Enhancement of transcription from the Ad2 major late promoter by upstream elements of the fibroin- and sericin-1-encoding genes in silk gland extracts.

The activity of the upstream promoter element (UPE) of the Bombyx mori fibroin-encoding gene was analyzed by using the major late promoter (MLP) of adenovirus 2 (Ad2) as the basal promoter. The UPE reveals remarkable stimulatory activity on the transcription of Ad2 MLP in tissue extracts prepared from the posterior silk gland where the fibroin-encoding gene is specifically expressed. This stimulation of transcription is due to an increase of initiation complex formation as shown by a one-round transcription assay. The UPE of the sericin-1-encoding gene, which is specifically expressed in the middle silk gland, also stimulates Ad2 MLP transcription in a similar manner. The results of the competition experiments indicate that the stimulatory effect is mediated by trans-acting factors which bind to these UPE. These two UPE are also active in tissue extracts of the middle silk gland, but are nearly inactive in extracts from the ovarian tissue and an embryonic cell line, Bm-e21. These observations further support the hypothesis that a silk-gland-specific factor(s) is responsible for this tissue-specific transcription in vitro. The possible molecular basis of this silk-gland specificity is discussed in light of the various trans-acting factors known to bind these UPE.