Activation of transcription by HMGN proteins in vitro

The C-terminal domain of HMGN was required for transcriptional enhancement in both SV40 minichromosomes and in minichromosomes assembled using 

HMGN proteins are thus able to decompact chromatin and activate transcription from chromatin templates. Both activities are dependent on the C-terminal domain, and it is likely that the transcriptional enhancement is a direct result of the ability of these proteins to unfold chromatin. 

Models for chromatin unfolding by HMGN proteins 7.1. Interaction with core histone tails 

A second model for HMGN action is that it counteracts chromatin compaction by linker histones. The ability of HMGN proteins to unfold SV40 minichromosomes and stimulate transcription from them is dependent on the presence of linker histones, and the data is consistent with HMGN counteracting the repressive 

The two models for HMGN action are not mutually exclusive, as HMGN may use both mechanisms to alter chromatin structure. Several observations point to an interplay between linker histones and the core histone tails. For example, linker histone inhibits the acetylation of H3 by P/CAF [93]. The inhibition was shown to result from steric hindrance of the tail by linker histones, rather than tail inaccessibility due to chromatin folding. It has also been shown that the removal of core histone tails may reduce the affinity of linker histone HI for the nucleosome [94], and that the interaction of the C-terminal tail of core histone H2A with linker DNA is rearranged in the presence of HI [47]. It is conceivable, therefore, that HMGN modulates the activity both of the core histone tails and of linker histones within the same nucleosome. 

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