Figure 1.

Types of accessibility. (a) Permeability of a nuclear volume (‘compartment’) determines whether proteins can diffuse into the same space as a genomic locus. (b) Broad-based accessibility of chromatin in a set of genomic loci determines the ability of proteins to nonspecifically access and scan the genomic DNA. Chromatin fiber compaction is shown as a mechanism for modulating broad-based accessibility, but many other mechanisms are possible. (c) Local hyper-accessible sites are narrow loci where proteins can access DNA with similar ease as de-chromatinized DNA. These sites are thought to have low nucleosome occupancy for several reasons, including competition from transcription factors (TFs) (illustrated, blue and red) that exclude nucleosomes.