The Zab domain of the human RNA editing enzyme ADAR1 recognizes Z-DNA whensurrounded by B-DNA

The Zab domain of the editing enzyme ADAR1 binds tightly and specifically t o Z-DNA stabilized by bromination or supercoiling. A stoichiometric amount of protein has been shown to convert a substrate of suitable sequence to th e 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;), embedded in B-DNA, The binding of Zab to such sequences results in a complex including Z-DNA B-DNA, and two B-Z junc tions. In this complex, the d(CG), sequence, but not the flanking region, i s in the Z conformation. The presence of Z-DNA was detected by cleavage wit h 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-D NA with low affinity and flips any favorable sequence into Z-DNA, resulting in a high affinity complex. Alternatively, Zab may capture Z-DNA that exis ts transiently in solution. The binding of Zab to potential as well as esta blished Z-DNA segments suggests that the range of biological substrates mig ht be wider than previously thought.