ESCHERICHIA COLI B RESTRICTION ENDONUCLEASE

1. 1. The restriction endonuclease of Escherichia coli B has been purified 1000-fold from crude extracts. 2. 2. It has been found to be similar to the K12 and P1 restriction endonucleases8, i.e., it requires ATP, S-adenosyl-l-methionine and Mg2+ and unmodified DNA for enzymatic activity and has an estimated large molecular weight (approx. 300 000 daltons). 3. 3. It introduces a limited number of double strand scissions in unmodified λvir DNA, Escherichia coli chromosomal DNA and probably one double strand scission per fd replicative form DNA. The double strand scission in the unmodified fd replicative form DNA occurs by a two-step mechanism. 4. 4. Replicative form DNA generated from an fd mutant which is only restricted by Escherichia coli B by a factor of 1·10-2 (versus 1·10-4 for wild type fd) is also a substrate for the B restriction endonuclease. Cosedimentation of the endonuclease-treated wild type and mutant replicative form DNA results in qualitatively identical patterns of DNA distribution. © 1969.