Potential mechanisms of H1-mediated regulation of transcription. (a) Linker histones stabilize and/or promote the formation of transcriptionally inert higher-order chromatin structures. (b) The removal or modification of the binding of histone H1 to relaxed chromatin structures may be a prerequisite for initiating a series of events resulting in transcriptional activation. These events may include covalent modification (for example acetylation) of core histones, chromatin remodeling by sliding or removal of nucleosome cores, and the establishment of a transcription preinitiation complex. Differences among H1 variants in their ability to promote or antagonize any of these transitions could contribute to transcriptional regulation.