Substrate specificity of the DNA unwinding activity of the RecBC enzyme of Escherichia coli.

The RecBC enzyme of Escherichia coli promotes genetic recombination of phage or bacterial chromosomes. The purified enzyme travels through duplex DNA, unwinding and rewinding the DNA with the transient production of potentially recombinogenic single-stranded DNA. The studies reported here are aimed at understanding which chromosomal forms allow the entry of RecBC enzyme and hence may undergo RecBC enzyme-mediated recombination. Circular duplex molecules, whether covalently closed, nicked or containing single-stranded gaps of 10 to 774 nucleotides, are not detectably unwound by RecBC enzyme. Linear duplex molecules are readily unwound if they have a nearly flush-ended terminus whose 5' and 3' ends are offset by no more than about 25 nucleotides; molecules with longer single-stranded tails are poorly bound by RecBC enzyme and are infrequently unwound. The single-strand endonuclease activity of RecBC enzyme can slowly cleave gapped circles to produce molecules presumably capable of being unwound. These results provide an enzymatic basis for the recombinogenicity of double-stranded DNA ends established from genetic studies of RecBC enzyme and Chi sites, recognition sites for RecBC enzyme-mediated DNA strand cleavage.