The Zab Domain of the Human RNA Editing Enzyme ADAR1 Recognizes Z-DNA When Surrounded by B-DNA

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) n sequence, 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 Sss I 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 capture Z-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.