How the BfiI restriction enzyme uses one active site to cut two DNA strands.

Unlike other restriction enzymes, BfiI functions without metal ions. It recognizes an asymmetric DNA sequence, 5'-ACTGGG-3', and cuts top and bottom strands at fixed positions downstream of this sequence. Many restriction enzymes are dimers of identical subunits, with one active site for each DNA strand. Others, like FokI, dimerize transiently during catalysis. BfiI is also a dimer but it has only one active site, at the dimer interface. We show here that BfiI remains a dimer as it makes double-strand breaks in DNA and that its single active site acts sequentially, first on the bottom and then the top strand. Hence, after cutting the bottom strand, a rearrangement of either the protein and/or the DNA in the BfiI-DNA complex must switch the active site to the top strand. Low pH values selectively block top-strand cleavage, converting BfiI into a nicking enzyme that cleaves only the bottom strand. The switch to the top strand may depend on the ionization of the cleaved 5' phosphate in the bottom strand. BfiI thus uses a mechanism for making double-strand breaks that is novel among restriction enzymes.