Nucleosome dynamics and histone variants

In eukaryotes, DNA is organized into chromatin, a dynamic structure that enables DNA to be accessed for processes such as transcription, replication and repair. To form, maintain or alter this organization according to cellular needs, histones, the main protein component of chromatin, are deposited, replaced, exchanged and post-translationally modified. Histone variants, which exhibit specialized deposition modes in relation to the cell cycle and possibly particular chromatin regions, add an additional level of complexity in the regulation of histone flow. During their metabolism, from their synthesis to their delivery for nucleosome formation, the histones are escorted by proteins called histone chaperones. In the present chapter we summarize our current knowledge concerning histone chaperones and their interaction with particular histones based on key structural properties. From a compilation of selected histone chaperones identified to date, we discuss how they may be placed in a network to regulate histone dynamics. Finally, we provide working models to explain how H3 variants, deposited on to DNA using different histone chaperone machineries, can be transmitted or lost through cell divisions.