Characterization of ribonucleotide reductase activity from mouse L cells

We describe some fundamental properties of the cytidine 5'-diphosphate (CDP) and guanosine 5'-diphosphate (GDP) reductase activity from mouse L cells. Both activities increased in a nonlinear fashion at low protein concentrations; this may be due to dissociation of two protein subunits of the enzyme at very low concentrations. CDP reductase activity was greatly stimulated in the presence of ATP and required magnesium and iron for maximum activity. GDP reductase required 2'-deoxythymidine 5'-triphosphate for maximum activity. Also apparent Km values of 0.14 mmol/l for CDP and 0.05 mmol/l for GDP were determined from double reciprocal plots of velocity against substrate concentrations. Activity in extracts of logarithmically growing mouse L cells was very high indicating that attempts to purify the enzyme from this source should be rewarding.

Differential effect of hydroxyurea on the replication of plasmid and chromosomal DNA in Bacillus subtilis

The replication in Bacillus subtilis of the staphylococcal R plasmids pE194, pBD15, pUB110, pSA0501, and pSA2100 has been studied in the presence of hydroxyurea. In all cases, an enrichment for covalently closed circular DNA compared with chromosomal DNA was observed. In this respect, hydroxyurea mimics the effect previously observed with pUB110, using strains carrying the conditional mutation dnaA13. This mutation has been reported to affect ribonucleotide reductase (G. W. Bazill and D. Karamata, Mol. Gen. Genet. 117:19-29, 1972). An explanation for these effects is offered, together with some supporting evidence.

Deoxyribonucleoside-requiring mutants of Bacillus subtilis

A number of deoxyribonucleoside-requiring mutants (dns) of Bacillus subtilis were isolated and their growth characteristics and ribonucleotide reductase activities were compared with those of the wild type and of a dna mutant (tsA13). Both tsA13 and dns mutants required the presence of a mixture of deoxyribonucleosides for growth at 45 degrees C but not at 25 degrees C. All the mutant strains tested contained ribonucleotide reductase activity which showed heat sensitivity similar to that of the enzyme from a wild-type strain. The reductase in B. subtilis seemed to reduce ribonucleoside triphosphates in a similar manner to the enzyme in Lactobacillus leichmannii.

Multiple functions of thioredoxins

Reduced thioredoxins from microbial and plant cells, both of cytoplasmic or chloroplast origin, are interchangeable in stimulating such diverse enzyme activities as ribonucleoside diphosphate reductase (E. coli), PAPS sulfotransferase (Synechococcus), and fructose-1,6-bis-phosphatase (from spinach) in vitro. It is suggested that reduced thioredoxins are unspecific, multifunctional cellular proteins while in contrast the oxidized froms require specific enzymes for their reduction.

Reconstitution of the ribonucleotide reductase enzyme from Ehrlich tumor cells

Ribonucleotide reductase from Ehrlich tumor cells was separated by chromatography on blue dextran/Sepharose into two protein fractions (Tris and Dye fractions). Neither fraction alone had reductase activity, but when combined, constituted an active enzyme system. Heat treatment of either fraction resulted in an inactive combination. The approximate molecular size of the active component of the Tris and Dye fractions was determined to be 5.7 S and 6.5 S, respectively, compared to 9 S for the intact enzyme. The Tris fraction was inactivated by hydroxylamine while the dye fraction was inactivated by pyridoxal phosphate/BH4-treatment. The inactivation of the Dye fraction was prevented by ATP. These data would indicate that the Tris and Dye fractions were comparable in function to the B2 and B1 proteins, respectively, of the Escherichia coli ribonucleotide reductase.

Differential effect of hydroxyurea on a ribonucleotide reductase system

Infection of Escherichia coli with phage T4 induces a large increase in ribonucleotide reductase activity. We show that hydroxyurea inhibits T4-induced CDP, ADP, UDP, and GDP reductase activities in vitro. Moreover, there are significant differences in the degree of inhibition of each ribonucleotide reductase activity. The reductase activities for CDP and ADP are more sensitive to hydroxyurea than those for UDP and GDP, particularly at high hydroxyurea molarities. As little as 5 x 10(-4)M hydroxyurea lowers CDP and ADP reductase activities to 25 to 30% whereas as much as 0.5 M hydroxyurea is needed to lower UDP and GDP reductase activities to 50%.

Assay of ribonucleotide reduction in nucleotide-permeable hamster cells

Ribonucleotide reduction was measured in Chinese hamster ovary cells made permeable to nucleotides by treatment with the detergent Tween-80. When compared to the respective ribonucleotide reductase activity in partially purified cell extracts, CDP and GDP reductase activities in permeabilized cells responded in a similar fashion to dithiothreitol, pH, MgCl2, FeCl3, substrate concentration and the presence of positive or negative allosteric effectors. At low protein concentrations both CDP and GDP reduction with whole cells increased linearly with cell number and was greater than the activity in corresponding cell extracts. Permeabilized cells were used to measure the level of CDP and GDP reductase in a hamster cell line resistant to the cytotoxic effects of hydroxyurea. The hydroxyurea-resistant cell line contained four to ten times more CDP and GDP reductase activity compared to parental or revertant cell lines. The permeabilized cell assay was also used to measure CDP and GDP reductase activities in Chinese hamster ovary cells synchronized by isoleucine starvation. CDP reductase activity was low in G1 arrested cells but increased 10-fold by 16 hours after the readdition of isoleucine to the growth medium. GDP reductase, which is present at much higher levels, is similarly induced after isoleucine addition, but only by 2-fold. The maximum activity of both CDP and GDP reductase occurred from 14 to 16 hours after isoleucine addition, which corresponded to the period of maximum DNA synthesis.

