In vivo existence of left-handed DNA.

A genetic-biochemical assay has been developed to investigate the in vivo existence and consequences of unusual DNA structures. Left-handed DNA was shown to exist in living Escherichia coli. The EcoRI methyltransferase gene (temperature-sensitive) was cloned to serve as a probe for perturbed GAATTC sites in vivo. This plasmid was cotransformed with different plasmids containing inserts that had varying capacities to form left-handed helices or cruciforms with a target EcoRI site in the center or at the ends of the inserts. Inhibition of methylation in vivo was found for the stable inserts with the longest left-handed helices. In vitro methylation with the purified M.EcoRI enzyme agreed with the in vivo results.