Sequence and functional-group specificity for cleavage of DNA junctions by RuvC of Escherichia coli.

RuvC is the DNA junction-resolving enzyme of Escherichia coli. While the enzyme binds to DNA junctions independently of base sequence, it exhibits considerable sequence selectivity for the phosphodiester cleavage reaction. We have analyzed the sequence specificity using a panel of DNA junctions, measuring the rate of cleavage of each under single-turnover conditions. We have found that the optimal sequence for cleavage can be described by (A approximately T)TT downward arrow(C>G approximately A), where downward arrow denotes the position of backbone scission. Cleavage is fastest when the cleaved phosphodiester linkage is located at the point of strand exchange. However, cleavage is possible one nucleotide 3' of this position when directed by the sequence, with a rate that is 1 order of magnitude slower than the optimal. The maximum sequence discrimination occurs at the central TT in the tetranucleotide site, where any alteration of sequence results in a rate reduction of at least 100-fold and cleavage is undetectable for some changes. However, certain sequences in the outer nucleotides are strongly inhibitory to cleavage. Introduction of base analogues around the cleavage site reveals a number of important functional groups and suggests that major-groove contacts in the center of the tetranucleotide are important for the cleavage process. Since RuvC binds to all the variant junctions with very similar affinity, any contacts affecting the rate of cleavage must be primarily important in the transition state. Introduction of the optimal cleavage sequence into a three-way DNA junction led to relatively efficient cleavage by RuvC, at a rate only 3-fold slower than the optimal four-way junction. This is consistent with a protein-induced alteration in the conformation of the DNA.