Interaction of the Zalpha domain of human ADAR1 with a negatively supercoiled plasmid visualized by atomic force microscopy.

Interest to the left-handed DNA conformation has been recently boosted by the findings that a number of proteins contain the Zalpha domain, which has been shown to specifically recognize Z-DNA. The biological function of Zalpha is presently unknown, but it has been suggested that it may specifically direct protein regions of Z-DNA induced by negative supercoiling in actively transcribing genes. Many studies, including a crystal structure in complex with Z-DNA, have focused on the human ADAR1 Zalpha domain in isolation. We have hypothesized that the recognition of a Z-DNA sequence by the Zalpha(ADAR1) domain is context specific, occurring under energetic conditions, which favor Z-DNA formation. To test this hypothesis, we have applied atomic force microscopy to image Zalpha(ADAR1) complexed with supercoiled plasmid DNAs. We have demonstrated that the Zalpha(ADAR1) binds specifically to Z-DNA and preferentially to d(CG)(n) inserts, which require less energy for Z-DNA induction compared to other sequences. A notable finding is that site-specific Zalpha binding to d(GC)(13) or d(GC)(2)C(GC)(10) inserts is observed when DNA supercoiling is insufficient to induce Z-DNA formation. These results indicate that Zalpha(ADAR1) binding facilities the B-to-Z transition and provides additional support to the model that Z-DNA binding proteins may regulate biological processes through structure-specific recognition.