Assembling heterochromatin in the appropriate places: A boost is needed.

Heterochromatin, or condensed chromatin, has the potential to encroach into what ordinarily would be euchromatin and repress resident genes. We explore how heterochromatin is restricted to the appropriate regions of the genome, using Saccharomyces cerevisiae as a case study and emphasizing two under-appreciated aspects of silenced chromatin. First, the capacity of silenced chromatin to propagate along a chromosome is limited by the intrinsic instability of the structure. We argue that this limited potential to spread is an important factor restricting silenced chromatin to the vicinity of recruitment sites (silencers). Second, this limited capacity to spread creates the need for additional mechanisms to stabilize silenced chromatin at the required locations. Such mechanisms include the use of multiple silencers and higher-order arrangements of the chromatin fiber. Therefore, to understand how silenced chromatin is restricted to the appropriate genomic locations, researchers must take into account the mechanisms by which silenced chromatin is stabilized in appropriate locations. J. Cell. Physiol. 219: 525-528, 2009. (c) 2009 Wiley-Liss, Inc.