Repressor induced site-specific binding of HU for transcriptional regulation.

Transcription from two overlapping gal promoters is repressed by Gal repressor binding to bipartite gal operators, O(E) and O(I), which flank the promoters. Concurrent repression of the gal promoters also requires the bacterial histone-like protein HU which acts as a co-factor. Footprinting experiments using iron-EDTA-coupled HU show that HU binding to gal DNA is orientation specific and is specifically dependent upon binding of GalR to both O(E) and O(I). We propose that HU, in concert with GalR, forms a specific nucleoprotein higher order complex containing a DNA loop. This way, HU deforms the promoter to make the latter inactive for transcription initiation while remaining sensitive to inducer. The example of gal repression provides a model for studying how a 'condensed' DNA becomes available for transcription.