INFLUENCE OF DNA GEOMETRY ON TRANSCRIPTIONAL ACTIVATION IN ESCHERICHIA-COLI

Transcription from many Escherichia coli promoters can be activated by the cAMP-CRP complex bound at different locations upstream of the pro moter. At some locations the mechanism of activation involves direct p rotein-protein contacts between CRP and the RNA polymerase. We positio ned the CRP binding site at various distances from the transcription s tart site of the malT promoter and measured the in vivo activities of these promoter variants. From the activation profiles we deduce that t he protein-protein interactions involved in transcriptional activation are rather rigid. A heterologous protein (IHF) that bends the DNA to a similar degree as does CRP activates transcription when bound at sit es equivalent to activating positions for CRP. DNA geometry makes a ma jor contribution to the process of transcriptional activation and DNA upstream of the activator binding site participates in this process. R emoval of this DNA decreases the capacity of the malT promoter to be a ctivated by CRP in vitro. We conclude that both DNA topology and direc t protein-protein contacts contribute to transcriptional activation an d that the relative importance of these two modes of activation depend s on the nature of the activator and on the location of the activator binding site.