Nuclear Mobility and Mitotic Chromosome Binding: Similarities between Pioneer Transcription Factor FoxA and Linker Histone H1

There exists a hierarchy by which transcription factors can engage their target sites in chromatin, in that a subset of factors can bind transcriptionally silent, nucleosomal DNA, whereas most factors cannot, and this hierarchy is reflected, at least in part, in the developmental function of the factors. For example, transcription factors possessing the Forkhead box (Fox) DNA-binding domain contain an overall fold resembling that of linker histone and thus are structured to bind DNA, site specifically, in a nucleosomal context. Where tested, Fox factors bind early in the developmental or physiological activation of target genes, thereby enabling the binding of other factors that cannot engage chromatin on their own. To investigate the basis for early chromatin binding, we have used fluorescence recovery after photobleaching (FRAP) to analyze the mobility, in the live cell nucleus, of FoxA factors in comparison to linker histone and other transcription factors. We have further analyzed the factors for their ability to bind to chromatin in mitosis and thereby serve as epigenetic marks. The results indicate that the "pioneer" features of FoxA factors involve various chromatin-binding parameters seen in linker histones and that distinguish the factors with respect to their regulatory and mechanistic functions.