Fork head alternative binding drives stage-specific gene expression in the silk gland of Bombyx mori.

Here, we identified the main transactivator of fhx, the gene encoding the silk protein fibrohexamerin in posterior silk gland cells (PSG), as the homeotic SGF1/fork head factor. The same factor also stimulates sericin-1, another silk protein encoding gene, in the middle silk gland cells. SGF1/fork head is present in the silk gland nuclei during the whole course of larval life, but its binding to the fhx promoter occurs at intermolt and not during molt, when fhx is respectively turned on and off. The alternative binding of the factor is associated with specific changes in the fhx chromatin topology in PSG cells. Taken together, our results show that stabilization of SGF1/fork head to its target sequence is critical to promote fhx transcription at each intermolt. We also found that fhx is characterized by a PSG-specific DNase I hypersensitive site in the first intron, present during molt and intermolt, i.e. independent of the transcriptional status of the gene. All these data suggest that differential chromatin accessibility and fork head activation are crucial in controlling the spatial and temporal regulation of the fhx gene in the posterior silk gland cells.