) against HCMV. The IC90 of GCV is reduced from 65 microM when GCV alone is given to 2.8 microM when GCV is combined with 1110U81 at a molar ratio of 1:1.

Herpes and human ribonucleotide reductases. Inhibition by 2-acetylpyridine 5-[(2-chloroanilino)-thiocarbonyl]-thiocarbonohydrazone (348U87)

The mode of inactivation of herpes simplex virus type 1 and human ribonucleotide reductases by 2-acetylpyridine 5-[(2-chloroanilino)-thiocarbonyl]-thiocarbonohydrazone++ + (348U87) was determined and compared to that described previously [Porter et al. Biochem Pharmacol 39: 639-646, 1990] for 2-acetylpyridine 5-[(dimethylamino)thiocarbonyl]-thiocarbonohydrazone (A1110U). 348U87 inactivated herpes ribonucleotide reductase faster than did A1110U. Moreover, iron-complexed 348U87 was a considerably more potent inactivator than iron-complexed A1110U. It appeared to efficiently form an initial complex with the viral enzyme prior to rapid enzyme inactivation. The combination of 348U87 and iron-complexed 348U87 inactivated with a rate constant that was slightly greater than the sum of their individual rate constants of inactivation. The corresponding combination of A1110U species inactivated with a rate constant that was much greater than the sum of the individual rate constants of inactivation. Herpes ribonucleotide reductase that had been inactivated by either species of 348U87 was reactivated by diluting the enzyme and inactivators into assay medium containing excess iron. 348U87 was also an effective inactivator of herpes simplex virus type 2 and varicella zoster virus ribonucleotide reductases. The iron-complexed forms of 348U87 and A1110U exhibited very different modes of inactivation of human ribonucleotide reductase. Iron-complexed 348U87 was a tight-binding inactivator, whereas iron-complexed A1110U was only a weak, non-inactivating, inhibitor. Furthermore, the inactivation by iron-complexed 348U87 was not stimulated by either 348U87 or A1110U, whereas the weak inhibition by iron-complexed A1110U was converted to rapid inactivation by A1110U. Excess iron prevented the inactivation by iron-complexed 348U87. Uncomplexed 348U87 was similar to uncomplexed A1110U in that it was not an inhibitor of the human enzyme.

The large subunit of herpes simplex virus type 1 ribonucleotide reductase: expression in Escherichia coli and purification

The open reading frame of the large subunit (R1) of herpes simplex virus type 1 (HSV-1) ribonucleotide reductase has been positioned downstream of the phage T7 gene 10 promoter in the expression vector, pET. Transformation of this recombinant plasmid into Escherichia coli BL21 DE3 cells containing the T7 RNA polymerase, under the control of the lac UV5 promoter, allows expression of the subunit on induction of the T7 RNA polymerase by isopropyl thiodigalactoside. The expressed protein is soluble and can be purified with yields up to 0.5 mg of R1 per litre of bacterial culture. The subunit can complement R2 produced in BHK cells or E. coli to give specific activities comparable to that produced in BHK cells infected with HSV-1. Enzyme activity reconstituted from E. coli-expressed R1 and R2 is inhibited by the nonapeptide YAGAVVNDL with an IC50 comparable to that obtained with enzyme extracted from BHK cells infected with HSV-1. Results suggest that the E. coli produced enzyme is a good source of protein for further structural and functional studies.

2-Acetylpyridine 5-[(dimethylamino)thiocarbonyl]-thiocarbonohydrazone (A1110U), a potent inactivator of ribonucleotide reductases of herpes simplex and varicella-zoster viruses and a potentiator of acyclovir

