Obstacles May Facilitate and Direct DNA Search by Proteins

DNA recognition by DNA-binding proteins (DBPs), which is a pivotal event in most gene regulatory processes, is often preceded by an extensive search for the correct site. A facilitated diffusion process in which a DBP combines three-dimensional diffusion in solution with one-dimensional sliding along DNA has been suggested to explain how proteins can locate their target sites on DNA much faster than predicted by three-dimensional diffusion alone. Although experimental and theoretical studies have recently advanced understanding of the biophysical principles underlying the search mechanism, the process under in vivo cellular conditions is poorly understood. In this study, we used various computational approaches to explore how the presence of obstacle proteins on the DNA influences search efficiency. At a low obstacle occupancy (i.e., when few obstacles occupy sites on the DNA), sliding by the searching DBP may be confined, which may impair search efficiency. The obstacles, however, can be bypassed during hopping events, and the number of bypasses is larger for higher obstacle occupancies. Dynamism on the part of the obstacles may even further facilitate search kinetics. Our study shows that the nature and efficiency of the search process may be governed not only by the intrinsic properties of the DBP and the salt concentration of the medium, but also by the in vivo association of DNA with other macromolecular obstacles, their location, and occupancy.