Chromatin Scanning by Dynamic Binding of Pioneer Factors.

Pioneer factors such as FoxA target nucleosomal DNA and initiate cooperative interactions at silent genes during development, cellular reprogramming, and steroid hormone induction. Biophysical studies previously showed that the nuclear mobility of FoxA1 is slower than for many other transcription factors, whereas a new single molecule study (Swinstead et al., 2016, Cell) shows comparable chromatin residence times for FoxA1 and steroid receptors. Despite that steroid receptors engage nucleosome-remodeling complexes, the vast majority of co-bound sites with FoxA are dependent upon FoxA, not vice versa. Taken together, the distinguishing feature of pioneer factors remains nucleosomal access rather than an exceptional residence time in chromatin.