Decisive factors: a transcription activator can overcome heterochromatin silencing.

Eukaryotes organize certain chromosomal intervals into domains capable of silencing most genes. Examples of silencing domains include the HML/HMR loci and subtelomeric chromatin in yeast, the Barr body X chromosome in mammals, and the pericentric heterochromatin of Drosophila. Silencing chromatin is often correlated with more regularized nucleosomal array than that found in active chromatin, and transcriptional activators appear to be missing from their target sites in silent chromatin. In Drosophila, gene silencing by heterochromatin is often variegated, indicating that a gene may escape silencing in some cells. In a recent study, Ahmad and Henikoff(1) show that a yeast activator can compete successfully with Drosophila heterochromatic silencing factors for target sites in DNA. This competition, together with developmental change in the stability of heterochromatin itself, decides the transcriptional state for a gene subject to heterochromatin repression.