PROTEIN-PHOSPHORYLATION AND CONTROL OF CHORION GENE ACTIVATION THROUGH TEMPORAL MOBILIZATION OF A PROMOTER DNA-BINDING FACTOR FROM THE CYTOPLASM INTO THE NUCLEUS

The transcriptional activation of high cysteine chorion genes in the f ollicular cells of the silkworm Bombyx mori occurs at the end of oogen esis and coincides with the appearance of a chorion promoter DNA bindi ng factor, BCFI, in follicular cell nuclei. Follicular cells of vitell ogenic and choriogenic follicles that do not express high cysteine cho rion genes contain high levels of a latent form of BCFI in their cytop lasm. The abundance of the cytoplasmic factor, termed cBCFI, is dramat ically reduced during late choriogenesis, coincident with the appearan ce of factor BCFI in the nucleus and the transcriptional activation of high cysteine genes. Mobility shift assays performed with partially p roteolyzed nuclear and cytoplasmic extracts of follicular cells, DNA b inding assays carried out in the presence of anti- BCFI antibodies, an d electrophoretic analyses of the proteins present in the nuclear and cytoplasmic fractions of follicular cells and recognized by the same a ntibodies suggest that factor cBCFI represents a covalently modified v ersion of BCFI. The DNA-binding sites of BCFI and cBCFI include a core sequence, AGATAA, but, while this sequence is sufficient for specific binding of BCFI, it only constitutes part of the DNA-binding site of cBCFI. Dephosphorylation of cBCFI results in a change of its binding s pecificity to that of BCFI. The cytoplasmic sequestration of cBCFI app ears to be mediated by a phosphorylation-dependent, reversible associa tion of this factor with an ancillary cytoplasmic factor.