We are interested in the effect of proteins on the flexibility of DNA within living cells. We hypothesize that the intrinsic longitudinal and torsional inflexibility of DNA is overcome in cells by non-specific DNA bending proteins (i.e., bacterial HU and eukaryotic HMGB). We have adapted classic lac repressor DNA looping studies within living E. coli cells to monitor DNA flexibility. Using this system we can test how apparent DNA flexibility responds to changes in the concentrations of endogenous or exogenous architectural proteins expressed in E. coli. We present the effects on DNA looping of deletion of E. coli genes encoding HU, IHF, and H-NS, and upon expression of exogenous HMGB yeast proteins. These data address the fundamental question of how the intrinsic physical properties of DNA are managed in a biological context.