The linker-protein network: control of nucleosomal DNA accessibility.

Numerous studies have recently addressed the accessibility of nucleosomal DNA to protein factors. Two popular concepts - the histone code and chromatin remodeling - consider the nucleosome as a passive entity that 'waits' to be marked by histone modifications and is 'mobilized' by ATP-dependent remodelers. Here, we propose a holistic view of the nucleosome as an active, dynamic entity, the accessibility of which is controlled by binding of different linker proteins to the DNA entry/exit site. The linker proteins might directly compete for this binding site; alternatively, protein chaperones and/or chromatin remodelers might exchange one linker protein for another. Finally, according to our proposed model, the exchange factors are themselves controlled by post-translational modifications or binding of protein partners, to respond to the ever-changing intra- and extra-cellular environment.