All of eight strains of examined contained between two and six plasmids ranging from 7 to > 200 MDal in size. Strain MCC-1, a derivative of NCIMB 8003, was cured of various of the four largest of its five plasmids and the phenotypes of the strains compared. All fixed nitrogen and exhibited uptake hydrogenase activity. No differences were observed in carbon source utilization or antibiotic, heavy metal or UV resistance. The genome sizes of two strains of were determined by two-dimensional electrophoresis. Strain CW8, an isolate from local soil containing two small plasmids of 6 and 6·5 MDal contained unique DNA sequences equivalent to 1·78 × 10 (·20%) bp (1·2 × 10 Dal). In strain MCD-1, a derivative of MCC-1, containing a 190 MDal and 7 MDal plasmid, the genome size was 1·94 × 10 (·20%) bp. In exponential batch cultures, both contained 20 to 25 genome equivalents per cell. MCD-1 exhibited complex UV kill kinetics with a marked plateau of resistance; CW8 showed a simple response inconsistent with the possibility of organization of its DNA into identical chromosome copies capable of independent segregation.


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