The nucleosome, the main structural fold of eukaryotic chromatin, is far from being the well-defined and inert histone-DNA complex depicted in most textbooks. On one side, nucleosome particles are very heterogeneous due to differences in DNA base-pair sequence, histone variants and their multiple biochemical modifications. Moreover, histones may combine with other DNA binding proteins to form >nucleosome-like> complexes. On the other side, we showed in recent years that nucleosomes have huge conformational plasticity to absorb DNA twisting waves and to accommodate positive or negative DNA supercoils. Thus, deciphering the complex interplay between chromatin structure and function will not be possible without understanding the mechanical properties and DNA topology constrains associated to specific nucleosome particles. To initiate the above, we revisited here the >linking number paradox> of nucleosomal DNA, which states: While biochemical and structural data show that DNA coils ~1.7 negative turns around the histone octamer, topological analyses indicate that each nucleosome constrains in average only ~1 negative DNA supercoil. To approach the paradox, we have examined the DNA topology (DNA linking number) of yeast circular mini-chromosomes containing different nucleosome number and types. We found that native yeast chromatin constrains ~1.3 negative supercoils per nucleosome (more than commonly assumed). Our results show also that a yeast centromere, a paradigm of non-canonical nucleosome-like particle, does constrain ~ 0.5 positive supercoils, thus in partial agreement with previous reports.

Comunicación presentada en la III Jornada de Cromatina i Epigenètica, celebrada el 4 de marzo de 2013 en Barcelona (España)

Peer Reviewed