Fig. 1

Transcription bursting and the contribution of epigenetic chromatin composition. A Cartoon representation of transcription bursting in time. On the Y-axis the state of transcriptional access is marked in colour. Green represents the ON state in which transcription takes place, orange the OFF permissive state in which chromatin is accessible for transcription to occur but does not yet take place, red the OFF non-permissive state in which chromatin is inaccessible for transcription to occur. On the X-axis the time illustrating moments of a microburst and macroburst are shown. The large horizontal arrow represents a macroburst whereas the small arrow represents one of the microbursts within a macroburst. The numbers 1–3 represent different moments of transcription dynamics. B Cartoon (simplified) representations of a suggested relationship between the epigenetic composition of the chromatin and the transcriptional state allowing transcription micro- and/or macrobursting with RNAPII progression to occur or resist. (I) DNA methylation of the promoter region will not allow transcription (RNAPII) to access. The transcriptional state will remain in state 1. (II) Turnover and displacement of H3 into H3.3 variant histone is related to transcriptional activity allowing micro- or macrobursts at state 3. (III) Nucleosome decondensation and histone acetylation induce decompaction of the chromatin allowing transcription to occur and produce micro- or macrobursts, going from the OFF permissive state (2) to the ON state (3). (IV) Histone H3K79 methylation relates active transcription. An increase in transcription burst frequency in state 3 is known to be related to the presence of H3K79me2. (V) H3K4 demethylation by histone demethylase KDM5B creates variability in transcription, i.e. going from a state 3 with micro, macrobursts to state 2, being either ON or OFF. (VI) Chromatin looping induces a connection between the enhancer with the promoter thereby allowing transcription into micro- or macrobursts at state 3