(A) FRH large cell (cyan), Mtr4 (pdb 2XGJ, green, RNA backbone in orange), and FRH small alternate cell (blue) differ primarily in the orientation of the arch domain. The FRH RNA-bound structure also crystallizes in the small-cell configuration. The two different FRH structures have an overall RMSD of 3.9 Å and the RMSD between Mtr4 and FRH is 2.9 Å (over 734 residues, for large cell FRH). 