Stabilization and Detection of Natural Left-Handed Z-DNA

Left-handed Z-DNA is favored by nucleotide sequences with alternating purines and pyrimidines. The structural basis of this preference is described, as well as the factors which stabilize Z-DNA. The most important biological factors stabilizing Z-DNA are negative supercoiling and Z-DNA binding proteins. Experiments are described which make it possible to determine the nucleotide sequences of segments which form Z-DNA through the use of specific antibodies. Parts of the plasmid pBR322 form Z-DNA upon negative supercoiling. Similarly, three segments of the SV40 simian tumor virus form Z-DNA. They are located in the transcriptional enhancer region and may function, together with Z-DNA binding proteins, in facilitating the attachment of RNA polymerase to the DNA in chromatin. This is the first specific example suggesting a role of Z-DNA in regulating transcription.