The lac operator nucleosomes. 1. Repressor binds specifically to operator within the nucleosome core

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Abstract
We have shown that the lac repressor can recognize and bind specifically to the lac operator contained in short restriction fragments which have been complexed with the four core histones to form artificial nucleosomes and core particles. These lac reconstitutes have been well characterized, and it is apparent that the operator DNA itself is associated fully and normally with the octameric histone cores. The binding of repressor to these reconstitutes is operator dependent since nucleosomes lacking the operator sequence fail completely to bind repressor under our conditions. Moreover, binding is abolished by IPTG (isopropyl thiogalactoside), further demonstrating operator specificity. Nevertheless, sedimentation studies show that repressor binding does not involve displacement of the histone octamer. Thus, the lac repressor and the histone octamer bind simultaneously to the same DNA. lac reconstitutes, in which the DNA has been cross-linked to the histones with formaldehyde, also support simultaneous specific binding by lac repressor. Since all particles among the reconstitutes, cross-linked or not, bind repressor quantitatively, we infer that the repressor binding surface of the operator DNA always faces generally outward rather than inward toward the histone core. It is likely to be this feature of lac operator particle structure, dictated in an unknown manner by DNA sequence, that allows the simultaneous binding of histones and repressor to the same DNA region.

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