Chromatin: Hanging on to histones

The core histones have long been known to have an important role in the packaging of DNA into nucleosomes. Today, nucleosome assembly is recognized as an integral component of both transcriptional activation and repression [1]. Transcriptional control is mediated through the assembly of extensive nucleoprotein structures containing both conventional transcription factors and architectural factors [2]. The latter include high mobility group proteins (HMGs), histones and proteins of similar structure that direct the folding of DNA necessary for the appropriate assembly of regulatory complexes. Recent progress has determined that the inclusion of histones into regulatory complexes depends upon specific protein–protein interactions with trans-acting regulatory factors [3,4]. These observations indicate that an intimate relationship exists between regulatory proteins and the histones, both within and outside the nucleosome.