Purification and characterization of an enhancer-binding protein of the fibroin gene. II. Functional analyses of fibroin factor 1.

Fibroin factor 1 (FF1) when coupled with fibroin factor 2 (FF2) is an enhancer binding protein of the fibroin gene. FF1a, one type of FF1, has been purified to homogeneity from crude nuclear extracts of posterior silk gland cells and identified as a protein with molecular mass 125 kDa (Suzuki, T., Matsuno, K., Takiya, S., Ohno, K., Ueno, K., and Suzuki, Y. (1991) J. Biol. Chem. 266, 16935-16941). FF1 is able to recognize and bind a sequence of -205 to -185 in the enhancer I of the fibroin gene in the presence of FF2. The binding sequence of FF1 with FF2 contains two repeats of the derivative of consensus sequence which is recognized by homeobox-containing proteins. Though FF1 activity to construct the major band complex I is specific to posterior silk gland cells, use of a specific antibody raised against FF1a showed that FF1 protein is ubiquitous in Bombyx cells. These results suggest the possibility that FF1 molecules present as multiple proteins might be specifically modified and activated for the binding to enhancer DNA in posterior silk gland cells. Since the FF1a antibody also inhibited a transcriptional enhancement activity governed by the enhancer sequence, we conclude that FF1 is one of the transcriptional factors of the fibroin gene. The functions of FF1 on fibroin gene transcription are discussed.