SAR-DEPENDENT MOBILIZATION OF HISTONE H1 BY HMG-I Y IN-VITRO - HMG-I/Y IS ENRICHED IN H1-DEPLETED CHROMATIN/

An experimental assay was developed to search for proteins capable of antagonizing histone H1-mediated general repression of transcription. T7 RNA polymerase templates containing an upstream scaffold-associated region (SAR) were highly selectively repressed by H1 relative to non- SAR control templates. This is due to the nucleation of H1 assembly in to flanking DNA brought about by the numerous A-tracts (AT-rich sequen ces containing short homopolymeric runs of dA.dT base pairs) of the SA R. Partial, selective titration of these A-tracts by the high mobility group (HMG) protein HMG-I/Y led to the complete derepression of trans cription from the SAR template by inducing the redistribution of H1 on to non-SAR templates. SARs are associated with many highly transcribe d regulated genes where they may serve to facilitate the HMG-I/Y-media ted displacement of histone H1 in chromatin. Indeed, HMG-I/Y was found to be strongly enriched in the H1-depleted subfraction which can be i solated from chromatin.