Functional instability allows access to DNA in longer transcription Activator-Like effector (TALE) arrays

Transcription activator-like effectors (TALEs) bind DNA through an array of tandem 34-residue repeats. How TALE repeat domains wrap around DNA, often extending more than 1.5 helical turns, without using external energy is not well understood. Here, we examine the kinetics of DNA binding of TALE arrays with varying numbers of identical repeats. Single molecule fluorescence analysis and deterministic modeling reveal conformational heterogeneity in both the free- and DNA-bound TALE arrays. Our findings, combined with previously identified partly folded states, indicate a TALE instability that is functionally important for DNA binding. For TALEs forming less than one superhelical turn around DNA, partly folded states inhibit DNA binding. In contrast, for TALEs forming more than one turn, partly folded states facilitate DNA binding, demonstrating a mode of ‘functional instability’ that facilitates macromolecular assembly. Increasing repeat number slows down interconversion between the various DNA-free and DNA-bound states.

The bending of DNA in nucleosomes and its wider implications

The DNA of a nucleosome core particle is wrapped tightly around a histone octamer with approximately 80 base pairs per superhelical turn. Studies of both naturally occurring and reconstituted systems have shown that DNA sequences very often adopt well-defined locations with respect to the octamer. Recent work in this laboratory has provided a structural explanation for this sequence-dependent positioning in terms of the differential flexibility of different sequences and of departures from smooth bending. The ‘rules’ that are emerging for DNA bendability and, from the results of other workers, on intrinsically bent DNA, are likely to be useful in considering looping and bending of DNA in other processes in which it is thought to be wrapped around a protein core.