Fig. 1.

Experimental models implicating histone modifications in the propagation of epigenetic states. a A simplified illustration of Polycomb/Trithorax regulation in Drosophila development. In the embryo the expression of Ultrabithorax (UBX) is set by the activity of Hunchback (Hb), which acts as a repressor, and Fushi tarazu (Ftz), which acts as an activator. However, in the adult, neither Hb nor Ftz is present and the embryonic pattern of UBX expression is maintained by binding of the Polycomb and Trithorax complexes. b Position effect variegation in the Drosophila eye. The Drosophila eye in wild type flies has a red colour due to the expression of the gene white. An inversion mutation can place the white gene in proximity to heterochromatin. This can lead, through spreading of the heterochromatin, to stochastic silencing of white and a variegated eye colour phenotype. Suppression of variegation, or Su(var), mutants are identified through reversion of this phenotype. c The S. cerevisiae mating type locus. The mating type is determined by the presence of the a or α mating type cassette within the expressed domain (green), allowing the two genes determining mating type, Mat a/α1 and Mat a/α2, to be expressed. The expressed domain is flanked by silent copies of both a and α mating type cassettes, adjacent to cis-acting silencer elements. Gene conversion can introduce either of the silent cassettes into the expressed cassette to switch mating type. d Live cell imaging to reveal the effect of H3K4 methylation on inheritance of transcriptionally active states in Dictyostelium. The transcription frequency of a reporter locus can be measured directly by observation of RNA production in vivo via binding of the MS2 bacteriophage coat protein to MS2 repeats engineered into the transcript. Cell division gives two daughter cells with good correlation between the transcriptional rates. However, reducing H3K4 methylation by introducing the H3K4A mutation leads to failure of this correlation [38]