Heptapeptide repeat in CTD of RNA pol II is
Largest subunit of eukaryotic RNA polymerase II possesses unique C-terminal domain extension absent in Pol I and Pol III. Domain consists of tandem repeats of conserved heptapeptide Tyr-Ser-Pro-Thr-Ser-Pro-Ser, written YSPTSPS, twenty-six copies in yeast Saccharomyces cerevisiae and fifty-two copies in mammals providing extended tail. Conserved tyrosine one, serines two, five, seven can be phosphorylated, threonine four phosphorylated, prolines three and six isomerized, lysine seven acetylated, creating combinatorial CTD code read by processing factors coordinating transcription with capping, splicing, and termination.
Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 10: Heptapeptide repeat YSPTSPS in CTD; Cell Biology of CTD code