Yang-Gyun Kim, Ky Lowenhaupt, Stefan Maas, Alan Herbert, Thomas Schwartz^§, and Alexander Rich^¶

From the Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 

The Zab domain of the editing enzyme ADAR1 binds tightly and specifically to Z-DNA stabilized by bromination or supercoiling. A stoichiometric amount of protein has been shown to convert a substrate of suitable sequence to the Z form, as demonstrated by a characteristic change in the CD spectrum of the DNA. Now we show that Zab can bind not only to isolated Z-forming d(CG)_n sequences but also to d(CG)_n embedded in B-DNA. The binding of Zab to such sequences results in a complex including Z-DNA, B-DNA, and two B-Z junctions. In this complex, the d(CG)_nsequence, but not the flanking region, is in the Z conformation. The presence of  Z-DNA was detected by cleavage with a Z-DNA specific nuclease, by undermethylation using Z-DNA sensitive SssI methylase, and by circular dichroism. It is possible that Zab binds to B-DNA with low affinity and flips any favorable sequence into Z-DNA, resulting in a high affinity complex. Alternatively, Zab may captureZ-DNA that exists transiently in solution. The binding of Zab to potential as well as established Z-DNA segments suggests that the range of biological substrates might be wider than previously thought.

^§ Present address: Crystallography Group, Max-Delbrück-Center for Molecular Medicine, Robert-Rössle-Str. 10, Berlin, D-13125, Germany.

^¶ To whom correspondence should be addressed. Tel.: 617-258-9299; Fax: 617-253-8699;
E-mail: cbeckman@mit.edu

1. "Mated Models of Gene Regulation in Eukaryotes".

2. "Oncogenes as Molecular Targets within Active Chromatin".

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