KLF1 Acts As a Pioneer Transcription Factor to Open Chromatin and Facilitate Recruitment of GATA1

Only a small subset of transcription factors (TFs) can act as pioneer factors; i.e. those that can 'open' otherwise 'closed' chromatin to facilitate assembly of TF complexes and co-factors to enable transcription. The KLF/SP family of TFs bind to a 9-10 bp consensus motif in DNA to activate or repress target gene expression. We have studied the potential for KLF1, which is essential for erythropoiesis, to provide a pioneering function in erythroid progentior cells. Previous ChIP-seq studies have shown KLF1 binds a few thousand enhancers and promoters to activate erythroid cell gene expression ¹. It often binds near to other key erythroid TFs such as GATA1 and SCL/TAL1, so is likely to work in concert with them in some contexts. We have employed an inducible stable KLF1-ER^TM construct to rescue gene expression and differentiation of Klf1^-/- erythroid cell lines ². We employed ChIP-seq, ATAC-seq and DNAse1 HS to show KLF1 can bind to closed sites in chromatin and induce an open state. We show this is essential for recruitment of the settler transcription, GATA1, at certain co-bound sites but not others. This pioneering function occurs at ~300 key erythroid enhancers and super-enhancers such the one at -26kb in the a-globin LCR and one within the body of the E2f2 gene ³ but rarely at promoters. We further show that two different neomorphic mutations in the KLF1 DNA-binding domain lead to ectopic pioneering (opening of closed chromatin) and aberrant gene activation ⁴. We generated a series of N-terminal deletions in KLF1 and employed ATAC-seq to map the domain/s within KLF1 responsible for the pioneering activity and show it is distinct from DNA-binding activity. The domain is responsible for bromodomain protein recruitment, the likely effector of chromatin remodelling. We have also examined whether KLF3, which acts as a transcription repressor via recruitment of the co-repressor, CtBP2, can force the closure of otherwise open chromatin ⁵. We find it cannot. Rather, KLF3 (and likely other members of this subclade) works via active recruitment of co-repressors rather than rendering chromatin inaccessible. This likely enables rapid reactivation of pioneered enhancers without the need to reprogram chromatin. This work has broad implications for how the KLF/SP family of TFs work in vivo to reprogram cells and direct differentiation. We will present data for such activity in non-erythroid cell systems.

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