Figure 2. Model for Cdc28-mediated phosphorylation of the CTD. (A) In late G1 Kin28 (or potentially a still uncharacterized kinase) primes the CTD. (B) Phosphorylated TP sites (threonine followed by a proline) and/or multiple phospho-S₅ serve as a docking site for Cks1/Cdc28. As Cdc28 becomes active in late G1, Cdc28 further phosphorylates S₅ to ensure high rates of transcription and mRNA capping.