Genome Size and Complexity in Azotobacter chroococcum

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Abstract
All of 8 strains of A. chroococcum examined contained 2-6 plasmids ranging from 7 to > 200 megadaltons (MDal) in size. Strain MCC-1, a derivative of NCIMB 8003, was cured of the 4 largest of its 5 plasmids and the phenotypes of the strains were compared. All fixed Na and exhibited uptake hydrogenase activity. No differences were observed in C source utilization or antibiotic, heavy metal or UV resistance. The genome sizes of 2 strains of A. chroococcum were determined by 2-dimensional electrophoresis. Strain CW8, an isolate from local soil containing 2 small plasmids of 6 and 6.5 MDal, contained unique DNA sequences equivalent to 1.78 .times. 106 (.+-. 20%) base pairs (bp) (1.2 .times. 109 daltons). In strain MCD-1, a derivative of MCC-1, containing a 190 MDal and a 7 MDal plasmid, the genome size was 1.94 .times. 106 (.+-. 20%) bp. In exponential batch cultures, both contained 20-25 genome equivalents/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.