Is protein kinase a subunit of RNA polymerase II, which is responsible for the specificity of transcription?

Rat liver nuclear protein kinases NI and NII. Purification, subunit composition, substrate specificity, possible levels of regulation

Rat liver nuclear protein kinases NI and NII have been purified to homogeneity by an improved method. This method includes a casein-phosvitin-Sepharose column step, which separates the enzymes from the other chromosomal non-histone proteins, and a gel filtration at high ionic strength in the presence of a high concentration of protease inhibitors to separate the two enzymes from each other. NI has an apparent molecular mass of approximately 50 kDa and is composed of a single subunit. NII has an apparent molecular mass of 133 kDa and is composed of two subunits of identical molecular mass. The V and the Km of the two enzymes were determined for several substrates. Both enzymes phosphorylate chromosomal non-histone proteins with partly different specificities as shown by two-dimensional electrophoreses. When incubated in the absence of protease inhibitors, the enzymes were degraded into discrete polypeptides. Autophosphorylation of a polypeptide derived from NII was observed after incubation of the enzyme with ATP. This phosphorylation stimulated the enzyme activity. Several chromosomal proteins coeluted with NII from the casein-phosvitin-Sepharose column. They remained associated with the enzyme in sucrose gradients, during gel filtration performed at physiological ionic strength, and are dissociated at high ionic strength. These proteins were highly phosphorylated when the protein-NII complex was incubated with ATP.

Eukaryotic RNA polymerases

This review will attempt to cover the present information on the multiple forms of eukaryotic DNA-dependent RNA polymerases, both at the structural and functional level. Nuclear RNA polymerases constitute a group of three large multimeric enzymes, each with a different and complex subunit structure and distinct specificity. The review will include a detailed description of their molecular structure. The current approaches to elucidate subunit function via chemical modification, phosphorylation, enzyme reconstitution, immunological studies, and mutant analysis will be described. In vitro reconstituted systems are available for the accurate transcription of cloned genes coding for rRNA, tRNA, 5 SRNA, and mRNA. These systems will be described with special attention to the cellular factors required for specific transcription. A section on future prospects will address questions concerning the significance of the complex subunit structure of the nuclear enzymes; the organization and regulation of the gene coding for RNA polymerase subunits; the obtention of mutants affected at the level of factors, or RNA polymerases; the mechanism of template recognition by factors and RNA polymerase.

DNA-dependent RNA polymerase II from nuclei of suspension-cultured tobacco cells

From isolated nuclei of suspension cultured cells of Nicotiana tabacum. DNA-dependent RNA polymerase II (E.C. 2.7.76) has been purified to homogeneity as evidenced by polyacrylamidegel electrophoresis under non-denaturing conditions. The purified enzyme had a specific activity of more than 15 nmol min(-1)·mg(-1) with denatured calf thymus DNA as template. Sodium-dodecyl-sulfate gel electrophoresis and protein highperformance liquid chromatography revealed a subunit composition of four proteins with molecular weights of 165 000, 135 000, 35 000 and 25 000 and with a stoichiometry of 1:1:2:2. The RNA polymerase did not exhibit any detectable proteinkinase activity. The 25 000 subunit binds ADP in a molar ratio of 1:1; it could not be decided whether this subunit has an ATPase activity or is merely an acceptor of ADP.