Chromatin is a barrier to the binding of many transcription factors. By contrast, pioneer factors access nucleosomal targets and promote chromatin opening. Despite binding to target motifs in closed chromatin, many pioneer factors display cell-type-specific binding and activity. The mechanisms governing pioneer factor occupancy and the relationship between chromatin occupancy and opening remain unclear. We studied three Drosophila transcription factors with distinct DNA-binding domains and biological functions: Zelda, Grainy head and Twist. We demonstrated that the level of chromatin occupancy is a key determinant of pioneering activity. Multiple factors regulate occupancy, including motif content, local chromatin and protein concentration. Regions outside the DNA-binding domain are required for binding and chromatin opening. Our results show that pioneering activity is not a binary feature intrinsic to a protein but occurs on a spectrum and is regulated by a variety of protein-intrinsic and cell-type-specific features.

染色质是许多转录因子结合的屏障。相比之下，先锋因子进入核小体靶标并促进染色质开放。尽管与封闭染色质中的靶基序结合，许多先锋因子仍表现出细胞类型特异性的结合和活性。控制先锋因子占据的机制以及染色质占据和开放之间的关系仍不清楚。我们研究了三种具有不同 DNA 结合域和生物学功能的果蝇转录因子：Zelda、Grainy head 和 Twist。我们证明染色质占据水平是开拓活动的关键决定因素。多种因素调节占据，包括基序内容、局部染色质和蛋白质浓度。 DNA 结合结构域之外的区域是结合和染色质打开所必需的。我们的结果表明，开拓活动不是蛋白质固有的二元特征，而是发生在一个光谱上，并受到各种蛋白质固有和细胞类型特异性特征的调节。