X-ray diffraction data collection and refinement

Crystal diffraction data were collected at the PROXIMA 1 beamline of the SOLEIL synchrotron, France. Native datasets were collected over 360° rotation ranges at 0.1° oscillation range (Table ​(Table2).2). Data was processed using the XDS software package (42). Initially, the data were processed in P2₁ space group. Molecular replacement was done using the Z-G4 crystal structure (PDB ID: 4U5M) as a search model to obtain initial phases. Two copies per asymmetric unit were found. Reconstruction in the molecular replacement density map clearly indicated the presence of one right-handed and one left-handed block. The model was iteratively built through cycles of refinement using Phenix (43,44) and manual rebuilding in Coot (45). However, the refinement could not be improved beyond R_work and R_free values of 23 and 27%, respectively. Moreover, strong signs of twinning were observed in the electron density of T residues from the right-handed block. The data were therefore further processed in the P1 space group. In order to increase completeness, data from two isomorphic crystals fished in the same crystallization drop were merged. The twin law -h, k, -l was used during the refinement procedure in Phenix. The structure was refined to final R_work, R_free values of 16.42% and 20.19% respectively (Table ​(Table2)2) which contained four molecules in the asymmetric unit.

Data collection and refinement statistics for the X-ray crystal structure of TBA-T-Block2

Statistics for the highest-resolution shell (1.9–1.8 Å) are shown in parentheses.