DUAL REGULATION OF THE DROSOPHILA HSP26 PROMOTER IN-VITRO

Efficient heat shock induction of Drosophila hsp26 gene transcription in vivo requires binding sites for heat shock factor (HSF) and GAGA fa ctor (GAF) close to the TATA box (proximal elements) as well as 350 bp upstream of the start site of transcription (distal elements), We hav e evaluated the contribution of hsp26 promoter sequences to transcript ional activity in extracts from either heat shocked or unstressed fly embryos, Efficient transcription in either extract was governed by dis tinct regulatory principles, Transcription in extracts from unstressed embryos relied solely on GAGA elements which efficiently counteracted repression by abundant non-specific DNA-binding proteins, Transcripti on in extracts from heat shocked embryos depended only a little on GAG A elements, relying mainly on functional HSEs, Constitutively active r ecombinant HSF or native factor in an extract from heat shocked embryo s was able to truly activate transcription essentially via proximal HS Es, but not when bound to distal sites, These two modes of regulation in vitro may correspond to the two functional states of the promoter b efore and after heat shock in vivo,