From deoxynucleotides to DNA synthesis

The deoxyribonucleotides required for DNA synthesis are provided by reduction of ribonucleotides. Our studies aim at an understanding of the interplay between the processes involved in the production of deoxyribonucleotides and in their consumption. The enzyme system reducing ribonucleotides has been characterized in some detail, and its regulatory aspects were investigated with pure enzymes. Knowledge gained in this way made it possible to interfere specifically with the activity of ribonucleotide reductase in intact cells and to study effects on DNA synthesis. In this connection, the replication of polyoma DNA was used as a model that allowed dissection of DNA synthesis into different steps involved in the initiation and elongation processes.

Ribonucleotide reduction and the possible role of cobalamin in evolution

The biological pathways of ribonucleotide reduction are briefly reviewed. The hypothesis is presented that reduction of ribonucleoside triphosphates to their deoxynucleotide analogs through the mediation of vitamin B12 or a similar corrinoid preceded and was necessary for the subsequent development of a DNA-type genome. There are two known biological systems for ribonucleotide reduction: (1) The ribonucleoside diphosphate reduction system which utilizes a nonheme iron ribonucleotide reductase enzyme, thioredoxin and its reductase, and NADPH. This enzyme complex is found in most bacteria, some higher organisms, and in all animals. (2) The ribonucleoside triphosphate reduction system which utilizes adenosyl cobalamin, ribonucleotide reductase and either thioredoxin or a disulfhydryl compound. The cobalamin-dependent reductase is restricted to a few species of bacteria and blue-gree algae. This system is considered more primitive than the iron reductase one based on their differences in distribution, components, and products.

[Correlation of mitochondrial ribonucleotide reductase and thymidine kinase activities with the synthesis of mitochondrial DNA in rat liver during regeneration]

3H-thymidine incorporation into DNA of heavy mitochondria from regenerating rat liver and the change of mitochondrial thymidine kinase and ribonucleotide reductase activities are studied in vivo in regenerating rat liver within 6--48 hours after hepatectomy. Synthesis of mitochondrial DNA and changes in the activity of the enzymes studied are found to be undulate. Thymidine kinase activity maxima coincide with those of 3H-thymidine incorporation. Maximal activity of ribonucleotide reductase pre-exists maxima of mitochondrial DNA synthesis.

Ribonucleotide reductase from Escherichia coli. Identification of allosteric effector sites by chromatography on immobilized effectors

Ribonucleotide reductase is responsible for the production of deoxyribonucleotides by catalyzing the reduction of ribonucleoside diphosphates. The enzyme is allosterically regulated in a complex way by the nucleoside triphosphates, ATP, dTTP, dGTP, dCTP, and dATP. Ribonucleotide reductase consists of two nonidentical subunits, proteins B1 and B2. Both substrates and allosteric effectors bind exclusively to B1. Binding of protein B1 to dTTP or dATP covalently coupled to Sepharose and elution with concentration gradients of the different nucleoside triphosphate effectors gave information about (1) the arrangement of the effector binding sites on protein B1 and (2) the affinity of the effectors for these sites. Protein B1 thus has two classes of effector binding sites. One class binds all effectors, as demonstrated by elution of the protein from dTTP-Sepharose with dATP, dGTP, ATP, or dCTP. The second class binds only dATP or ATP, since dATP and ATP were the only nucleotides which eluted protein B1 from dATP-Sepharose. These results confirm earlier data obtained by dialysis binding experiments. The eluting concentrations obtained for the different nucleoside triphosphates in experiments with dTTP-Sepharose could be used to calculate unknown dissociation constants for protein B1 -effector binary complexes. This was possible, since a plot of the eluting concentrations vs. known dissociation constants was linear.

Porphyrin and corrinoid mutants of Bacillus subtilis

Porphyrin auxotrophs of Bacillus subtilis can be divided into two groups. Strains belonging to the first group (hemA, hemB, or hemC) are not able to synthesize or metabolize porphobilinogen. These strains require cysteine, cystine, and methionine, respectively. Traces of aminolevulinic acid, in a hemin-containing medium, can replace the cysteine requirement in a mutant lacking aminolevulinic acid synthetase. In bacteria belonging to the second group (hemE, hemF, or hemG), porphyrin biosynthesis is blocked at later steps, and the amino acids mentioned above are not required. It is of interest that both the activity of ribonucleotide reductase and the amount of vitamin B12 were significantly lower in the first group. The addition of vitamin B12 to the medium did not promote the growth of strains examined. We assume that porphobilinogen deaminase is essential for the synthesis of corrinoids.

Ribonucleotide reductase from herpes simplex virus (types 1 and 2) infected and uninfected KB cells: properties of the partially purified enzymes

Mammalian ribonucleotide reductase is a complex enzyme modified in its activity by a complex regulatory system involving adenosine triphosphate (ATP) and deoxyribonucleoside triphosphates. Infection of KB cells with herpes simplex virus (HSV) type 1 or 2 induces the formation of an altered ribonucleotide reductase. The properties of partially purified reductase from uninfected KB cells have been compared with the enzymes obtained from HSV-1 and HSV-2 infected KB cells. We found that the virus-induced enzymes are similar to the KB enzyme in some properties but differed significantly from the host enzyme in three respects: (1) virus induced reductase was not inhibited significantly by deoxythymidine triphosphate regardless of ATP concentration, (2) magnesium was not required for virus enzyme activity although 2 mM-Mg2+ did stimulate the reaction, and (3) magnesium concentration required for optimal activity was different for virus and host enzymes. These changes are evidence that the enzyme molecules present after infection by HSV-1 or HSV-2 differ from those present before infection.