Natural plasmid pSD89(Cm-r) from Salmonella derby K89 enhancing radiation resistance of Escherichia coli K-12 strains

Several plasmids with molecular mass of 1.3-9 MDa were found in a clinical isolate of Salmonella derby K89 by electrophoresis in agarose gel. One of t hese plasmids, designated pSD89(Cm-1), was derived from the K89 strain via transformation of the plasmidless recipient S. derby K82 to chloramphenicol resistance. The plasmid-carrying strain K89 and the K82 strain completely cured of plasmids were equally sensitive to the lethal action of UV light, whereas the plasmid-carrying strain was even more sensitive to ionizing rad iation than the plasmidless variant. Nevertheless, transformants carrying o nly plasmid pSD89(Cm-1) were found to be more resistant to gamma-rays and U V light than the recipient. By using an intermediate host Escherichia coli Z805(r(-)m(+)), plasmid pSD89(Cm-1) was introduced into different E. coli K -12 strains: polA(-), recA(-) uvrA(-), umuC(-), and the wild-type strain. A slight increase in radioresistance off. coil wild-type cells and a signifi cant complementation of a repair defect in recA and polA mutants, but not i n uvrA and umuC, were observed.