Fig. 2. Tyrosine-proline contacts in CTD heptad repeats.

a Superposition of ¹H-¹⁵N- TROSY spectra of human (dark blue) and yeast (cyan) CTD. The region highlighted displays the most intense peak resonances from residues of the canonical heptad repeats (above 50% of the initial threshold). Typical chemical shifts from canonical residues (*) are indicated in the inset. b Residue-specific normalized cross peak intensities observed in ¹H-¹⁵N-HSQC spectra of yCTD and CTD peptides composed of one to six canonical heptad repeats YSPTSPS. For longer sequences, the cross peaks of individual heptad repeats overlap (labeled e.g. as “S7/S14…”). c NOE contacts between Tyr ring protons and aliphatic proline protons in two-dimensional ¹H-¹H NOESY spectra of 3R-CTD (purple) and yCTD (cyan) at non-phase separating conditions (5 °C). In addition to Tyr-Pro contacts, Tyr-Thr and Tyr-Ser cross-peaks were assigned in the NOESY spectrum of 3R-CTD. d ¹H-¹⁵N residual dipolar couplings (RDCs) for 2R- and 3R-CTD. Due to the increased resolution in the IPAP-HSQC experiments, RDCs could be determined for individual residues in the repeats. e Secondary structure propensity in yCTD derived from experimental NMR chemical shifts. The location of Tyr residues is marked by red triangles. Non-assigned/overlapping residues were excluded from the analysis.