Type II restriction endonucleases have attracted attention for two main reasons: firstly, their many applications in the dissection of DNA and in the construction of novel DNA molecules; secondly, as systems for studying the interactions of proteins with specific DNA sequences. With respect to the latter, the EcoR I restriction endonuclease has been examined in greater depth than any other type II enzyme [1-3]. However, the EcoR I enzyme has a major disadvantage as a system for studying DNA-protein interactions: the protein has a remarkably low solubility. The solutions in which EcoR I shows maximal activity, and also affinity for its recognition site, are saturated at less than 0.5 microM of this protein [4]. Consequently, many techniques that have been developed to study protein-ligand interactions but which require high concentrations of the protein in solution, such as NMR spectroscopy, cannot be used on EcoR I. But this drawback does not apply to all type II restriction enzymes. A different enzyme, the EcoR V restriction endonuclease [5-7], has special advantages as a system for studying DNA-protein interactions. In particular, this is the only type II restriction enzyme (apart from EcoR I [3]) for which crystals of the protein have been reported [7].