Structure determination of a truncated dimeric splicing endonuclease in pseudo-face-centered space group P2(1)2(1)2

RNA-splicing endonuclease is responsible for the excision of introns in transfer RNA and archaeal ribosomal RNAs. The archaeal form of the enzyme recognizes a unique RNA motif that consists of two three-nucleotide bulges separated by a four base-paired helix, known as the bulge-helix-bulge (BHB) motif. A crystal structure of the RNA-splicing endonuclease from Archaeoglobus fulgidus (AF) has been reported previously at 2.8 A. A truncated but fully active form of AF endonuclease that lacks the N-terminal domain was expressed and crystallized in an orthorhombic space group with two dimers in the asymmetric unit. The calculated native Patterson map suggests strong pseudo-face-centering characteristics, which lead to incorrect space-group assignment by the autoindexing program. The correct space group was determined to be P2(1)2(1)2 after reindexing. The structure was solved using molecular replacement and was refined to 2.0 A. The truncated AF endonuclease structure is essentially identical to the corresponding portion of the wild-type AF endonuclease structure in space group P4(3)2(1)2 as reported previously, with the exception of loop L9, which differs owing to different crystallographic packing. These results confirm the previously described structural features of dimeric splicing endonuclease.