Complex structure of Drosophila Ssu72-symplekin bound to doubly phosphorylated CTD peptide at Thr4/Ser5.
a. Composite annealing omit map of CTD peptide with doubly phosphorylated Thr4 and Ser5, contoured at 3 σ.
b. Overlay of structures of Drosophila Ssu72-symplekin in complex with 19-mer CTD peptides, singly phosphorylated Ser5 with carbon atoms colored yellow and doubly phosphorylated Thr4 and Ser5 with carbon shown in green.
c. Model of combinatorial code of phosphorylation and isomerization. Thr4 phosphorylation occurring after the Ser2 phosphorylation will delay the dephosphorylation of Ser5.