To learn more about the effects of transcription on DNA structure in E. coli topA

-

mutant

the plasmid pJGX15A

carrying a tandem transcription unit of tetA-cfaABCE

was used as the reporter plasmid. The topology of pJGX15A from HB101 (topA

+

gyrB

+

) and DM800 (topA

-

gyrB225) cells was then analyzed by chloroquine agarose gel electrophoresis. And

a type of irregular condensed structures of pJGX15A was found in DM800 cells. The condensed plasmid DNA had a high mobility rate and appeared to be smear bands in agarose gel electrophoresis. The topology of pJGX15A was also greatly affected by both cell growth phase and concentration of tetracycline in medium. The condensed structures were only formed in DM800 cells of exponential phase cultured in the medium with the concentration of tetracycline  below 6 μg/mL. AFM images showed that these plasmid DNA molecules contained intramolecular conde nsation to different levels instead of normal supercoils. Some of the molecules were even completely condensed to a coil.