Human RNA editing enzyme ADAR1 deaminates adenosine in pre-mRNA to yield inosine. The Zα domain of human ADAR1 (hZαADAR1) binds specifically to left-handed Z-RNA as well as Z-DNA and stabilizes the Z-conformation. To answer the question of how hZαADAR1 can induce both the B–Z transition of DNA and the A–Z transition of RNA, we investigated the structure and dynamics of hZαADAR1 in complex with 6-base-pair Z-DNA or Z-RNA. We performed chemical shift perturbation and relaxation dispersion experiments on hZαADAR1 upon binding to Z-DNA as well as Z-RNA. Our study demonstrates the unique dynamics of hZαADAR1 during the A–Z transition of RNA, in which the hZαADAR1 protein forms a thermodynamically stable complex with Z-RNA, similar to Z-DNA, but kinetically converts RNA to the Z-form more slowly than DNA. We also discovered some distinct structural features of hZαADAR1 in the Z-RNA binding conformation. Our results suggest that the A–Z transition of RNA facilitated by hZαADAR1 displays unique structural and dynamic features that may be involved in targeting ADAR1 for a role in recognition of RNA substrates.

I-motif or C4 is a four-stranded DNA structure with a protonated cytosine:cytosine base pair (C+:C) found in cytosine-rich sequences. We have found that oligodeoxynucleotides containing adenine and cytosine repeats form a stable secondary structure at a physiological pH with magnesium ion, which is similar to i-motif structure, and have named this structure ‘adenine:cytosine-motif (AC-motif)’. AC-motif contains C+:C base pairs intercalated with putative A+:C base pairs between protonated adenine and cytosine. By investigation of the AC-motif present in the CDKL3 promoter (AC-motifCDKL3), one of AC-motifs found in the genome, we confirmed that AC-motifCDKL3 has a key role in regulating CDKL3 gene expression in response to magnesium. This is further supported by confirming that genome-edited mutant cell lines, lacking the AC-motif formation, lost this regulation effect. Our results verify that adenine-cytosine repeats commonly present in the genome can form a stable non-canonical secondary structure with a non-Watson–Crick base pair and have regulatory roles in cells, which expand non-canonical DNA repertoires.

Z‐DNA, which is left‐handed double‐stranded DNA, is involved in various cellular processes. However, its biological roles have not been fully evaluated due to the lack of tools available that can control the precise conformational change to Z‐DNA in vitro and in vivo. Therefore, the need for identifying new Z‐DNA inducers is high. We developed an assay system to monitor the conformational change in DNA utilizing the fluorescence of SYBR green I integrated into a double‐stranded oligonucleotide. By applying this assay to screen for compounds that induce the B‐DNA to Z‐DNA transition, we identified the natural compound aklavin as a novel Z‐DNA inducer.