Identification of transcription factors required for the expression of mammalian U6 genes in vitro.

Transcription factors, required for the basal expression of the mouse U6 gene were identified in extracts from HeLa cells. This gene is transcribed at least four times more efficiently than its human counterpart in extracts from mouse or HeLa cells and hence provides an excellent in vitro system for the identification of transcription factors involved in the basal expression of mammalian U6 genes. At least four separate protein components were found to be required in addition to RNA polymerase III for correct synthesis of U6 RNA in vitro. These correspond to: (i) TFIIIB; (ii) a heat labile activity contained in a protein fraction enriched in TFIID; (iii) an, as yet, uncharacterized component contained in the flow-through upon rechromatography on phosphocellulose, and finally; (iv) a protein specifically binding to the mouse U6 gene promoter and transactivating its expression. Transcription factors IIIA and IIIC are not involved in mammalian U6 transcription in vitro. The U6-specific transcription factor has a molecular mass of approximately 90 +/- 10 kDa. It specifically binds to the U6 gene from bp -42 to -78 on the coding and from bp -37 to -79 on the non-coding strand thereby centrally encompassing the PSE motif of the mouse U6 promoter. The binding activity of this protein is correlated with the efficiency with which the U6 gene is transcribed in vitro, thereby indicating a crucial role of the PSE-binding protein for U6 transcription.