Incorporation of CC Steps into Z‑DNA: Interplay between B–Z Junction and Z‑DNA Helical Formation

The left-handed DNA structure, Z-DNA, is believed to play important roles in gene expression and regulation. Z-DNA forms sequence-specifically with a preference for sequences rich in pyrimidine/purine dinucleotide steps. In vivo, Z-DNA is generated in the presence of negative supercoiling or upon binding proteins that absorb the high energetic cost of the B-to-Z transition, including the creation of distorted junctions between B-DNA and Z-DNA. To date, the sequence preferences for the B-to-Z transition have primarily been studied in the context of sequence repeats lacking B–Z junctions. Here, we develop a method for characterizing sequence-specific preferences for Z-DNA formation and B–Z junction localization within heterogeneous DNA duplexes that is based on combining 2-aminopurine fluorescence measurements with a new quantitative application of circular dichroism spectroscopy for determining the fraction of B- versus Z-DNA. Using this approach, we show that at least three consecutive CC dinucleotide steps, traditionally thought to disfavor Z-DNA, can be incorporated within heterogeneous Z-DNA containing B–Z junctions upon binding to the Zα domain of the RNA adenosine deaminase protein. Our results indicate that the incorporation of CC steps into Z-DNA is driven by favorable sequence-specific Z–Z and B–Z stacking interactions as well as by sequence-specific energetics that localize the distorted B–Z junction at flexible sites. Together, our results expose higher-order complexities in the Z-DNA code within heterogeneous sequences and suggest that Z-DNA can in principle propagate into a wider range of genomic sequence elements than previously thought