Direct activation and direct versus indirect repression of genes. (A) Up-regulation via direct activation. (B) Down-regulation via direct repression. (C) Down-regulation as a consequence of up-regulation. Gene transcription is diagrammed as occurring in a transcription factory (orange disk with red center); genes not in contact with the factory are not expressed. Genes are shown as boxed arrows, with a bright solid fill indicating active transcription and a light fill indicating no transcription (red for induced, blue for repressed genes). Circles along the line (representing the DNA fiber) are transcription factor binding sites. Open circles indicate a lack of occupancy, and solid colors indicate occupancy; the color code is in the key. The situations prior to and subsequent to restoring GATA1 are on the left and right, respectively. Repressor proteins can recruit the Polycomb repressor complex 2 to methylate histone H3K27, but the chromatin structure is not shown explicitly.
