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 nucleo-protein 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.