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Multiple-Size Plasmids in Agrobacterium radiobacter and A. tumefaciens. S. Sheikholeslam, Department of Plant Pathology, University of California, Davis, CA 95616; B.-C. Lin(2), and C. I. Kado(3). (2)(3)Department of Plant Pathology, University of California, Davis, CA 95616, (2)Present address: Biology Research Center, Academia Sinica, Nanking, Taipei, Taiwan. Phytopathology 69:54-58. Accepted for publication 3 August 1978. Copyright 1979 The American Phytopathological Society. DOI: 10.1094/Phyto-69-54.

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