A stereospecific alignment between the promoter and the cis-acting sequence is required for Lrp-dependent activation of ilvIH transcription in Escherichia coli.

The leucine-responsive regulatory protein (Lrp) is a DNA binding protein that affects, either positively or negatively, the expression of several E. coli genes. The ilvIH operon is positively regulated by Lrp and leucine counteracts this effect reducing 5- to 10-fold the efficiency of ilvIH transcription. An investigation of the mechanism of transcription activation of the ilvIH operon by Lrp indicated that: (i) a stereospecific alignment between the ilvIH promoter and the cis-acting sequence upstream of it is required for activation; (ii) a correct distance between the promoter and the adjacent cis-acting sequence is needed for leucine to counteract the positive role of Lrp; (iii) Lrp fails to activate transcription when the cis-acting region is placed several hundred base pairs upstream of the ilvIH promoter.