EXPERIMENTAL FIGURE 9-5 DNA looping permits interaction of bound NtrC and σ⁵⁴-RNA polymerase. (a) Drawing (left) and electron micrograph (right) of DNA restriction fragment with phosphorylated NtrC dimers bound to the enhancer region near one end and σ⁵⁴-RNA polymerase bound to the glnA promoter near the other end. (b) Drawing (left) and electron micrograph (right) of the same fragment preparation, showing NtrC dimers and σ⁵⁴-RNA polymerase bound to each other, with the intervening DNA forming a loop between them. See W. Su et al., 1990, Proc. Natl. Acad. Sci. USA 87:5504.

[Micrographs courtesy Harrison Echols and Carol Gross.]