TRANSCRIPTIONAL ACTIVATION BY PROTEIN-INDUCED DNA BENDING - EVIDENCE FOR A DNA STRUCTURAL TRANSMISSION MODEL

Integration host factor (IHF) is a DNA-bending protein that binds to a n upstream activating sequence (UAS1) and, on a negatively supercoiled DNA template, activates transcription from the ilvP(G) promoter of th e ilvGMEDA operon of Escherichia coil, The transcriptional initiation site of the ilvGMEDA operon is located 92 bp downstream of UAS1, Activ ation is still observed when the orientation of the upstream II-IF bin ding site is reversed, This manipulation places the IHF binding site o n the opposite face of the DNA helix, directs the IHF-induced DNA bend in the opposite direction, and presents the opposite face of the nons ymmetrical, heterodimeric, IHF molecule to the downstream RNA polymera se, Lymphoid enhancer-binding factor, LEF-1, is a DNA-bending, lymphoi d-specific, mammalian transcription factor that shares no amino acid s equence similarity with IHF, When the IHF site in UAS1 is replaced wit h a LEF-1 site, LEF-1 activates transcription from the downstream ilvP (G) promoter in E, coli as well as it is activated by its natural acti vator, II-IF, These results suggest that specific interactions between IHF and RNA polymerase are not required for activation, The results o f DNA structural studies show that IHF forms a protein-DNA complex in the UAS1 region that, in the absence of RNA polymerase, alters the str ucture of the DNA helix in the -10 hexanucleotide region of the downst ream ilvP(G) promoter, The results of in vitro abortive transcription assays show that IHF also increases the apparent rate of RNA polymeras e isomerization from a closed to an open complex, We suggest, therefor e, that IHF activates transcription by forming a higher-order protein- DNA complex in the UAS1 region that structurally alters the DNA helix in a way that facilitates open complex formation at the downstream ilv P(G) promoter site.