Filament of SgrAI bound to its primary recognition sequence and induced conformational change.
(A) Left-handed helical filament formed from DNA bound SgrAI dimers (DBD, colored in red, green, purple or yellow). Approximately four DBDs are found per turn, with helical parameters −85.8° twist and 21.2 Å rise between subunits. Within each DBD, the DNA and two SgrAI chains are colored in slightly different hues. (B) As in A, but without the DNA shown. (C) As in A, but with only the DNA shown. The dotted line shows one way two duplexes could be connected within the filament. Note that within biologically relevant DNA, this loop may be ∼500–1000 bp in length. (D) Close-up of the region between adjacent DBDs of the green boxed region in A showing the position of the R31 loop between the red and purple DBDs, the position of the cleavage site, the primary recognition sequence (cyan), and bp flanking the recognition sequence that contact neighboring DBDs (salmon). (E) Comparison of DBD from filamentous (cyan) and non-filamentous (magenta) structural states reveals an 11° rotation of one chain of the SgrAI dimer relative to the other approximately parallel to the path of the DNA.