THE Z-ALPHA DOMAIN FROM HUMAN ADAR1 BINDS TO THE Z-DNA CONFORMER OF MANY DIFFERENT SEQUENCES

Z-DNA, the left-handed conformer of DNA, is stabilized by the negative supercoiling generated during the movement of an RNA polymerase throu gh a gene. Recently, we have shown that the editing enzyme ADAR1 (doub le-stranded RNA adenosine deaminase, type 1) has two Z-DNA binding mot ifs, Z alpha and Z beta, the function of which is currently unknown. H ere we show that a peptide containing the Z alpha motif binds with hig h affinity to Z-DNA as a dimer, that the binding site is no larger tha n 6 bp and that the Z alpha domain can flip a range of sequences, incl uding d(TA)(3), into the Z-DNA conformation. Evidence is also presente d to show that Z alpha and Z beta interact to form a functional DNA bi nding site. Studies with atomic force microscopy reveal that binding o f Z alpha to supercoiled plasmids is associated with relaxation of the plasmid. Pronounced kinking of DNA is observed, and appears to be ind uced by binding of Z alpha. The results reported here support a model where the Z-DNA binding motifs target ADAR1 to regions of negative sup ercoiling in actively transcribing genes. In this situation, binding b y Z alpha would be dependent upon the local level of negative superhel icity rather than the presence of any particular sequence.