DNA bending creates favored sites for retroviral integration: an explanation for preferred insertion sites in nucleosomes.

The choice of retroviral integration sites is strongly influenced by chromatin: integration in vitro occurs more efficiently into nucleosomal DNA than into naked DNA, and a characteristic pattern of preferred insertion sites with a 10 bp periodicity is observed at the outer face of the nucleosomal DNA. At least three features of nucleosomal DNA could be responsible for the creation of these favored sites: the presence of histones, attachment of the DNA to a protein surface, and DNA bending. To test each of these possibilities, we studied integration in vitro with human immunodeficiency virus and murine leukemia virus integrases into four model targets that mimic features of nucleosomal DNA: (i) catabolite activator protein-DNA complexes; (ii) lac repressor-operator complexes; (iii) lac repressor-induced loops; and (iv) intrinsically bent A-tract DNA. We found that bending of the target DNA can create favored integration sites at the outer face of the helix, irrespective of whether the bent DNA is attached to a protein surface. Our findings offer an explanation for the preferred usage of nucleosomes as integration targets. In addition, they suggest that bending of the target DNA might be an intrinsic feature of the integration reaction.