Gamma subunit of the CTDK-I complex, which hyperphosphorylates the C-terminal heptapeptide repeat domain (CTD) of the largest RNA polymerase II subunit. CTDK-I phosphorylates 'Ser-5' if the CTD substrate is not phosphorylated at 'Ser-5', but will phosphorylate 'Ser-2' of a CTD substrate if 'Ser-5' is already phosphorylated. CTDK-I is also more reactive toward substrates that are prephosphorylated at 'Ser-2' or 'Ser-5' compared with an unphosphorylated CTD substrate, therefore efficiently creating doubly phosphorylated CTD repeats. Involved in RNA polymerase I transcription and RNA polymerase II transcriptional elongation, and as part of the CTDK-I complex, pre-mRNA 3'-end processing and SET2 mediated H3K36 methylation. Together with CTK2, required for CTK1 CTD kinase activation. Required for DNA damage induced transcription. Involved in the adaptation to alternative carbon sources, including galactose, glycerol and ethanol, but not raffinose. Required for the integrity of the rDNA locus. CTDK-I consists of three subunits, CTK1, CTK2 and CTK3 (also called alpha, beta and gamma). Interacts with CTK1. Heterodimerization with CTK2 is required to protect this subunit from degradation. Ubiquitinated. Ubiquitination leads to degradation by the 26S proteasome pathway. Null mutants are viable, but grow more slowly than wild-type cells at 30 degrees Celsius. They are cold-sensitive, failing to grow at 12 degrees Celsius. They display flocculent growth in liquid media and they show abnormal cell morphologies, for example, a significant fraction of the cells are greatly enlarged. Deletion mutant is sensitive to the DNA synthesis inhibitor hydroxyurea (HU) and UV irradiation. Present with 2640 molecules/cell in log phase SD medium. Belongs to the CTK3 family.