Fig. 7.

Transcription elongation model. Compared with a prokaryotic system, eukaryotic DNA is wrapped around histones and not accessible for Pol II. The highly positively charged histone tails play critical roles in the packing of chromatin. Acetylation of the tails could loosen up the tight packing while cleavage of histone tails by JMJD5, JMJD7, and possibly others could lead to complete depletion of nucleosomes for the accessibility of Pol II, or Pol II is able to overcome the tailless nucleosomes as that in Archaea.