2.7 Crystallization, data collection, structure determination, and refinement

The initial crystallization screening was done with Hampton crystal screen HT using hanging-drop vapor diffusion method at 17 °C. The reservoir solution containing 0.2 M lithium sulfate, 0.1 M Tris HCl pH 8.5, and 30% PEG 4,000 mixed with the protein at ~10 mg/mL concentration (2 μL each) yielded crystals that were rhomboid-shaped (~80 μm each side) in 1-2 days. For data collection, the crystals were frozen in liquid N₂. X-ray diffraction data were collected with R-AXIS IV++ image-plate detector using Rigaku RU-200 rotating anode generator. The diffraction images were indexed and scaled with XDS [19].

The FnLpxA structure was solved by the molecular replacement method in PHASER using E. coli LpxA (PDB id: 1LXA) as the search model. Regions outside the hexapeptide repeats were removed from the initial molecular replacement solution, and manually rebuilt using COOT [20]. The structure refinement was carried out with REFMAC5 [21] within the CCP4 software suite [22]. The final model was validated using MOLPROBITY [23]. Structural alignments and figures were prepared using PyMol [24], and the atomic coordinates and structure factor have been deposited in the Protein Data Bank (PDB id: 5F42). The crystallographic information is summarized in Table S2.