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. 2014 Oct 6;111(42):E4449–E4457. doi: 10.1073/pnas.1410764111

Fig. 2.

Fig. 2.

Measuring looping in vivo. (A, Left) A LacI tetramer represses the promoter by binding to a proximal lac operator (OP). Repression can be improved by DNA looping between OP and a distal lac operator (OD) because a tetramer bound at OD constitutes a high local concentration at OP due to DNA tethering. The strength of the OD effect depends on the operator combination, the LacI concentration and the length of the spacer DNA. (Right) The effect of OD on reporter expression can be used to measure the fraction of time that the system spends in the looped state, FLac. In the absence of looping, FLac = 0, activity is the same with or without OD. If the system were fully looped, FLac = 1, the activity of the OD–OP reporter would be reduced to background. (B) CI. (Left) Repression of the PRM promoter is dependent on DNA looping between OR and OL, because OR3 is too weak to be occupied by a CI dimer at physiological concentrations. (Right) As with LacI, the effect of OL on reporter expression can be used to measure the fraction of time that the system spends in the looped state, FCI.