In enzymology, a [RNA-polymerase]-subunit kinase () is an enzyme that catalyzes the chemical reaction

ATP + [DNA-directed RNA polymerase] \rightleftharpoons ADP + phospho-[DNA-directed RNA polymerase]

Thus, the two substrates of this enzyme are ATP and DNA-directed RNA polymerase, whereas its two products are ADP and phospho-[DNA-directed RNA polymerase].

This enzyme belongs to the family of transferases, specifically those transferring a phosphate group to the sidechain oxygen atom of serine or threonine residues in proteins (protein-serine/threonine kinases). The systematic name of this enzyme class is ATP:[DNA-directed RNA polymerase] phosphotransferase. Other names in common use include CTD kinase, and STK9.

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