Table 3. The effect of amino acid substitutions on the DNA cleavage activity of N.BstNBI, PleI and MlyI.

N.BstNBI	D389A	E392A	E418A	D441A	D456A	E469A	E482A
	N, 100%	N, 100%	C, 0%	N, 100%	C, 0%	C, 1%	C, 1%
Ple1	D355A	E358A	E377A	D399A	D414A	E427A	E440A
	N, 100%	N, 100%	C, 10%	S, 0%	C, 0%	C, 0%	N, 100%
Mly1	D359A	D363A	E380A	D403A	D418A	E430A	E444A
	N, 100%	N, 100%	C, 0%	S, 0%	C, 0%	C, 10%	C, 1%

N, non-essential residues and substitution of these residues to alanine did not affect DNA cleavage activity. C, catalytic residues and changing these residues to alanine abolished or significantly reduced the DNA cleavage activity, but not the DNA binding activity, of the endonuclease. S, structurally important residues, which when changed to alanine affected both DNA cleavage and binding activities. The percentage represents the remaining activity and is shown under each residue. 0%, no cleavage activity can be detected in the crude cell extract. Conserved catalytic residues among the three endonucleases are shown in bold.