Protein phosphorylation and control of chorion gene activation through temporal mobilization of a promoter DNA binding factor from the cytoplasm into the nucleus.

Abstract

The transcriptional activation of high cysteine chorion genes in the follicular cells of the silkworm Bombyx mori occurs at the end of oogenesis and coincides with the appearance of a chorion promoter DNA binding factor, BCFI, in follicular cell nuclei. Follicular cells of vitellogenic and choriogenic follicles that do not express high cysteine chorion genes contain high levels of a latent form of BCFI in their cytoplasm. The abundance of the cytoplasmic factor, termed cBCFI, is dramatically reduced during late choriogenesis, coincident with the appearance of factor BCFI in the nucleus and the transcriptional activation of high cysteine genes. Mobility shift assays performed with partially proteolyzed nuclear and cytoplasmic extracts of follicular cells, DNA binding assays carried out in the presence of anti-BCFI antibodies, and electrophoretic analyses of the proteins present in the nuclear and cytoplasmic fractions of follicular cells and recognized by the same antibodies suggest that factor cBCFI represents a covalently modified version 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 specificity to that of BCFI. The cytoplasmic sequestration of cBCFI appears to be mediated by a phosphorylation-dependent, reversible association of this factor with an ancillary cytoplasmic factor.