SUMMARY: Studies have been carried out on the molecular nature of the plasmid DNA in four strains of carrying different mutants of the K88 factor. The factor was derepressed in three of the strains, one of which had kept the K88 antigen determinant, while the antigen was no longer demonstrable in the two other strains. The fourth strain possessed the K88 antigen but was transfer defective. The purified plasmid DNA was studied by electron microscopy and analytical CsCl density gradient centrifugation.

All the plasmid DNA from the four strains formed a single band in the neutral CsCl gradient at a density of 1.709 g/cm. The distributions of the contour lengths of the circular DNA molecules indicate three principal size classes of about 5, 20 and 25 µm. The small, 5 µm circles presumably contain the K88 determinant. The results seem to support the theory of a dissociation of composite plasmids.


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