The in-vivo occurrence of Z DNA

The energetics of the B-Z transition of two different types of cloned alternating purine/pyrimidine DNA sequences have been analysed by a two dimensional electrophoretic technique. Since the transition between right handed and left handed forms of these polymers is detected by alterations of electrophoretic mobilities of topoisomers of the plasmid DNA molecules, the method is not dependent on Z-DNA binding ligands. The measurements reflect intrinsic properties of the DNA unperturbed by the free energy of binding such a ligand. Direct evidence from the analysis of topoisomer distributions is presented which shows that d(GC)n.d(GC)n sequence elements within an E. coli plasmid will adopt a Z conformation in-vivo under conditions of blocked protein synthesis. Evidence for the in-vivo occurrence of Z-DNA was not detected in plasmid DNA isolated from bacterial cells growing in the absence of protein synthesis inhibitors. A model is proposed for a function for the B-Z transition in ensuring the correct pairing of homologous chromosomes during meiosis.