Chromatin and transcription--how transcription factors battle with a repressive chromatin environment.

The last year has seen much progress in our understanding of chromatin and transcription. Transcriptionally active chromatin has long been correlated with a higher level of histone acetlyation. The discovery of a nuclear histone acetyltransferase activity encoded by factors with a role in transcription raises the possibility that the cell is able to dynamically modulate the (local) level of histone acteylation, switching chromatin templates from inactive to transcriptionally active states. Furthermore, histone acetylation states have shown to play a role in determining the efficacy of transcriptionally silenced chromatin in both yeast and Drosophila. The advances in our knowledge regarding the role of the origin-recognition complex in the establishment of silencing, and the requirement for a locally concentrated zone of the silence information regulator proteins in the nucleus has provided insights into the complex architecture of silenced chromatin. The goal of understanding the mechanisms by which the cell is able to 'open' repressive chromatin structures has prompted the discovery of multiple chromatin remodelling activities. These large protein complexes identified from organisms as diverse as yeast, mouse, fly and man demonstrate the ubiquity and fundamental importance of the ability to perturb the structure of chromatin allowing transcription of the desired genes. These data provide the latest and potentially most significant demonstration of the importance of the nucleosome in the regulation of transcription.