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. 2014 May 13;28(7):989–998. doi: 10.1210/me.2014-1084

Figure 2.

Figure 2.

The active pioneer transcription factor Pax7 opens the chromatin structure at intermediate pituitary melanotrope-specific enhancers. A, The protein convertase 2 (PCSK2) gene is specifically expressed in pituitary intermediate lobe melanotropes and not in the anterior lobe corticotropes, the other POMC-expressing lineage. Expression of this gene is critically dependent on an enhancer sequence that is located at −146 kb and that includes 1.4 kb of evolutionarily conserved sequences. B and C, Chromatin changes at enhancers pioneered by Pax7. The average distribution of chromatin marks at enhancers similar to the PCSK2 −146-kb enhancer in corticotrope AtT-20 cells (C) where the enhancers are inactive and after their activation through the pioneering action of Pax7 (B). The action of Pax7 on these enhancers results in chromatin opening as revealed by FAIRE and by the appearance of a bimodal distribution of H3K4me1 flanking the enhancer. Activation is also accompanied by recruitment of p300. D and E, Recruitment of Pax7 (D) and Tpit (E) is revealed by ChIPseq in AtT-20 cells expressing Pax7; neither protein is present at these positions in normal cells. Pax7 binds a composite sequence that is the hallmark of the pioneering targets of Pax7, whereas Tpit binds a complex palindrome (half-site motifs indicated by brown arrows over the sequence) similar to that of other Tpit-binding sites. The Pax7 and Tpit binding sites are separated by 337 bp and a small peak of Tpit recruitment coincident with the Pax7 peak reveals direct interactions between these proteins. F, Structure of Pax7 showing the presence of two DNA-binding domains, the Prd and HD. Each of these DNA-binding domains binds a unique sequence motif and either or both of these motifs are found at most Pax7 binding sites observed by ChIPseq in AtT-20 cells (67). Some Pax7 ChIPseq peaks harbor a unique composite Pax7 target sequence that includes the Prd and HD motifs side by side. This composite target site is uniquely associated with sites of pioneering by Pax7, such as the PCSK2 −146-kb enhancer where Pax7 binding leads to chromatin remodeling and enhancer activation. a.a., amino acids.