Factors involved in preferential transcription of the fibroin gene.

Transcription factors for class II genes from a tissue source, the posterior silk glands of Bombyx mori, were fractionated by stepwise elution on a phosphocellulose column into five fractions: A (0.04 M KCl flow through), A' (0.1 M KCl eluate), B (0.3 M KCl eluate), C (0.5 M KCl eluate) and D (1 M KCl eluate). The minimal requirement for reconstruction of accurate initiation of transcription of the fibroin gene, as well as of the adenovirus 2 major late promoter, was the combination of fractions A, B and D, suggesting that transcription factors from B. mori can recognize general signals of the promoters for class II genes and that basic transcription factors are conserved even in distantly diverged species of eukaryotes. To detect activities stimulating the transcription governed by the promoter of fibroin gene, each fraction was tested for its function by supplementing a basal amount of HeLa cell extract. When circular templates were used, stimulatory activities specific for the fibroin gene were detected in fraction D. This preferential transcription is composed of at least three activities; the first and the second dependent on the upstream elements and the third dependent on the sequence downstream from the TATA box. However, when linear templates were used the preference for the fibroin gene was apparently lost and transcription activation by fraction D became general.