Left-handed DNA crossovers. Implications for DNA-DNA recognition and structural alterations.

The close approach of DNA segments participates in many biological function s including DNA condensation and DNA processing. Previous crystallographic studies have shown that B-DNA self-fitting by mutual groove-backbone intera ction produces right-handed DNA crossovers. These structures have opened ne w perspectives on the role of close DNA-DNA interactions in the architectur e and activity the DNA molecule. In the present study, the analysis of the crystal packing of two B-DNA decamer duplexes d(CCIIICCCGG) and d(CCCCCGGCG G) reveals the existence of new modes of DNA crossing. Symmetric left-hande d crossovers are produced by mutual fitting of DNA grooves at the crossing point. New sequence patterns contribute to stabilize longitudinal fitting o f the sugar-phosphate backbone into the major groove. In addition, the clos e approach of DNA segments greatly influences the DNA conformation in a seq uence dependent manner. This study provides new insights into the role of D NA sequence and structure in DNA-DNA recognition. In providing detailed mol ecular views of DNA crossovers of opposite chirality, this study can also h elp to elucidate the role of symmetry and chirality in the recognition of c omplex DNA structures by protein dimers or tetramers, such as topoisomerase II and recombinase enzymes. These results are discussed in the context of the possible relationships between DNA condensation and DNA processing.