Transcription signals and protein binding sites for sericin gene transcription in vitro

Using nuclear extracts from Bombyx mori middle silk glands (MSG) where the sericin-1 (Ser-1) mRNA is produced specifically, we have studied the in vitro transcription of the Ser-1 gene. To determine the sites required for the promoter function of the Ser-1 gene and characterize the factors binding to these sites, the upstream region was dissected. Footprinting assays revealed the presence of at least three specific protein binding sites, SA (-103 to -85), SB (-149 to -135) and SC (-204 to -183). Removal of SA and SC by 5'-deletions decreased the promoter activity. DNase I footprinting using four extracts of B. mori origin indicated that the factor binding to SA is specific to the silk gland extracts, and the factor binding to SC is very much abundant in the MSG extracts. Stimulation of the promoter activity with the region from -239 to -174 that accommodates the SC site, is stronger in MSG extracts than in the extracts from the posterior silk glands. These results suggest that the SC binding factor abundant in MSG may play an important role in the MSG-specific expression of the Ser-1 gene.