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Abstract

Crystal structures of Type 11 restriction endonucleases demonstrate a conserved common core and active site residues but diverse structural elements involved in DNA sequence discrimination. Comparative structural analysis of restriction enzymes recognizing the same nucleotide sequence might therefore contribute to our understanding of the structural diversity of specificity determinants within restriction enzymes. We have solved the crystal structure of the Bacillus stearothermophilus restriction endonuclease Bse63411 by the multiple isomorphous replacement technique to 2.17 Angstrom resolution. Bse6341 is an isoschisomer of the Cfr101 restriction enzyme whose crystal structure has been reported previously. Comparative structural analysis of the first pair of isoschisomeric enzymes revealed conserved structural determinants of sequence recognition and catalysis. However, conformations of the N-terminal subdomains differed between Bse6341/Cfr101, suggesting a rigid body movement that might couple DNA recognition and catalysis. Structural similarities extend to the quaternary structure level: crystal contacts suggest that Bse6341 similarly to Cfr101 is arranged as a tetramer. Kinetic analysis reveals that Bse6341 is able to interact simultaneously with two recognition sites supporting the tetrameric architecture of the protein. Thus, restriction enzymes Bse6341, Cfr101 and NgoMIV, recognizing overlapping nucleotide sequences, exhibit a conserved tetrameric architecture that is of functional importance.