RNA Pol II exhibits different post-translational modifications at various stages of transcription. (a) The CTD is highly phosphorylated at Ser5 at the beginning of transcription. Ser2 gets phosphorylated during the pausing release. At the end of transcription, it is believed that both phosphorylation marks are removed. (b) Sequences of the yeast, Drosophila and human CTD are shown in a heptad-wise manner. Heptads with light blue background are consensus heptads (Sequence - YSPTSPS). Conservation maps for yeast and human CTD are shown using LogOdds Sequence Logo.
https://www.ncbi.nlm.nih.gov/CBBresearch/Yu/logoddslogo/
(c) Frames of references for heptads are shown with potential starting residue for each heptad. (d) All mono-phosphorylated heptad possibilities are shown. (e) All bis-phosphorylated heptad possibilities with the total number of combinations are shown.