RNA polymerase I in hepatoma 3924A: Mechanism of enhanced activity relative to liver

Endotoxicosis induced by Coxiella burnetii lipopolysaccharide stimulates a ribosomal protein S6 kinase: some properties of the partially purified enzyme

Guinea pig endotoxicosis induced by lipopolysaccharide from Coxiella burnetii Nine Mile phase I stimulates phosphorylation of liver ribosomal protein S6, with a 50% increase at 12 h postinoculation. The responsible protein kinase (S6PK) has been partially purified from liver; its activity is independent of cyclic AMP and of Ca2+ plus phosphatidyl serine or diacylglycerol. The preparation has an apparent optimum concentration of 20 mM Mg2+, while Ca2+ and Mn2+ are each inhibitory at 2 mM. The apparent Km for ATP is 30 microM with intact ribosomes. Because of the central role of phosphorylation in metabolic regulation and a purported role of phosphorylated S6 in protein synthesis, the lipopolysaccharide-induced stimulation of S6PK suggests a significant regulatory role of such enzymes in the pathobiochemistry of Q fever infection and endotoxicosis.

Association of DNA topoisomerase I and RNA polymerase I: a possible role for topoisomerase I in ribosomal gene transcription

RNA polymerase I preparations purified from a rat hepatoma contained DNA topoisomerase activity. The DNA topoisomerase associated with the polymerase had an Mr of 110,000, required Mg2+ but not ATP, and was recognized by anti-topoisomerase I antibodies. When added to RNA polymerase I preparations containing topoisomerase activity, anti-topoisomerase I antibodies were able to inhibit the DNA relaxing activity of the preparation as well as RNA synthesis in vitro. RNA polymerase II prepared by analogous procedures did not contain topoisomerase activity and was not recognized by the antibodies. The topoisomerase I: polymerase I complex was reversibly dissociated by column chromatography on Sephacryl S200 in the presence of 0.25 M (NH4)2SO4. Topoisomerase I was immunolocalized in the transcriptionally active ribosomal gene complex containing RNA polymerase I in situ. These data indicate that topoisomerase I and RNA polymerase I are tightly complexed both in vivo and in vitro, and suggest a role for DNA topoisomerase I in the transcription of ribosomal genes.

Could the loss of regulation of genetic expression in cancer cells be used to cause their necrosis?

In cancer cells the control over the genetic message involved in the induction of mitosis is irreversibly lost. This fact is indicated by certain phenomena displayed by cancer cells under restricted nutritional conditions. Cells transformed by DNA viruses (which stabilize "p53") keep on cycling and die. In starving cells at the inside of tumors the synthesis of pre-rRNA still proceeds while all other anabolic processes are already at a standstill. The reason is that glutamine, glycine and aspartate are channelled into the enzymatic pathways for the synthesis of nucleosides: thus, protein synthesis is denied those aminoacids. Such situations might be imitated through the administration of excess nucleosides and (within limits) the simultaneous restriction of some selected aminoacids. DNA replication depends on the stabilization of p53, but an accumulation of pre-rRNA might occur, which ultimately might be harmful for cancer cells. Several ways to improve this rationale might be tested on cultured cells and on research animals. They include the destruction of methionine with bacterial enzymes, or the addition of ornithine, a precursor of putrescine, which is an important factor of DNA and pre-rRNA synthesis.

Phosphorylation of RNA polymerases: specific association of protein kinase NII with RNA polymerase I

The activities of the three DNA-dependent RNA polymerases from a rapidly growing rat tumour, Morris hepatoma 3924A, and from rat liver were examined. The activity of RNA polymerase I was higher in the tumour than in the liver. The enhanced capacity for RNA synthesis was a result of a higher concentration of polymerase I in the tumour as well as of an activation of this enzyme in vivo. The possibility that the high specific activity of the hepatoma polymerase I resulted from phosphorylation was investigated. Two major cyclic-AMP-independent nuclear casein kinases (NI and NII) were identified; the activity of protein kinase NII in the tumour was ten times that in liver. Protein kinase NII was capable of activating and phosphorylating RNA polymerase I in vitro. This kinase could also stimulate RNA polymerase II activity, although to a lesser extent than RNA polymerase I. RNA polymerase III was not affected by protein kinase NII. Protein kinase NII was tightly associated with polymerase I and was found even in purified preparations of the polymerase. Antibodies against both RNA polymerase I and protein kinase NII were present in sera of patients with certain rheumatic autoimmune diseases. These results imply that RNA polymerase I and protein kinase NII are in close association in vivo as well as in vitro and that polymerase phosphorylation may regulate the rate of ribosomal RNA synthesis in the cell.