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. 2009 May 18;29(14):4002–4013. doi: 10.1128/MCB.00189-09

FIG. 6.

FIG. 6.

Nucleosome depletion circumvents the need for P-TEFb for efficient elongation of transcription. (A) Within the transcription units of protein-coding genes, the presence of nucleosomes causes a “road block” for pol II that becomes a checkpoint due to NELF recruitment. NELF recruitment may be due to polymerase stalling and/or the increase in nucleosomes. P-TEFb activity is required to overcome NELF/nucleosome-induced arrest. CTCF may play a role in positioning the NELF/nucleosome checkpoint. Nucleosomes are depicted as cylinders with two turns of DNA wrapped around them. The angled arrow indicates the start of transcription. (B) In the absence of nucleosomes on the U2 transcription unit, NELF is not recruited. NELF recruitment together with an increase in nucleosome occupancy causes termination of transcription. CTCF may play a role in positioning the NELF/nucleosome terminator.