Figure 7.

Diverse mechanisms for rapid activation of genes during development. The top panel shows that rapidly activated genes can be further subdivided into distinct categories, A–C. (A) The Hoxb1 gene newly recruits Pol II and GTFs in a classical gene activation mechanism, where RAR/RXR binds in the presence of RA and, with the help of coactivators, recruits GTFs and Pol II. (B) Paused Pol II, with DSIF/NELF, is present at the TSS of developmentally regulated genes, such as Hoxa1. In the presence of RA, RAR/RXR recruits SEC to stimulate transcription elongation through phosphorylation of the DSIF/NELF and the Pol II CTD. (C) Cyp26a1, a developmentally regulated gene that lacks paused Pol II, is induced by RA in a SEC-dependent manner. All of the same factors are present after RA treatment as seen at Hoxa1, but Cyp26a1 is induced to higher levels, suggesting that paused Pol II may serve to help regulate activation to equivalent levels.