Different Z DNA forming sequences are revealed in phi X174 RFI by high resolution darkfield immuno‐electron microscopy.

The specific interaction between left‐handed Z DNA sequences in negatively supercoiled bacteriophage phi X174 replicative form I (RFI) DNA and anti‐Z DNA immunoglobulin G (IgG) was investigated by high resolution darkfield immuno‐electron microscopy. DNA‐antibody complexes were formed and maintained under optimal binding conditions, purified by column chromatography, and visualized after uranyl acetate staining without using aldehyde fixation, shadowing, or second antibody. Bivalent anti‐Z DNA IgGs bound to RFI molecules, thus forming intramolecular bridges. They could also oligomerize separate molecules by intermolecular linking of Z DNA sequences. At relatively low ionic strength and low temperature, high affinity anti‐Z IgG was retained at certain loci even after restriction endonuclease cleavage of the DNA. In these cleaved molecules some superhelices could be visualized in the loops generated by the bivalent IgG. To our knowledge this is the first example of polypeptide stabilization of local superhelical strain in a cut molecule. Z DNA sequences in phi X174 RFI DNA were mapped. Alternating tracts of purines and pyrimidines starting at nucleotides 763, 1027, 1714, 2146, 2363, 3504, 4161, 4911 and 5345 occur within the nine different anti‐Z IgG binding sites which were expressed with varying frequencies (53‐3%) on the molecules. Usually, a limited number of sites (generally less than or equal to 2) exists on any one molecule. The formation of multiple Z sites (at the extracted superhelix density) in a given molecule is probably non‐cooperative due to relaxation of torsional stress by the B–‐Z transition. Z sites occur in several different genes, including regions where transcription is attenuated and, in one case, in front of a promoter of transcription.