SUPPLEMENTARY DATA Supplementary Data are available at NAR Online. ACKNOWLEDGEMENTS The authors would like to thank New England Biolabs (Ipswitch, MA, USA) and Vera and Alfred Pingoud (Justus-Liebig-University, Giessen, Germany) for provid- ing PspGI clones. They are grateful to Hans Bartunik for generous allocation of beamtime on BW6 (DESY, Hamburg) and to John SantaLucia (Wayne State University, Detroit, USA) for useful discussions regarding the article. FUNDING EMBO/HHMI Young Investigator Award (to M.B.); Polish Ministry of Science and Higher Education (0295/ B/P01/2008/34 to M.B.); National Institutes of Health of the United States (GM 57200 and CA 97899 to A.B.); Polish Ministry of Science and Higher Education (PBZ/ MEiN/01/2006/24 to H.C.); Lithuania State Science and Studies Foundation (to V.S.). Funding for open access charge: Polish Ministry of Science and Higher Education (0295/B/PO1/2008/34) Conﬂict of interest statement. None declared. REFERENCES 1. 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