Chromatine remodelling factors

Chromatine remodelling factors are protein that alter chromatin structure, either by chemical modification of the histones or by alteration of the inter-histone distance. In particular ATP-dependent remodelling factors are supposed to displace the nucleosome on DNA, possibly by translocating and twisting the DNA wound around the core histone.

In order to directly monitor the action of an ATP-dependent remodeller, we have studied the activity of the RSC chromatin remodelling complex (Cairns et al., 1996) on single DNA molecules using an approach similar to one that has previously proven to be effective in the study of topoisomerases (Strick et al., 2000), DNA helicases (Dessinges et al., 2004) and DNA translocases such as FtsK (Saleh et al., 2004) and RuvAB (Dawid et al., 2004). We find that translocation by the RSC complex is able to generate large loops (hundreds of base pairs long) within which DNA is underwound. The size of these loops was found to depend upon both the tension in DNA and the ATP concentration. Interestingly, the majority of loops were also relaxed in an ATP-dependent reaction which suggests that translocation of the complex is reversible. This ability to reversibly modify the DNA structure could provide a potent means by which ATP dependent remodeller of the Snf2 helicase family are able to control a number of protein DNA interactions.

 

Fig.1: a) Schematics of the action of a single RSC complex on DNA. b) in absnece of ATP no variation in the DNA extension is observed (left) while in presence of ATP one observes isolated short burst of decrease in extension due to ATP-dependent transient interaction of RSC and DNA. c) From these burst different parameters can be measured (the size of the burst, its on- and off-times). d,e) the histograms of these parameters can be evaluated. of particular interest is the size of the burst which exhibits a peak at about 500bps.  

The singe molecule data, see Fig.1 suggest that RSC acts on DNA by translocating and twisting the molecule, forming a loop of about 500 bps. This interpretation is validated by AFM images of the RSC-DNA complex, see Fig.2:

Fig.2: AFM images of RSC-DNA complexes where the enzyme seems to be associated with a large DNA loop (twisted or not) (bar=50 nm).