Multiple-Size Plasmids inAgrobacterium radiobacterandA. tumefaciens

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Abstract
Tumorigenic A. tumefaciens and naturally avirulent A. radiobacter strains harbor large plasmids with MW ranging 27-154 megadaltons, and mini plasmids with MW of 1.0-12.0 megadaltons. Replicating plasmid molecules were observed by EM, indicating that these molecules are autonomous. The guanine-cytosine composition of these large plasmids (58.8 mol %) is equivalent to that of chromosomal DNA. The number of total plasmid copies per cell is about 2; they may be either 2 large plasmids or 1 of each size class. The small plasmid probably arose from the large plasmid and represent a mechanism for amplification of virulence genes that could account for the various degrees of virulence that were observed commonly.