Cytosine methylation as an effector of right-handed to left-handed DNA structural transitions

Cytosine methylation has energetic and structural influences on left-handed Z-DNA formation in supercoiled plasmids. The restriction and modification enzymes from Haemophilus haemolyticus (HhaI and M · HhaI) provide a system to locate and analyze small segments of Z-DNA in large supercoiled plasmids. An approach is outlined that uses M · HhaI as an in vivo conformational probe for the detection of unusual DNA structures in a living cell. Also, characteristic features of the M · HhaI gene and protein are discussed. © 1988.