X-ray diffraction data were collected at X06SA at the Swiss Light Source, Villigen, Switzerland. Data were collected on several regions of any given crystal and the exposure time was adjusted to ensure that a complete data set could be obtained from a single crystal with minimal radiation damage. All data were integrated and scaled using XDS 42 . The previous high resolution 70S structure 11 (link) without its tRNA and mRNA ligands was used as a starting model for refinement. Refinement was conducted using CNS first through a rigid body refinement of each of the two 70S molecules in the asymmetric unit; an additional rigid body refinement where each domain of the ribosome, the tRNAs and ribosomal proteins were defined as separate rigid-body groups, followed by two rounds of energy minimization and B-factor refinement. The tRNA and mRNA ligands were built into the unbiased difference density from the initial round of refinement and the described refinement scheme was performed after the addition of each ligand. The amino acids attached to the tRNA were omitted until the remainder of the active site had been correctly built, and were then placed into the unbiased difference density using previous 50S structures as a guide 43 (link). The N-terminal tail of L27 was initially refined with the polyalanine model published in our previous structure 11 (link). A registry error was corrected and side chains were placed based on difference Fourier maps, which showed clear positive difference density for the amino acid side chains for the first 9 residues. The side chain orientation was confirmed by difference Fourier density obtained from a parallel refinement in which the first 15 residues of L27 were completely omitted from the initial model. Data and refinement statistics are reported in Table 1.