Supercoiled circular DNA in crown-gall inducing Agrobacterium strains

Abstract

By alkaline sucrose gradient, neutral sucrose gradient and dye-buoyant density centrifugation a large plasmid was shown to be present in the crown-gall inducing Agrobacterium tumefaciens strain B6-S3. Measurements of contour lengths carried out on electron micrographs resulted in a mean length of 54·1 μ m, corresponding to a molecular weight of 112 × 10 6 . Only one or a few copies of this plasmid are present per bacterial chromosome. Mitomycin C induction has no influence on the amount of plasmid DNA in the cell. At present this plasmid must be considered as cryptic, for no genetic markers on it are known. Furthermore, large plasmids were isolated from crown-gall inducing strains belonging to seven Agrobacterium groups described by Kersters et al. (1973). Contour length measurements carried out on the plasmids isolated from the various crown-gall inducing strains fell in the range from 54·1 μ m to 75·4 μ m, depending on the strain examined. We were not able to find such plasmids in eight non-pathogenic strains belonging to four of the same groups. The hypothesis is formulated that the large plasmid present in crown-gall inducing bacteria could be the “tumor-inducing principle”.