D) Scheme showing contacts (putative H-bonds and hydrophobic contacts) and energetics between the Sen1 NIM peptide and Nrd1 CID. Equilibrium binding experiments with the protein mutants were monitored by FA. L20D mutant disrupts the hydrophobic contact with F17 and impairs the overall geometry of the α1-α2 loop that contributes to the interaction with the upstream electronegative stretch of NIM. KD (wild type Nrd1 CID-Sen1 NIM) equals 1.20 ± 0.02 μM. E) Alignment of RNAPII CTD and CTD mimics (Trf4 NIM, Sen1 NIM, and Mpp6 CTD mimic). Blue and grey boxes highlight the upstream electronegative stretches and hydrophobic regions, respectively. Previous structural works reported that S/NPXX motifs form the β-turn conformation (Kubicek et al., 2012; Tudek et al., 2014). 