Phasing of protein-induced DNA bends in a recombination complex.

Many of the structures responsible for replication, transcription initiation and recombination arise from complex sets of protein-protein interactions and the folding of DNA in three dimensions, with protein-induced bending of DNA often playing an integral role. The magnitude and orientation of DNA bending induced by various single proteins has been estimated by gel mobility shift methods and by modelling of crystallographic data. The site-specific recombination by which bacteriophage lambda (phage lambda) integrates into the chromosome of its host Escherichia coli requires a host protein, 'integration host factor' (IHF), which is known to be able to bend the DNA to which it binds. To determine the three-dimensional path of DNA within the higher order structure responsible for phage lambda site-specific recombination, we have determined the relative direction of IHF-induced bending at each of the three binding sites within the complex. IHF, which appears to bend DNA by more than 140 degrees, is a major determinant of the DNA path in the recombination complex and is also involved in a wide range of other cellular events.