Etudes sur le mécanisme de remodelage des nucléosomes par ...

tel-00413908, version 1 - 7 Sep 2009
I.5.1.3.1 Generation of superhelical torsion
Since the remodelers belong to SF2 superfamily of helicases it was expected that some
helicases like behaviour would be exhibited by these. The evidence was provided by Havas
and colleagues (Havas et al., 2000) by testing various chromatin remodelers for the ability to
generate super helical torsion in DNA and chromatin substrates. The assay measured
extrusion of cruciform from a DNA construct containing an inverted [AT]34 repeat. Any
superhelical torsion created by the enzyme would result in formation of a cruciform, cleavable
by the junction resolving enzyme, T4 Endonuclease VII. It was shown that, SWI/SNF,
Xenopus Mi-2, ISWI, and recombinant BRG1 were all able to generate superhelical torsion in
an ATP dependent manner. However, only BRG1 and SWI/SNF were able to generate torsion
on chromatin templates while Mi-2 and ISWI only functioned on nucleosomal template. It
must be noted that, however, the generation of superhelical torsion could either be
consequence of remodeling or may represent a way by which histone DNA contacts are
disrupted.
I.5.1.3.2 Nucleosome sliding
Passive movement of nucleosomes along DNA i.e translational repositioning can occur in
response to elevated temperatures or ionic conditions (Meersseman et al., 1992; Pennings et
al., 1991). Given the strong interaction between histone octamer and DNA, this process is
energetically unfavourable. To achieve this ATP dependent chromatin remodelers use the
energy of ATP. In fact, it is a common feature of all the remodelers to mobilize the histone
octamer along the DNA (Längst and Becker, 2001). This was first demonstrated in initial
studies testing undefined ATP dependent activities in Drosophila extracts (Tsukiyama et al.,
1994; Varga-Weisz et al., 1995). Later on NURF, CHRAC and ISWI were shown to
directionally reposition the mononucleosomes reconstituted on DNA fragments longer than
200 bp in length (Hamiche et al., 1999; Längst et al, 1999). Similarly, yeast and human
SWI/SNF complexes as well as the Mi-2 complexes were shown to reposition nucleosomes
on short linear as well as small circular plasmid DNAs (Brehm et al., 2000; Gavin et al.,
2001; Goschin et al., 2000; Guyon et al., 2001; Jaskelioff et al., 2000; Whitehouse et al.,
1999). Additionally, ISWI group of remodelers exhibit the ability to generate regularly spaced
arrays (Längst and Becker, 2001). This property was not shared by other families of
remodelers indicating that ISWI remodelers may have a role in chromatin assembly.
74