Abstract: After treating the plasmid of Escherichia coli HB101 by CdCl₂ in vitro or in vivo,the effect of Cd on the structure of plasmid pWH58 DNA was studied through argarose gel electrophesis and restriction fragment length polymorphism analysis.By comparing the growth of E.coli with and without plasmid cultured in the medium with Amp LB and non anti LB under different concentrations of CdCl₂,the effect of Cd on plasmid E.coli of in vivo and the role of plasmid in Cd-tolerance of its host were studied.Cd treatments of E.coli in vitro or in vivo didn't cause an obvious change of the structure of plasmid pWH58 DNA.Plasmid pWh58 could replicate for regeneration and gene express under Cd stress.Plasmid pWH58 of E.coli in vivo lowered Cd-tolerance of its host form 75mg稬^-1 to 50mg稬^-1.Cd-tolerance of E.coli ascended markedly after Cd taming,but had a trend of reversing to original level after restoration,indicating that Cd-tolerance of E.coli could be tamed,but the tamed Cd tolerance still had no genetic basis.

Key words: CdCl₂, Plasmid, Escherichia coli, Tolerance