Fig. 6.

Models for Nun and Gre binding in TEC. (A) The relative arrangement of the RNA:DNA hybrid, active center, β′ Flap domain, and Nun in TEC. The basic features on the DNA:DNA hybrid surrounding the hybrid are indicated. Active center Mg²⁺ ions are shown as green circles. The blue arrow indicates the proposed displacement of the Flap loop by Nun, which opens the RNA exit channel. In the first model, two TEC-bound Nun molecules (blue spheres) cross-link to the 3′ and 5′ RNA ends. In the second model, a single Nun molecule (cyan sphere) cross-links to both RNA ends. The red arrow indicates a possible 3′ cross-link to Nun through the main channel. (B) High resolution model for GreA (Right) and GreB (Left) binding in the vicinity of the active center. The Black arrow indicates the possible exit for the 3′ backtracked RNA segment in the GreB complex, which is blocked in the GreA complex.