Diguanosine nucleotides of fungi that regulate RNA polymerases isolated and partially characterised

Synthesis of Gp4N and Gp3N compounds by guanylyltransferase purified from yeast

Guanylyltransferase that catalyzes mRNA capping by the reaction, ppNpN + GTP----GpppNpN was purified from S. cerevisiae. The enzyme forms a nucleotidyl intermediate by phosphoamide linkage of GMP. Two guanylylated polypeptides of MR approximately 52,000 and 46,000 were obtained, the latter apparently by proteolysis of the larger component. Both forms transferred the covalently bound GMP to ppApG, yielding GpppApG. Dinucleoside tri- and tetraphosphates of the type Gp3N and Gp4N were also produced by using ribonucleoside 5'-di and triphosphates as acceptors. The purified yeast guanylyltransferase contained little or no RNA 5'-triphosphatase or methyltransferase.

Priming of reovirus transcription by GppppG and formation of CpG(5')GpC

Compounds of general structure N(5')pn(5')N were used by the reovirus-associated RNA polymerase as primers for template-directed synthesis of virus-specific oligonucleotides and RNA. Structural requirements for activity included a guanosine residue and at least four phosphates--i.e., G(5')pppp(5')N. Gp4G incubated with viral cores in the presence of CTP yielded Gp4GpC and CpGp4GpC. In a complete transcription mixture, Gp4G was also incorporated into the 5' termini of full-length transcripts in the unmethylated forms Gp4GpC and CpGp4GpC, in contrast to viral mRNAs that contain 5'-terminal m7GpppGmpC formed de novo. Gp5G, Gp6G, and Gp4A but not Gp2G, Gp3G, and Ap4A also primed reovirus transcription and inhibited RNA methylation.

Inhibition of mammalian ribonucleotide reductase by a dinucleotide produced in eucaryotic cells

HS3, a highly phosphorylated dinucleoside originally purified from the fungus Achlya, has been isolated from Chinese hamster ovary cells undergoing glutamine starvation. The HS3 compounds obtained from the fungal and mammalian sources exhibited similar physical and chemical properties. This unusual dinucleotide may be an important regulator of eucaryotic ribonucleoside diphosphate reductase activity; for 50 micrometer HS3, isolated from either mammalian or fungal cells, significantly inhibited CDP reduction in Achlya or hamster cell preparations, but only marginally affected the activity of the enzyme from E. coli. Studies with HS3 isolated from Achlya and partially purified mammalian ribonucleotide reductase indicated that the compound noncompetitively inhibited the reduction of varying concentrations of the substrates CDP, ADP and GDP with Ki values of 23 micrometer, 14 micron and 16 micron respectively. These inhibitor concentrations are well below the estimated intracellular levels of HS3 in glutamine starved cells and suggest that HS3 inhibition of ribonucleotide reduction may be responsible for the rapid inhibition of DNA synthesis seen under these culture conditions.

Dinucleosidasetetraphosphatase in rat liver and Artemia salina

A comparative study of an enzymatic activity present in Artemia salina and rat liver which specifically splits dinucleoside tetraphosphates is presented. All the purine and pyrimidine dinucleoside tetraphosphates tested, i.e. diadenosine, diguanosine, dixanthosine and diuridine tetraphosphates, were substrates of both enzymes with similar maximum velocities and Km values, (around 10 muM). The inhibition by nucleotides of the enzyme from the two sources is also similar. Particularly relevant is the strong inhibition caused by nucleoside tetraphosphates which have Ki values in the nanomolar range. The Artemia enzyme has a slightly lower molecular weight (17 500) than the liver enzyme (21 000) and is more resistant to acidic pH. Based on previous findings, the enzyme from Artemia salina was named diguanosinetetraphosphatase (EC 3.6.1.17) by the Enzyme Commission. The results presented in this paper show that the liver and Artemia enzymes are similar, and we propose to name this enzyme as dinucleosidetetraphosphatase or dinucleoside-tetraphosphate nucleotidehydrolase.

A novel nucleotide implicated in the response of E. coli to energy source downshift

When E. coli cells are subjected to energy source downshift, the accumulation of RNA (and overall cell growth) is drastically restricted within 1 to 2 min. However, the identity of the primary metabolic signal for this adjustment is a mystery. Earlier studies, and further evidence presented here, show that there is no satisfactory correlation between the sudden adjustment of RNA accumulation and the kinetics of changes in the levels of prospective signalling compounds, such as glycolytic intermediates, ppGpp, ATP, or the three adenylate nucleotides. We have discovered an unusual nucleotide, which we call the phantom spot, whose level decreases dramatically within a minute of downshift, correlating well with the adjustment of RNA accumulation. Preliminary characterization of the phantom spot indicates that it is a triphosphate derived from the guanylate pathway, and suggests that it is a form of GTP with a modification of the imidazole portion of the purine ring. We postulate that this nucleotide serves as a regulatory facsimile of ATP, linking the rate of RNA accumulation and other anabolic processes to the overall rate of phosphorylation.

Occurence of nucleoside polyphosphates in rapidly labelled RNA preparations from radish seedlings (Raphanus sativus.)

Preparations of rapidly labelled RNA are usually heavily contaminated by rapidly labelled low molecular weight components. This contaminating fraction mainly contains nucleoside triphosphates, as previously reported, but also nucleoside tetra, penta and hexaphosphates. These highly phosphorylated nucleosides are, according to recent literature, likely candidates for an important regulatory role in the coordination of a variety of biochemical reactions. This is the first demonstration of their occurence in higher plants and a method is presented to separate them from other usual nucleotides.