Bacterial repression loops require enhanced DNA flexibility.

The Escherichia coli lac operon provides a classic paradigm for understanding regulation of gene transcription. It is now appreciated that lac promoter repression involves cooperative binding of the bidentate lac repressor tetramer to pairs of lac operators via DNA looping. We have adapted components of this system to create an artificial assay of DNA flexibility in E.coli. This approach allows for systematic study of endogenous and exogenous proteins as architectural factors that enhance apparent DNA flexibility in vivo. We show that inducer binding does not completely remove repression loops but it does alter their geometries. Deletion of the E.coli HU protein drastically destabilizes small repression loops, an effect that can be partially overcome by expression of a heterologous mammalian HMG protein. These results emphasize that the inherent torsional inflexibility of DNA restrains looping and must be modulated in vivo.