[Unpaired bases in the B--Z junction of the supercoiled plasmid]

The restriction analysis has been used to establish that O-beta-diethylaminoethylhydroxylamine (OHA) produces modification of unpaired cytidines in the polylinker region adjacent to the Z-insert (dG-dC)10. (dG-dC)10 in the negatively supercoiled plasmid pGC20. The length of the transition region between B- and Z-portions of DNA is not less than 36 bps. The reaction of OHA with the unpaired cytidines in the B-Z junction is a fixing one and produces no secondary despiralling of the neighboring regions. The reaction with DNA proceeds much slower than the one with monomers and single-strand polynucleotides. The structural nonuniformity has been observed, which is manifested in the alternating B and "non-B" form DNA in the B-Z junction. It is suggested that these junctions may contain nucleotide sequences which are stable to violation of the B structure during the change in superhelical density of DNA.