Ribonucleotide reductase from Escherichia coli: demonstration of a highly active form of the enzyme

Ribonucleotide reductase from Escherichia coli consists of two nonidentical subunits, proteins B1 and B2. The activity of the enzyme in crude extracts prepared from mechanically disrupted bacteria is very low. Enzyme activity is stimulated 5 to 10-fold by addition of an excess of either subunit. Concentrated extracts from cells lysed gently on Cellophane discs (Schaller et al.) contained 10 to 20-fold higher activity than extracts from mechanically disrupted cells. This activity was not further stimulated by either B1 or B2. The system is suitable for complementation tests for the analysis of temperature-sensitive mutants affecting the ribonucleotide reductase system. Concentrated high-speed supernatants from E. coli treated with lysozyme (Wickner et al.) also contained a high ribonucleotide reductase activity, which was stimulated slightly or not at all by addition of B1 and B2. This active form of the enzyme was unstable and could not be purified. The results suggest that the intracellular form of the enzyme consists of a tight complex of proteins B1 and B2, possibly stabilized by other intracellular structures.

[Ribonucleotide reductase in Propionibacterium shermani]

The cell-free extract of Propionibacterium shermanii was found to contain B12-dependent ribonucleotide reductase. The extract of the cells grown under the conditions of the inhibited synthesis of vitamin B12 reduces ribonucleotides with the participation of B12-independent enzyme. The synthesis of B12-dependent apoenzyme of ribonucleotide reductase is partially maintained under these conditions. Both enzymes reduce preferably ribonucleoside diphosphates. The reducing agent of nucleotides in vitro is lipoic acid or dithiothreitol, in the B12-dependent pathway, and NADPH and thioredoxin, in the B12-independent pathway. Only B12-independent ribonucleotide reductase requires Mg2+ ions. Vitamin B12 in the coenzyme form inhibits the activity of B12-independent enzyme.

Ribonucleotide reductase activity during amphibian development

Uniformly labeled [3H] uridine is incorporated into DNA by dissociated Pleurodeles blastulae; the label is found in cytosine and to a much lesser extent in thymine. Ribonucleotide reductase activity cannot be detected in full grown oocytes of Xenopus and Pleurodeles, but is present in unfertilized egg. The enzyme is synthesized (or activated) when maturation is induced in Xenopus oocytes by in vitro hormonal treatment. The enzymatic activity increases after fertilization and reaches a peak at the 2--4 cell stage; it decreases at the blastula, gastrula and neurula stages to the low level initially present in unfertilized eggs. The enzyme is no longer detectable in swimming tadpoles. Addition of hydroxyurea (1 mg/ml) to fertilized eggs leads to complete loss of ribonucleotide reductase activity: cycloheximide (20 mug/ml) inhibits the rise in activity characteristic of early cleavage, while actinomycin D (20 mug/ml) has no effect. The significance of these results in discussed.

Inhibition of herpes simplex virus type 2 replication by thymidine

Replication of herpes simplex virus type 2 (HSV-2) was impeded in KB cells which were blocked in their capacity to synthesize DNA by 2 mM thymidine (TdR). The degree of inhibition was dependent upon the concentration of TdR. In marked contrast, HSV-1 is able to replicate under these conditions. The failure of HSV-2 to replicate is probably due to the inhibition of viral DNA synthesis; there was a marked reduction in the rate of DNA synthesis as well as the total amount of HSV-2 DNA made in the presence of 2 mM TdR. We postulated that the effect of TdR on viral replication occurs at the level of ribonucleotide reductase in a manner similar to KB cells. However, unlike KB cells, an altered ribonucleotide reductase activity, highly resistant to thymidine triphosphate inhibition, was found in extracts of HSV-2-infected KB cells. This activity was present in HSV-2-infected cells incubated in the presence or absence of TdR. Ribonucleotide reductase activity in extracts of HSV-1-infected KB cells showed a similar resistance to thymidine triphosphate inhibition. These results suggest that the effect of TdR on HSV-2 replication occurs at a stage of DNA synthesis other than reduction of cytidine nucleotides to deoxycytidine nucleotides.

In situ activity of enzymes on polyacrylamide gels of a deoxyribonucleic acid-membrane fraction extracted from pneumococci

A deoxyribonucleic acid (DNA)-membrane fraction extracted from Diplococcus pneumoniae was subjected to polyacrylamide gel electrophoresis after treatment with 0.16% sodium dodecyl sulfate. At least two DNA polymerase activities were detected by in situ assays with appropriate substrates, templates, and inhibitors, including a co-polymer of deoxyadenylic and thymidylic acid and N-ethylmaleimide. This activity coincided with a fraction in the gel containing 7.5, 9.4, and 24%, respectively of the DNA, phospholipid, and protein present in the DNA-membrane fraction before electrophoresis and sodium dodecyl sulfate treatment. Assays with minced gels showed that several nuclease activities, deoxyribonucleotide kinase activity, and DNA ligase activity also coincided with this fraction. However, ribonucleoside diphosphate reductase activity did not. These results demonstrate that a complex of enzymes involved in DNA replication is firmly bound to the DNA-membrane fraction in pneumococci.