The Biology of Left-handed Z-DNA

DNA encodes biological information in at least two different ways. First, through the linear order of nucleotides, it specifies the composition of proteins. Second, through its shape, DNA can provide information that is used directly or indirectly by a variety of macromolecules to regulate the assembly of cellular machines. DNA is capable of assuming many shapes (1). One of the most dramatic changes in shape is that which occurs in going from the familiar right-handed B-DNA double helix to the slightly thinner and elongated left-handed Z-DNA confor-mation (2). This conformational change occurs most readily in segments with specialized sequences, favored largely by alter-nations of purines and pyrimidines, especially alternating de-oxycytidine and deoxyguanosine residues (3–5). A requirement for specialized nucleotide sequences also appears to be true for other unusual DNA structures such as the TATA box, which