Fig 2. Model and representative electron-density for RNA-bound PUS1.

(a) The contents of the asymmetric unit, for both structures that were solved, corresponds to a single protein subunit (colored as a spectrum, from the blue N-terminal end of the refined model to the red C-terminal end) bound to a single R263 RNA oligonucleotide (black bases). The model of the catalytically inactive D134A enzyme is shown in two orientations related by a 90° rotation around the x-axis (PDB ID: 7R9G). (b) In both structures, the application of a crystallographic 2-fold rotation axis generates a dimeric complex in which two subunits are independently bound to an RNA duplex. The second protein subunit and second RNA strand are colored in dark teal and pale blue, respectively. (c) Representative simulated annealing composite omit 2Fo-Fc electron density contoured across the RNA duplex and (d) at the region of protein-RNA contacts observed at the 5’ end of one RNA strand. The structural features illustrated in this figure are replicated for the wild type enzyme bound to a closely related RNA complex, which was solved in an unrelated crystallographic space group and lattice (S7 Fig). The position of the bound sulfate ion mirrors a similarly placed sulfate in the previously described structure of unbound human PUS1.