Electron Microscopic Evidence for a Multimeric System of Plasmids in Fast-Growing Rhizobium Spp.

Abstract

Covalently closed circular (CCC)-DNA could be isolated, by dye-buoyant density centrifugation, from l3 out of 15 strains from three species of fast-growing rhizobia. The plasmid band was also present in ineffective mutants and non-infective mutants derived from symbiotically effective, plasmid-carrying parent strains. The buoyant density in caesium chloride was 1· 719 g/cm3 for chromosomal DNA and 1· 715-1· 716 g/cm3 for plasmid DNA in the four strains examined. Electron microscopy of the CCC-DNA from five strains revealed a wide size range of large circular molecules, with the contour length ranging from l3. 5 to c. 170 jim (mol. wt range 28 x 106-352 x 106 ). Analysis of the size distribution of open circular molecules from the five strains indicated that a multimeric series of plasmid molecules may occur in these bacteria, with an estimated mean monomer length of l3·5 jlin.