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Spermine-mediated phosphorylation of RNA polymerase I and its effect on transcription
Abstract:
A nuclear protein kinase, designated NII, was purified essentially to homogeneity from the Morris hepatoma 3924A. In the presence of excess Mg2+, phosphorylation of casein by the kinase was stimulated by spermine (1-5 mM) and was inhibited completely by 0.1 microgram/ml heparin. The apparent Km for casein was reduced in the presence of spermine. Spermine preferentially augmented phosphorylation of threonine residues. The kinase was also associated with highly purified RNA polymerase I and appears to correspond to two polypeptides (Mr 42,000 and 24,600) of the polymerase. RNA polymerase I polypeptides of Mr 120,000 (S2), Mr 65,000 (S3) and Mr 24,600 (S5) were phosphorylated by the endogenous kinase. Spermine enhanced phosphorylation of the RNA polymerase I subunits as much as 20-fold. Phosphorylation activated RNA polymerase I; the phosphorylated enzyme synthesized longer product with no apparent effect on the number of RNA chains initiated.
Insights
A novel nuclear protein kinase (NII) from Morris hepatoma 3924A enhances RNA polymerase I activity. This kinase phosphorylates specific subunits, increasing RNA product length and activity, with spermine acting as a potent stimulator.
Area of Science:
- Biochemistry
- Molecular Biology
- Cancer Research
Background:
- Nuclear protein kinases play crucial roles in cellular regulation.
- Morris hepatoma 3924A is a well-characterized experimental tumor model.
- RNA polymerase I is responsible for ribosomal RNA synthesis.
Purpose of the Study:
- To purify and characterize a novel nuclear protein kinase from Morris hepatoma 3924A.
- To investigate the kinase's interaction with and regulation of RNA polymerase I.
- To elucidate the role of spermine in modulating kinase activity and substrate phosphorylation.
Main Methods:
- Purification of nuclear protein kinase NII to homogeneity.
- Enzymatic assays to determine kinase activity, substrate specificity, and kinetic parameters.
- Association studies with purified RNA polymerase I.
- Phosphorylation analysis of RNA polymerase I subunits using SDS-PAGE and autoradiography.
Main Results:
- Nuclear protein kinase NII was purified from Morris hepatoma 3924A.
- Spermine (1-5 mM) stimulated NII activity, while heparin inhibited it.
- Spermine preferentially enhanced threonine residue phosphorylation and reduced the apparent Km for casein.
- NII was associated with RNA polymerase I, phosphorylating its S2, S3, and S5 subunits.
- Spermine significantly increased RNA polymerase I subunit phosphorylation (up to 20-fold).
- Phosphorylation activated RNA polymerase I, leading to longer RNA products without altering initiation rates.
Conclusions:
- Nuclear protein kinase NII is a key regulator of RNA polymerase I activity.
- Spermine acts as a potent activator of NII, enhancing RNA polymerase I function.
- Phosphorylation by NII modulates RNA synthesis, potentially impacting cellular growth and metabolism in hepatoma cells.