Repressor induced site-specific binding of HU for transcriptional regulation

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Abstract
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.