SYMMETRY AND ASYMMETRY IN THE FUNCTION OF ESCHERICHIA-COLI INTEGRATION HOST FACTOR - IMPLICATIONS FOR TARGET IDENTIFICATION BY DNA-BINDING PROTEINS

Background: Escherichia coli integration host factor (IHF) is a DNA-binding protein that participates in a wide variety of biochemical functions. In many of its activities, IHF appears to act as an architectural element, dramatically distorting the conformation of bound DNA. IHF is a dimer of non-identical subunits, each about 30 amino acids long. One dimer interacts specifically with a 30 base pair (bp) target, but well-conserved sequences are found in only half of this binding site. Thus, the IHF-DNA system has long been viewed as a paradigm of asymmetry in a protein-DNA interaction. Results: We have isolated the subunits of IHF and show that either subunit is capable of specifically recognizing natural IHF-binding sites and supporting lambda site-specific recombination in vitro. Mobility shift and footprinting data indicate that the isolated subunits interact with DNA as homodimers. We also describe the design of symmetric duplexes to which heterodimeric and homodimeric IHFs can bind by recognizing specific sequences. Conclusions: Our in vitro manipulation of the IHF system demonstrates that binding and bending of target DNA can be accomplished symmetrically. The prevalence of asymmetry found for this system in nature suggests that additional selective forces may operate. We suggest that these follow from the disparity between the size of the DNA that IHF protects (30 bp) and the length of DNA that the protein can initially contact (10 bp). This disparity implies that an IHF target is recognized in stages and may dispose the part of the protein-DNA system used for initial recognition to evolve distinctly from the remainder of the interaction surface. We suggest that a limitation in the length of DNA that can be initially contacted is a general property of DNA-binding proteins. In that case, many proteins can be expected to identify complex targets by step-wise, rather than simultaneous, contact between sequence elements and DNA-binding domains.