2-Acetylpyridine 5-[(dimethylamino)thiocarbonyl]thiocarbonohydrazone (A1110U) was found to be a potent inactivator of the ribonucleotide reductases (EC 1.17.4.1) encoded by herpes simplex virus types 1 and 2 and by varicella-zoster virus and to be a weaker inactivator of human ribonucleotide reductase. It also markedly potentiated the antiherpetic activity of acyclovir against these viruses in tissue culture. A1110U both decreased the dGTP pool that builds up when infected cells are treated with acyclovir and induced a large increase in the pool of acyclovir triphosphate. The resultant 100-fold increase in the ratio of the concentrations of acyclovir triphosphate to dGTP should facilitate the binding of the fraudulent nucleotide to its target enzyme, herpes virus-encoded DNA polymerase, and could account for the synergy between A1110U and acyclovir. A similar change in the acyclovir triphosphate-to-dGTP ratio was previously reported to be induced by another ribonucleotide reductase inhibitor, 2-acetylpyridine 4-(2-morpholinoethyl)thiosemicarbazone (A723U). However, A1110U is considerably more potent and may have better clinical potential. Synergistic toxic interactions between A1110U and acyclovir were not detected in uninfected cells.

Ribonucleotide reductase induced by varicella zoster virus. Characterization, and potentiation of acyclovir by its inhibition

An enzyme that catalyzes the conversion of CDP to 2'-dCDP in the presence of dithiothreitol (DTT) was detected in ammonium sulfate fractionated-extracts of varicella zoster virus (VZV)-infected cells. This ribonucleotide reductase was antigenically distinguishable from the isofunctional eucaryotic enzyme as well as the ribonucleotide reductases induced by herpes simplex virus types 1 and 2 (HSV-1 and HSV-2). The VZV-induced enzyme was purified to the extent that most of the contaminating enzymes, which would significantly deplete the substrate, were removed. The VZV-induced ribonucleotide reductase exhibited maximum activity in the absence of ATP and/or magnesium and was only weakly inhibited by 2'-deoxynucleoside triphosphates. Furthermore, ADP, UDP and GDP competitively inhibited CDP reduction with Ki (Km) values of 15, 20, 1.8 and 0.88 microM, respectively. These kinetic properties were very similar to those of the correspondingly purified ribonucleotide reductases induced by HSV-1 [Averett et al., J. biol. Chem. 258, 9831 (1983)] and HSV-2 [Averett et al., J. Virol. 52, 981 (1984)] and were dissimilar to the allosterically regulated mammalian enzyme. A723U, an inactivator of HSV-1 ribonucleotide reductase that potentiates the anti-HSV-1 activity of acyclovir [Spector et al., Proc. natn. Acad. Sci. U.S.A. 82, 4254 (1985)], also appeared to inactivate this VZV-induced ribonucleotide reductase and to potentiate the anti-VZV activity of acyclovir.

Structural features of ribonucleotide reductase

Herpes simplex virus type 1 (HSV-1) encodes a ribonucleotide reductase which comprises two polypeptides with sizes of 136,000 (RR1) and 38,000 mol. wt. (RR2). We have determined the entire DNA sequence specifying HSV-1 RR1 and have identified two adjacent open reading frames in varicella-zoster virus (VZV) which have homology to HSV RR1 and RR2; the predicted sizes for the VZV RR1 and RR2 polypeptides are 87,000 and 35,000 mol. wt. respectively. Amino acid comparisons with RR1 and RR2 polypeptides from other organisms indicate that HSV-1 RR1 contains a unique N-terminal domain which is absent from other RR1 polypeptides apart from HSV-2 RR1. These N-terminal amino acid sequences are poorly conserved between HSV-1 and HSV-2 in contrast to the remainder of the protein which shows greater than 90% homology. Polypeptide structural predictions suggest that the HSV-1 N-terminal domain may be separated into two regions, namely, a beta-sheet structure followed by a nonstructured area. Across the remainder of RR1 and RR2, comparisons also reveal blocks of amino acids conserved between the different ribonucleotide reductases, and these may be important for enzyme activity. From predictions on the structure of these conserved blocks, we have proposed that the location of a substrate binding site within RR1 is centered on three conserved glycine residues in a region which is predicted to adopt a beta-sheet/turn/alpha-helical structure; this approximates to the structure for ADP nucleotide binding folds. Finally, we propose that the promoters for the HSV and Epstein-Barr virus (EBV) RR2 transcripts have evolved by separate evolutionary routes.