How pioneer factors interface with chromatin to promote accessibility for transcription control is poorly understood in vivo. Here, we directly visualize chromatin association by the prototypical GAGA pioneer factor (GAF) in live Drosophila hemocytes. Single-particle tracking reveals that most GAF is chromatin bound, with a stable-binding fraction showing nucleosome-like confinement residing on chromatin for more than 2 min, far longer than the dynamic range of most transcription factors. These kinetic properties require the full complement of GAF’s DNA-binding, multimerization and intrinsically disordered domains, and are autonomous from recruited chromatin remodelers NURF and PBAP, whose activities primarily benefit GAF’s neighbors such as Heat Shock Factor. Evaluation of GAF kinetics together with its endogenous abundance indicates that, despite on–off dynamics, GAF constitutively and fully occupies major chromatin targets, thereby providing a temporal mechanism that sustains open chromatin for transcriptional responses to homeostatic, environmental and developmental signals.

先驱因子如何与染色质相互作用以促进转录控制的可及性在体内知之甚少。在这里，我们通过活果蝇中的原型 GAGA 先驱因子 (GAF) 直接可视化染色质关联血细胞。单粒子追踪显示大多数 GAF 是染色质结合的，稳定结合部分显示核小体样限制在染色质上停留超过 2 分钟，远长于大多数转录因子的动态范围。这些动力学特性需要 GAF 的 DNA 结合、多聚化和本质上无序的结构域的完整补充，并且独立于招募的染色质重塑者 NURF 和 PBAP，它们的活动主要有利于 GAF 的邻居，如热休克因子。对 GAF 动力学及其内源性丰度的评估表明，尽管有开-关动力学，GAF 组成型并完全占据主要染色质靶标，从而提供一种时间机制，维持开放染色质对稳态、环境和发育信号的转录反应。