Fig. 1. The histone acetylation switch. Targeted HAT and HDAC activities negotiate the acetylation status of chromatin. Acetylation establishes a structure that permits ATP-dependent chromatin remodeling factors to open promoters. Deacetylation, frequently followed by histone methylation, may form a solid base for highly repressive structures, such as heterochromatin. Acetylated histone tails are shown as yellow circles. Methylations are indicated as gray rectangles. HAT, histone acetyltransferase; HDAC, histone deacetylase; HMT, histone methyltransferase; HP1, heterochromatin protein 1.