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Fig. 2. Alternative strategies to link CDK activation to RNA polymerase II (Pol II) phosphorylation. (A) The TFIIH-associated CDK-cyclin-RING protein complex has been conserved throughout eukaryotic evolution but is incapable of activating cell-cycle CDKs in budding yeast. The monomeric kinase responsible for all known CDK-activating phosphorylation in S. cerevisiae (Cak1) has a non-essential ortholog (Csk1) with similar functions in S. pombe, but there is no such enzyme in metazoan cells. Thus, the CAK-CDK networks controlling cell division and gene expression are connected at different levels depending on the organism: directly by a common enzyme that is both a CAK and a CTD kinase in most eukaryotes, and by the upstream CAK common to both transcriptional and cell-cycle CDKs in budding yeast. Fission yeast has both connections. (B) The XPD helicase tethers, and may actively recruit, the CDK7 complex to the TFIIH core. XPD has also been proposed to sequester CAK in the cytoplasm during interphase in Drosophila embryos; its degradation by an unknown mechanism might release CDK7 to sustain the high levels of CDK activation needed during mitosis.
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