Effects of the antitumor drug VP16 (etoposide) on the archaebacterial Halobacterium GRB 1.7 kb plasmid in vivo.

The topoprofile of 1.7 kb plasmids from the archaebacterium Halobacterium GRB was analysed from cells growing with or without VP16 (etoposide). This drug interferes with the breakage-reunion reaction of eukaryotic DNA topoisomerase II by inhibiting the ligase activity of this enzyme. Addition of VP16 to the culture medium of Halobacterium GRB cells results in the introduction of single- and double-strand DNA breaks in part of the plasmid population, with proteins covalently associated at their 5' ends. While some of the remaining covalently closed circular DNA molecules are relaxed, VP16 treatment also gives rise to the production of positively supercoiled 1.7 kb plasmids. In contrast to adriamycin, VP16 does not intercalate into the 1.7 kb plasmid DNA in vivo. These results suggest that the VP16 target in halobacteria is a DNA topoisomerase II. Three major cleavage sites were detected on the 1.7 kb plasmid after VP16 treatment in vivo.