The type III restriction-modification enzyme EcoP15I requires the interaction of two unmethylated, inversely oriented recognition sites 5'-CAGCAG in head to head configuration to allow an efficient DNA cleavage. It has been hypothesized that two convergent DNA-translocating enzyme-substrate complexes interact to form the active cleavage complex and that translocation is driven by ATP hydrolysis. Using a half-automated, fluorescence-based detection method, we investigated how the distance between two inversely oriented recognition sites affects DNA cleavage efficiency. We determined that EcoP15I cleaves DNA efficiently even for two adjacent head to head or tail to tail oriented target sites. Hence, DNA translocation appears not to be required for initiating DNA cleavage in these cases. Furthermore, we report here that EcoP15I is able to cleave single-site substrates. When we analyzed the interaction of EcoP15I with DNA substrates containing adjacent target sites in the presence of non-hydrolyzable ATP analogues, we found that cleavage depended on the hydrolysis of ATP. Moreover, we show that cleavage occurs at only one of the two possible cleavage positions of an interacting pair of target sequences. When EcoP15I bound to a DNA substrate containing one recognition site in the absence of ATP, we observed a 36 nucleotide DNaseI-footprint that is asymmetric on both strands. All of our footprinting experiments showed that the enzyme did not cover the region around the cleavage site. Analyzing a DNA fragment with two head to head oriented recognition sites, EcoP15I protected 27-33 nucleotides around the recognition sequence, including an additional region of 26 bp between both cleavage sites. For all DNA substrates examined, the presence of ATP caused altered footprinting patterns. We assume that the altered patterns are most likely due to a conformational change of the enzyme. Overall, our data further refine the tracking-collision model for type III restriction enzymes.
Copyright 2001 Academic Press.